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Community and Q&A

Installed R-60 insulation in attic… now using AC more than before

jeffwatson | Posted in General Questions on

I had R60 blown into my 1000sqft attic where there was previously only about R11. Baffles were installed in almost every roof rafter. Air sealing of attic floor was performed. This is a low pitch asphalt shingled hip roof over the whole house.

As expected, the temperature in the house doesn’t fluctuate as much. However, I feel as if I’m using A/C more. Before the R60, the A/C would be on when it’s 80+ degrees out. Now I’m using it even when it’s low 70s outside because the house refuses to budge in temperature when trying to “air” it out by cracking some windows open. For example, daytime temp can be 85 degrees, and the A/C cools it down to 78 degrees. Overnight the outside temperature might by 70 degrees, and after leaving windows open all night, I wake up to the stat saying it’s 79 degrees.

What explains this? I have a hunch maybe I don’t have enough attic ventilation so all that hot mass stored in the insulation just releases itself instead of ventilating outside. I have 8 soffit vents and 2 turtle vents at the top of the roof. Do I need more?

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  1. jackofalltrades777 | | #1

    One would need hard data numbers. The problem with "feeling" is that there is no way to base any scientific results on it. What were your A/C bills prior to the R-60 insulation? Compare the R-11 A/C bill with the R-60 A/C bill and make sure the outside temps were similar as were the inside A/C control temps.

    As far as the attic venting goes. From what GBA stated in articles past, attic vents are mostly overrated and the main reason for vents is to DRY any moisture that may be on the wood, it doesn't really do much to "cool" an attic with more vents or fans based on the premise that it will somehow keep the home cooler. It was debunked as hype, especially those selling attic fans.

    Maybe your A/C is on the fritz and it is not cooling like it did before and therefore you are running it more often? Maybe the relative humidity is higher this year and you feel hotter, therefore you are running the A/C more often?

  2. davidmeiland | | #2

    Yeah, not enough data to draw a conclusion. You would need to track interior temps, exterior temps, probably some measure of solar loading, A/C runtimes or preferably actual output.

    An easy thing to do is check the A/C itself. Refrigerant charge and other specs should be verified.

  3. jeffwatson | | #3

    Thankyou Peter & David. I agree that if my goal was to determine if I'm using the A/C more, I'd need to collect some data & have a pre-R60 baseline to compare that data to.

    But my underlying goal is a step before that; along the lines of "why does it take forever to air out the house to drop interior temperature that I have to resort to using A/C," and that's assuming there's at least an 8-10 degree difference between exterior & interior temperature.

    I'm not too concerned with tracking my A/C usage; just the situations in which I actually turn it on. I don't consistently use A/C all day...I'm one of those "last resort" guys who turns it on if I(or the lady) just can't take it, and right after the R60 install, I'm hitting that scenario a lot more when outside temp would not necessitate A/C if we had the same temp before the R60.

    So I guess my question is less about A/C performance in its current environment, and more on the house environment & its resistance to cooling via natural means.

  4. SouthGeorgianBay | | #4

    Assuming that your A/C is mechanically sound, the biggest variable might be the air sealing.

    Did you do the work yourself, or better yet, was it tested with a blower door? Attics are awful places to work, and there is a strong incentive to cut corners, especially when the work will be buried beneath R60 of insulation.

    A second, related thought is whether there are any ducts in the attic that may have been damaged during the work, thereby connecting the super heated attic to your house. Even without the forced air system running, you would have a large pathway for heat to enter the conditioned part of your house. It's very easy for some ducts to become disconnected.

  5. jeffwatson | | #5

    Graham, I hired an insulation company to do the work. I do not have blower door numbers post-insulation, but what prompted the install was a recommendation by an energy audit that I had (that did include a blower door test). According to that test, my house was 1370 CFM @ 50 Pascals. When the insulation guys came & did their test before doing the insulation, it was 2500 CFm @ 50 Pascals, so not sure who to believe. They didn't take an 'after' test.

    I don't have any ducts in the attic; all my ducts run under the floors (single story home w/basement).

    I just don't understand why the temperature in the house rises overnight when it is colder outside & I've got literally all windows open.

  6. HD_DIY | | #6

    Very interesting and timely question! We are in the same boat. Had R49 blown into our attic about 6 weeks ago and got the first bill. Our kwh's used were higher this June vs last year. I'm going to post a similar question, but do you have any areas that aren't well insulated? Also having an energy audit performed in a couple of hours as well.

  7. GBA Editor
    Martin Holladay | | #7

    A few more points: while heat gain through ceilings can be significant, especially in a house that has very little ceiling insulation, two other sources of heat gain are often more significant: heat gain through windows and internal heat gains from lighting and other appliances (oven, kitchen range, refrigerator, TV, etc.).

    If any of these factors has changed in the last year, these will affect your cooling bill. Of course, so will the weather, which may be different this year than last year.

  8. bobhol | | #8

    Is it possible that the extra insulation is now trapping heat that otherwise could have escaped? Martin alluded to this as well...just a thought...

  9. GBA Editor
    Martin Holladay | | #9

    It's not really possible for the attic insulation to be causing any problems. If the outdoor temperature is in the low 70s, as reported, it's not as if you can cool a house by conduction through the ceiling drywall. The attic isn't going to be cool enough to pull heat from the house under these circumstances.

  10. gusfhb | | #10

    AC is not a window fan

    Using it the way you are it is impossible to judge how much energy you are using. It is likely you would use less energy leaving it on at a reasonable temp than the way you are using it.

    Look at the 3 day forecast, if it is going to be AC worthy turn it on and forget about it.

    You are not saving money making the AC pull all the moisture out then lowering the temp only to shut it off again

  11. jeffwatson | | #11

    Not sure if I'm getting my point across, so let's forget all about A/C for a minute. I'm not trying to compare cooling bills or the weather from last year. Just want to understand what is causing the heat flow.

    Since I'm talking about the temperature within the house rising overnight, this means all lights & heat-generating things are off, most windows are open, and the sun has already set. If it's 78 inside with windows open, how can the temperature rise overnight within the house if it's 70 or lower outside?

    Bob H, I am kind of thinking something along the same lines - that the insulation is somehow releasing the heat within the house instead of outside.

    Keith G, I'm not trying to gauge my energy usage, but I see where you're coming from. We only use A/C during the day and just don't see a point to using A/C overnight especially if it's 70 or below. That's when we crack the windows - only overnight. But that is what prompted this post - that now the house doesn't cool off overnight with natural ventilation.

    As an aside, we know the air within the house is at least circulating because our carbon dioxide meter might read 2000ppm right before we are about to go to sleep (from leaving the windows closed for A/C) and it'll be about 500ppm in the morning.

  12. wjrobinson | | #12

    Jeff.... I understand what you are thinking... If your roof is cooling way down maybe due to night sky radiational cooling then indeed it would have more effect on your living space with less insulation above the ceiling.

    Interesting.... I remember a scientist testing out whether he could concentrate night sky radiational cooling with a parabolic dish mirror he had that was about 3' in diameter.

  13. kevin_in_denver | | #13

    I think Jeff has narrowed his question down to the simplest terms:
    How come, prior to the insulation, he could cool his house at night by opening the windows, but now he can't?

    The answer is equally simple: There is a lot of heat stored by the thermal mass of the attic insulation, and this heat radiates downward.

    When you have a large, well insulated mass exposed to outside air, it will tend to hover within a few degrees of the average daily temperature, which Jeff says is 79F

    I've personally lived with this, and that's why for summer cooling I'd much rather have an unvented R60 foam roof than a vented attic full of fluff. Stop the heat at the roof plane, then you don't have to cool down the attic somehow before bedtime. A vented attic with fluff gets heated all day by the attic ventilation air.

    Now Jeff must install forced ventilation in the attic, to be used only at night. None of the articles and studies that denigrate forced attic ventilation have addressed this phenomenon.

    A whole house fan is worth considering, but they are usually a bad thermal bypass in winter.

    More soffit and turtle vents won't help because they don't move enough air volume, and can't be turned off during the day. Solar powered attic ventilators ditto.

    If not a whole house fan, then Jeff would benefit from window fans left on all night.

  14. GBA Editor
    Martin Holladay | | #14

    Q. "If it's 78 inside with windows open, how can the temperature rise overnight within the house if it's 70 or lower outside?"

    A. Refrigerator. Set-top box. Dishwasher. Small transformers. People. Dogs. Aquarium pumps. Internet routers. Computers.

  15. homedesign | | #15

    I am surprised at the wide range in the Blower Door Results...
    And wonder why the Contractor did a "Pre-Improvement" Blower door and didn't bother with an "After".
    Was the pre-test done mainly for the "Show" so he could get the "Dough"?
    How Airtight is the House Now?
    Did the Neutral Pressure Plane Shift After the Improvements?

    I agree with Keith Gustafson.
    Build Tight, Ventilate RIGHT ....
    Get the House and contents in "Good Condition".
    Maintain the "Good Condition" ...Coast when you can...
    Avoid STOP and GO (City Driving)

  16. kevin_in_denver | | #16


    The temperature didn't rise overnight before the attic insulation upgrade. Now it rises. Internal heat generation sources haven't changed.

  17. GBA Editor
    Martin Holladay | | #17

    There is a temperature gradient through the insulation on the attic floor. The bottom inch of insulation is at the same temperature as the ceiling drywall. Jeff reports running his air conditioner during the day to maintain the temperature of his indoor air (and his ceiling drywall) at 78 degrees. So when Jeff turns off his air conditioner and opens his windows to go to bed, the ceiling drywall and the bottom inch of attic insulation are at about 78 degrees.

    So, when it is beddy-by time for Jeff, is the top inch of attic insulation warmer or cooler than 78 degrees? I'm not sure -- that depends on how hot his attic is. If the weather is hot enough for Jeff to want to run his air conditioner, it's not cold up there. Jeff mentioned two outdoor temperatures: "the low 70s" (that's when he feels like turning on his air conditioner) and "85 degrees" (a typical day when he is running his air conditioner). Using this data, and my knowledge that attics are often hotter that the outdoors during the summer, I'm guessing that his attic air is at 76 degrees to 95 degrees on the days that we are discussing.

    So, the bottom inch of his attic insulation is at 78 degrees when he goes to bed. The top inch of insulation is somewhere between 76 degrees and 95 degrees when he goes to bed. My conclusion: the thick blanket of insulation on his attic floor either has no effect on his indoor temperature (when his attic is at 76 degrees) or is helping to keep his house cool (when his attic is 79 degrees or warmer).

  18. wjrobinson | | #18

    I think jeff is rightly mentioning some type of phenomena that is directly related to the insulation change.

    Time will tell and be on my side.

    Do as I do. Eat bacon and eggs and use real butter, and you will attain a life of joy and bliss... Positing ever so to your fellow man.

  19. Expert Member
    Dana Dorsett | | #19

    An R11 attic/roof will radiate quite a bit of heat to the (much colder than 70F radiation temp) sky at night, which will cool the house a bit. An R60 attic not so much, since the insulation slows the heat loss through the roof. The exterior roof temperatures will often reach the outdoor dew point, which can be 10F or more cooler than the overnight air temperatures, so you get a bit of "free" overnight cooling from this lossy roof. The thermal mass of the attic framing wood & roof deck store some of the daytime gains, and the amount of this "free" night time cooling isn't huge, but measurable

    But with an R11 attic there is a lot of heat gain from the roof in direct sun that makes it into the conditioned (much higher than the overnight freebie cooling effects), whereas with an R60 attic most of your heat gain inside of conditioned space will be from windows, with very little heat entering or leaving via the roof.

    The thermal mass of R60 fiberglass is negligible- about 0.2 BTU/ per degree F per lb., and for R60 you're looking at about 1lb per square foot of ceiling area for about 0.20 BTU/degree-foot^2 of ceiling area. The thermal mass of open-blown R60 cellulose is about 3x as much, but still small ~0.33 BTU/ degree-F-lb at about 2lbs per square foot of ceiling, for about 0.66 BTU/degree-foot^2 .

    For perspective, standard density 1/2" gypsum board at 1.7lbs per square foot and a specific heat of 0.26 BTU/degree-F-lb which makes makes it about about 0.44 BTU/degree-foot^2 of ceiling. That's more than half the thermal mass of the entire cellulose layer, and more than the thermal mass of R60 fiberglass.

    The mass effects of cellulose insulation are real, but don't get very "interesting" except in higher-R dense-packed (greater than 3 lbs per cubic foot) densities, not so much at open blown density. The mass effects of fiberglass insulation is small, almost "in noise" of measurement error when totaling up all of the thermal masses of different elements of the assembly.

    The thermal mass of all the stuff in your house, combined with interior heat sources like dogs & DVRs, refrigerators & girlfriends generally keeps the interior temps about 5F warmer than the outdoor air in an R11 house, but 10F or more warmer than the outdoor temps in very well-insulated houses. When the outdoor temp is lower than the indoor temp you can get some cooling via ventilation, but unless you have a tall house and big windows to take advantage of stack effects to drive the air exchange, simply opening the windows won't necessarily get you there.

  20. wjrobinson | | #20

    Night sky radiational roof cooling. I Bingo-ed first Dana.

    Peter Yost, yaa better chime in and agree when this thread gets a blog write up.

  21. fitchplate | | #21

    "The use of cellulose insulation material does not make HVAC installations superfluous, but it
    can decrease the calculated capacity of these installations, and therefore generate a positive
    effect upon the environment."

    Flexible buildings and cellulose insulation
    Stefan Hulsbosch
    Edwin J. van Dijk Ir.
    Elisa C. Boelman, Dr. Eng. MBA
    Christoph M. Ravesloot, M. Sc. A., M. STS

    These seem pretty low temperatures for air conditioning unless the humidity is uncomfortably high. Instead of wasting AC electricity, box fans in upstairs windows blowing outwards, with down stairs and cross-room windows open, will cool down the house rapidly and for pennies.

  22. gusfhb | | #22

    You used to have a house that needed 100kwh to cool during the day, and 0 kwh during the night

    Now you have a house that needs 50kwh to cool during the day and 10 kwh at night

    You need to start running it like the house you have not like the house you used to have

  23. Expert Member
    Dana Dorsett | | #23

    Flitch Plate:

    FWIW the referenced T.U. Delft paper states:

    "...normal density for cellulose insulation is 60 kg/m3..."

    which is about 3.74 lbs per cubic foot, somewhat higher density than most dense-packed wall applications in the US, and ~2.5x as dense as open-blown attic installations.

    But at that density it's not surprising that at that density they found:

    "When building with a light (flexible) construction, the use of cellulose insulation material
    yields an advantage of 10 to 20% over glass wool, under the conditions assumed in the

    Yes, there is a measurable difference due to the mass effect, but it's not going to be a huge factor in most assemblies. In figure 1 on p.3 (PDF pagination) the temperature differences between fiberglass & cellulose are negligible in the overnight hours, but about 2C cooler for the afternoon peaks. And that's with dense-packed cellulose, not open blown. But it's NOTHING like the mass effect temperature moderation you get with concrete (as seen in the paper), which yielded a 10C or greater difference in peak temps from the celluose & glass wool cases.

  24. user-659915 | | #24

    "You need to start running it like the house you have not like the house you used to have"
    Best comment award goes to Keith Gustavson. If you have a well-sealed and well-insulated house with decent air quality, night venting during the A/C season makes absolutely no sense.

  25. jeffwatson | | #25

    Ok, so I guess the gist of what you all are saying is that now that the attic is insulated pretty well, that small heat sources now have a more significant effect on the indoor temperature, and not necessarily the insulation having some huge amount of thermal mass that is slowly released.

    The only major heat sources at night: 2 people & a refrigerator. We turn stuff off when not using (e.g., cable box, TV, computers/internet) - only stuff left on would be the microwave/stove/dishwasher which are only powered for running the LED displays/clock & not actually doing anything.

    We did go on a 3 day vacation recently and left all windows closed. Upon coming back, the house smelled like cardboard on the inside. I presume we were just smelling the cellulose...but could this indicate air leakage from attic into the living space?

    Forgot to mention - this is a single story house; basement & attic. I have a feeling that since it's only a single story, that attic air sealing & insulation in a sense appear to lower the "hot zone" more towards the living space floor since before the sealing/insulation, that hot air could rise into & out of the attic, but now it is more trapped below the drywall & relies more on mechanical means (A/C) to exchange it. Does that make sense?

  26. jeffwatson | | #26

    James & Keith, put that way, I can sort of agree; It's just that I've never read about this phenomena before deciding on getting the extra insulation.

    When an insulation contractor or energy auditor tells you that you'll use less energy to cool the house, you don't necessarily picture in your mind that you will now have to run A/C overnight. Makes you feel like running A/C more often equates to spending more money. I just don't think these are accurate comparisons of energy bills if scenario 1 is: R11 attic + turn off A/C overnight while scenario 2 is: R60 attic + run A/C overnight.

    I thought it was common to turn the furnace down overnight, why isn't it the same for A/C (e.g., turning it off at night)?

  27. RZR | | #27

    BAD DESIGN - A misunderstanding of the heat capacity and moisture storage content(an electrically charged material that attracts moisture which causes it to expand at the surface, store and release moisture based on RH) and their ability to breathe. Has less to do with dew point, temp, attic insulation, etc...more to do with dynamic material properties (specific heat, density, thickness) design that most do not begin to understand that you have to understand the valuation and difference between dynamic mass and steady state r-value.

    Synthetic manufactured gypsum is a bad choice, most concretes with high levels of portland cement as the binder is another. Neither have much useful mass, high heat-cool storage capacity, or the ability to expand-store when wet, shrink when can not force these materials to perform in ways they are not design for by changing the environment they are placed in.....wrong approach!

    Hempcrete, limecrete, and especially earthcrete with the right types of clay are much better than OPC_concrete. Better IAQ and for the environment too. You won't find much complaining like this from these homeowners, house stays cool in summer, warm in winter, little heat-cool loads, no humidity control required.....allergy problems solved, no mold and mildew.

    Knowing where to place mass is the other challenge, The chemistry is a big challenge. There is more than BTU exchange and guessing or neglecting moisture effects on BTU loads going on here.

    Another way to cool mass if you had it is HR, pump water from the hot mass during the day to the metal roof at night to cool by night time radiation, send the cool water back to the mass during the day. Some have a reported a 10 degree load drop at the expense of running a water pump. Leaving your windows, some even doors, open at night makes perfect sense in the right design as proven by builds over all the world, mainly Europe. You have to think beyond r-value as the norm for a "well insulated house" you can have one of those that does not perform as you have found out the hard way.

  28. iLikeDirt | | #28

    The short version:
    After beefing its insulation up to R-60, your ceiling now gains less heat during the day than it did before (good), but it also loses far less heat, so any previous nighttime cooling through the ceiling now no longer works (bad). Since you were successfully practicing nighttime ventilation and benefiting from cooling through the attic, you have noticed the difference. If the roof/attic wasn't a major source of summer heat gain before (maybe it's very cloudy where you live or your roof is largely shaded by big trees), then you may have gotten very little summer benefit from the "upgrade."

    The long version:
    What you are discovering is IMHO the dirty little secret of the American building science and energy retrofit industries: the pitfall of a better-but-not-perfectly-insulated house with low-mass construction. With little thermal mass to store and release heat to buffer interior temperatures, that heat just stays in the air, rather than being stored in the mass itself, able to be released at a later time or cooled down by your A/C during the daytime. So now you need to run your A/C at night to evacuate that heat. Thermal mass is most useful during the cooling season when its trait of averaging out the diurnal temperature differential results in an average temperature that is very close to the human comfort level in many climates. With the outdoor temperatures you're describing, many people living in mass homes around the world would be laughing at your needing to use A/C when it's 85 degrees out.

    Insulation decouples the interior temperature from the exterior temperature such that you are less affected by its extremes (good) but also less able to use ambient temperatures to your advantage, whether deliberately or passively (bad). This takes the form of traditional strategies like nighttime ventilation, night sky cooling, and solar irradiation having less of an effect, since previously there was less insulation for those heat flows to contend with as they moved heat from one side of the building envelope to the other. Whether the gains outweigh the losses will largely depend on the climate, the level of insulation and mass in other building components, your temperature sensitivity, and how many traditional strategies you are currently employing with success. Because when you better insulate your house to stop uncomfortably high or low temperatures from making you sweat or feel chilly, you are also making the house less dependent on the world around it and more dependent on mechanical systems you control and pay for to provide comfort on demand.

    Now, if you insulate your house well enough (Pasivehaus level), you can size those mechanical systems to be so small as to be negligible or even remove some of them entirely. But before you hit that point, the benefit of more insulation is very climate-dependent. Anytime the world around your house is currently assisting in the house's interior comfort or temperature balance, insulation will reduce that. You have to weigh that against the upsides of reduced system sizing or runtime, or interior comfort. If the insulation you're thinking about beefing up won't make you able to reduce the size or cost of your mechanical systems, or won't increase your comfort, then it's a waste.

  29. RZR | | #29

    ORNL Thermal Mass Calculator shows that two inches on interior concrete mass drops cooling bills significantly compared to stick/r-value homes in most climate zones. Use their calculator to estimate your cost saving's. As I said, changing the environment or band aids based on peoples opinion is not going to change the results of their extensive testing and modeling, you need different materials. The best way to remedy your issue is to add mass in the form of natural plaster that breath to the interior walls, leave the well insulated-air sealed envelope as is. Earth plasters have the best ability to regulate moisture as well as heat-cold....plenty of proven examples on the internet. There are all kinds of books on natural plasters out there, do not use any OPC. You might test it on a room with some solar gain, open the windows at night, or see how the hvac makes a difference. I think you will be pleased at what you can do with plasters, the IAQ and beauty they create. You can create walls that are gorgeous! I agree with Nathaniel a pitfall of the industrial revolution and manufactured products that cost more and do not perform as well. If you DIY you can add the mass dirt cheap ;) loose a little floor space but lower your cooling and heating bills, reduce or eliminate VOC, cold-hot spots, moisture issues, etc....

  30. GBA Editor
    Martin Holladay | | #30

    Terry's opinion that "the best way to remedy your issue is to add mass" is not universally shared. In most homes, adding thermal mass costs so much money that you are unlikely to see any payback from the investment.

    Improved airtightness, high-quality insulation, and proper shading of windows (or the use of low-solar-gain glazing) are proven approaches.

    For more information on thermal mass, see All About Thermal Mass.

  31. user-659915 | | #31

    Jeff, two questions and one comment:

    1. What is your location? I don't recall any specifics about your local climate in this thread, I apologize if it's buried in there somewhere but I don't have time to go back and read every comment right now. Bottom line: if your climate is hot and dry in the summer, night flushing will help. If it's hot and humid, it won't, it'll just load up your house with more humid air for the a/c to deal with next day.

    2. Have you addressed other enclosure issues - specifically do you have a vented, unsealed crawl space? As a general rule of thumb we seem to find here in warm humid N. Carolina (a very broad rule, with many exceptions according to the specific condition of the home), attic insulation upgrades improve heating season performance, closing and insulating vented crawl spaces has the greatest impact on summer cooling performance.

    Finally, you ask "I thought it was common to turn the furnace down overnight, why isn't it the same for A/C (e.g., turning it off at night)?". Response: do you open all the windows in the house when you set back the furnace? if not, you shouldn't expect to do that when you turn off the a/c either. Here's an easy experiment that can increase your data set: continue to turn off the a/c at night but leave the house closed up. Continue to use ceiling and floor fans if you wish, and if you have them - in inhabited rooms only, of course - to keep a little cooling airflow over your bodies. What does this do for your overall summer a/c use?

  32. iLikeDirt | | #32

    Adding mass is mostly impractical in existing stick built construction unless you're willing to add big water jug pillars or a huge masonry heater or double up your interior drywall or something. But even that last suggestion often doesn't work well since the quest to make drywall lighter and cheaper has come at the expense of its mass, resulting in its largely being an additional layer of weak insulation rather than really having much heat-storing abilities.

    The solution to this problem is pretty much going to boil down to simply plowing ahead and continuing with the "conventional" insulation retrofits, addressing the new top source of heat gain or loss for every project.

    Jeff, why is your A/C unable to cool your house down below 78 when it's 85 outside? that strongly suggests to me that either your A/C was grossly undersized from the start (unlikely here in the USA) or there is a major unaddressed source of heat gain fighting the A/C all day. Are your windows old and bad and do they receive a lot of direct sunlight? That'll do it. Are your exterior walls dark-colored, have poor insulation, and receive a lot of direct sunlight? Those poorly-insulated walls will soak up and radiate a lot of heat into your house. Is your roof covered in dark shingles? Even with the R-60 attic upgrade, a dark shingle roof plus framing represents an enormous thermal mass receiving heat all day during summer that's located right where you don't want it (outside the insulation) that your attic floor insulation will be fighting long into the night as it cools off. Could there be any internal gains you're unaware of or not considering? A gas water heater that's within the thermal envelope and running for hours and hours a day, for example? Two plasma TVs? A gas oven and a gourmet chef?

    Those are some of the things you'll have to address next to stop some of that heat from entering and staying in your house.

  33. GBA Editor
    Martin Holladay | | #33

    I stand by my statement. While high-mass walls can lower air conditioning costs in some climates, adding mass to existing walls isn't easy or cheap. Most fans of thermal mass overstate the benefits (or underestimate the cost) of building high-mass walls.

  34. RZR | | #34

    See Martin's quote below: Inaccurate and lacks comparative data, simple because you can not make a comparison between mass effect and (windows, air sealing, r-value insulation, shading") although dependent on one another, the outcome of the two differ dramatically, it's like comparing apples to oranges.

    The mass effect I use and refer to, the pay back is immediate and the cost in most cases less than mainstream and other options, especially if DIY as I said. Adding mass to one wall can be effective depending on location, and it can be take less than a day labor, few hundred dollars, compared to air-sealing, replacing windows, insulation, etc.... with materials cost at close to nothing.

    All though there are some poor attempts above, it is very difficult to quantify adding mass and it's dynamic effect benefit in all climate zones, this is what ORNL attempted to do after lots of prototype builds and modeling from their results, 1000's of models, many builds, to establish at DBMS cost comparison value for manufacture products(I manufacture my own products) I'd like to see this 'universally' shared" collection of data in "most homes" where the remedies captured all the pay back parameters and did a comprehensive cost of performance analysis based on lower utility bills such as the field study and extensive modeling ORNL did. My guess is that has never been done or captured, submitted to code books, standards, etc....and this 'universally accepted" standard in just a matter of wide based limited US opinion based on mainstream bad construction methods using factory built products and assemble knowledge. There are other natural methods that have been "universally" proven world wide to be a higher cop just not in this country. The great thing for my business is this inability to think outside the mainstream box, and lack of supporting data, leaves the door wide open to remedy situations like this with better performing "alternate materials" per code.

    The best suggestion for Jeff is to not listen to any of it, educate yourself more, run some test if possible, look at your situation, then decide what is best. There are faster cheaper ways to add mass than plastering I mentioned. No one out here can make that judgement call for you, we simple do not have all the facts.

  35. Expert Member
    Dana Dorsett | | #35

    "My guess is that has never been done or captured, submitted to code books, standards, etc."

    Mass-effect benefits are enshrined in code, have been for at least the last three iterations of the IRC. Like all IRC codes it's somewhat blocky, not a perfectly tuned energy use model (or even an imperfectly tuned but still protty-good tool like DOE2, that's adequately predictive for these purposes), but it's in there. See the mass-wall allowances in TABLE N1102.1.1 (R402.1.1) in chapter 11 of IRC 2012.

    FWIW: ORNL vetted the thermal mass modeling of the DOE2 tool in one experiment by building two identical houses side-by side, one stick-built to (then) code min, the other with a bottom-of-the-line R16 ICF, and operated them for over a year while monitoring.

    Modeling the dynamic effects of fiber or foam insulation layers looking at the insulation's thermal mass isn't that important from a total point of determining peak or average loads of a house, the way it might be for say, controlling the temperature of a superluminescent diode in a fiber-optic gyroscope used for satellite or aircraft navigation, or any other system that needs very tight temperature control even when subjected to temperature swings/energy inputs over a wide range of time scales. The time scale of diurnal temperature swings are just too long for the very small amount of internal thermal mass of the insulation. to be relevant. While it's measurable in fairly thick layers of fiber insulation on those time scales, it's more of a "who cares?" third order effect.

    Even in ICFs or AAC wall assemblies the dynamic effects still really only a second-order effect, but significant enough to acknowledge in building codes.

  36. wjrobinson | | #36

    Terry Lee... Interesting posts... Curious as to your background... Engineer... Contractor?

  37. RZR | | #37

    Hi AJ, I have a long history in Engineering, structures & systems design, renovation, restoration, and testing in several industries. I recently quit a test lab to design buildings for our construction company. My son is the licensed contractor that manages our storm restoration and renovation division.

    Right now I am testing and developing methods to cast-pour in place, plaster, etc., mass of different types. I am finalizing a new product using a high silica cellulose and a pozzolanic hydraulic cement binder. It has been a challenge determining the percentages and chemical compositions for production homes. This particular mass design would offer moderate level breathability or regulation of heat-coolth-moisture, a high ability to absorb CO2 improving IAQ. It also surpasses burn rates of mainstream, sound proofing, insect and rodent. If is mainly an insulation for the envelope, but if those attributes are important to a client we can add to existing walls, or build new for less than $3.00 ft2 plus labor. So if you DIY, a 2" x 10' X 8' wall would cost less than $300 material. I can cast a 2” 8’ x 10’ foot wall in about 4 hours. Simple plywood forms casted in place to any shape desired fast…..If you had to extend an electrical box or register couple hours more. No need to remove moldings nor add them back or render. What makes Martin’s statement correct is you will not find this at Home Depot. Other counties that have a much better understanding of mass, since it was not abandoned post-industrial revolution, poor people, have a better understand and have developed this technology proven for centuries to be cost effective. I just updated and modified it. What effect, what value, what load drop, is what I stated that Dana and others misunderstand do you place on that. That is where it gets complex, ORNL attempted to quantify it using concrete but, fell far short. They realized that and state they had a lot more work to do. I imagine budget constraints, it does take lots of R&D money.

    Mass effect has little to do with R-value or the table that uses concrete Dana made reference to. The mass effect I described is not capture in any code, or if it is it is misrepresented, devalued, especially giving it and steady state “r-value”, not even remotely close. What needs to be listed there at a minimum is ORNLs “DBMS” values for different types and thickness of concrete mass which still falls short neglecting more effective mass, or whole house dynamic mass that may be used in total or partial construction, or alternative materials that perform much better than concrete with high levels of Portland cement, or gypsum wall board, ICFs; with high levels of junk, inert fillers and aggregates to cut manufacturing cost that result in low internal mass performance, plasticizers, VOCs…. etc. ORNL DBMS value falls short of the other benefits and design of mass that surpass heating and cooling loads. There is no design guide, or code, to provide a prescriptive path to the chemical composition, density, specific heat, thickness, for a given building in a certain climate zone. I am just getting started, there are other mass designs that offer a totally different set of “benefits” if you will, I offer clients depending on what they desire or the issues are. If it is cooling loads and moisture, the above design would not be the best choice. I’d use a material that had a high ability to regulate moisture and cold air loaded from night time air or hvac. If it were storm or debri resistance the client wanted we use a different ballistic mass. On MANY occasions home owners have reported a 30-80% reduction in utility bills from mass depending on quality of design, allergy issues solved, odor issues solved, sound issues solved, etc….many have noticed DRASTIC differences in IAQ in addition to hvac load drops, both dependent on the type, location, design of mass. To say it is not that important is absurd! To say that most have under-estimated its cost and performance or misunderstood it is accurate.

    Dana: I have written those test reports MANY times, very boring BTW. Let me help you understand the design-build process used by many companies world-wide. The modeling “tool” used, DEO2, WUFI, whatever, is a great tool for initially determining what design path say A vs B to fund, after that it takes a back seat. The models are usually not that accurate in the early stages of design especially if heat-cold-moisture-CO2-wind-hail-snow-seismic-etc is being modeled which is extremely difficult along with interior and exterior environments in many climates zones. Developing the modeling effort in man hours there is A LOT of R&D cost most builders cannot afford. Sometimes a team of engineers need to be present. In conjunction with the product design there is parallel test fixture design and test procedure effort, lots of R&D cost to. Test readiness reviews, collaborations with clients if the lab is a third party, etc…testing commences, the results are back calibrated to the models, results such as thermocouple and pressure gages, accelerometers, strain gages, etc……it can take months or years to go through all the hot box testing, lots of changes an iterations. The design is released for field testing that again back calibrates the models and test fixtures-procedures. 1000’s of test gage results, interpretations, and models later the final models are useful to some extent, hence the birth of ORNL “DBMS” value and “Mass Calculator”, REAL DATA that surpasses opinions.

    Got a meeting with a developer Friday to put some of these natural mass designs to use I need to prepare for. Wish me luck!

  38. jeffwatson | | #38

    Nathaniel G - I think your post (#28) explained it best to just your average concerned home owner who doesn't have a background in building science or construction. If I understand, you're basically saying that instead of the hot air being able to easily pass through my ceiling (since it has added insulation), that heat stays within the living space & thermal mass now plays a more important role. Mechanical means are now required to remove that heat. When my A/C is on, it has no problem cooling down the space; it is functional & effective when it is turned on - don't know where I mentioned that it couldn't cool the house.

    James Morgan - You ask:
    1) Location: I am in Chicago, where the summer daytime temps might peak in the upper 80s and overnight get down to the upper 60s or 70s. If it's upper 60s at night & not too humid, I crack open the windows & use fans overnight.

    2) I have a single-story brick house, with a basement and attic, and yes the roof is dark shingles. I do believe that I will see a much more obvious benefit during wintertime with the added insulation. Summertime - maybe the effect is not as easily observable, but can only be verified with electric bills. We're pretty energy conscious - we turn things off when not using, the water heater is set pretty low, we hardly cook, we have a single TV/cable box which is only 22" big that probably only gets a couple hours of use in the evening, no pets, we don't have any big furniture or interior decorations so maybe that means we have very little thermal mass. Adding mass just seems pointless if this is not a new construction.

    3) Opening windows overnight during heating season - you're correct; I don't crack the windows in the heating season overnight; however we're talking about two different phenomena, because I crack the windows during summer nights because it's hotter inside than outside. On the same thinking, I would be cracking the windows in winter if it was colder inside than outside overnight (an impossible scenario).

    This thread has shown me that I am "using my A/C the wrong way." It seems you guys are saying A/C should be left on all the time (all day, all night) with the windows closed. So I tried that for about a week, but then I ran into a different issue - CO2 levels get way too high. This was the second reason for opening windows overnight. Now I feel like I have to constantly run my 80cfm bathroom exhaust fan for ventilation & the levels still don't budge by much. This means more electricity to run the fan. I'm afraid of my upcoming electric bill because I've been running A/C nonstop (yes it cycles on & off) & the bathroom fan pretty much nonstop as well, and still haven't seen my CO2 levels go under 1200ppm when we're home. Now, of course if I was doing this pre-R60-insulation, I'm sure I will see a difference in electric usage; but I wasn't, so in a sense, adding insulation has forced me to use more energy.

    No offense to anyone who is an energy auditor, but they made it seem black & white - install this much insulation & you'll save X dollars. Where was it mentioned that I now have to run A/C 24/7, that I have to have my bathroom fan on 24/7, etc...prior to installing the insulation, we've never ran the A/C for more than a couple of hours a day - just enough to cool off & let the overnight temperatures take over. I guess auditors should be assessing the homeowner's usage habits before being able to turn an "improvement" into a dollar amount of savings. If I was told all this beforehand, it's one of those things that's like - hmmm...maybe I should just search for a house which already has a better design than this old house that's just going through upgrades & retrofits.

  39. iLikeDirt | | #39

    "This thread has shown me that I am "using my A/C the wrong way." "

    Welcome to the rabbit hole… It's insidious, isn't it?!

    First, you build a building without regard for energy efficiency in a climate with 1) a heating season, 2) a cooling season, and 3) a humidity load (ugh, Chicago).

    Then you add an air conditioner and a heater for comfort. All is well.

    Then OPEC turns off the oil! Energy prices go up by a factor of ten overnight! Your jaw hits the floor when you get the bills for running these things!

    So you add insulation to your attic and walls, and you replace your windows.

    Where you put the insulation in relation to other components (e.g. inside the brick wall or outside) and what type of insulation it is (massive insulation like cellulose vs lightweight insulation like foam) are critical details that must be correctly matched to the context, but nobody tells you this. They may not know themselves.

    Likewise with the windows; what kind of glazing, which orientations should receive the different glazings, etc.

    The added insulation helps during the winter but seems to make the summer worse.

    The new windows help during the summer but are a wash or have a disappointingly small effect in the winter. Your house also has less natural light, so you spend more money on electric lighting.

    You do some air sealing. You learn to use your air conditioner correctly. You keep your windows shut.

    Suddenly your indoor air quality is terrible and you're practically suffocating! You discover that your gas furnace is now backdrafting and poisoning you with carbon monoxide!

    You quickly replace your furnace with a sealed combustion, high-efficiency model (ka-ching!). But your indoor air is still stale and full of carbon dioxide.

    So now you have to add a mechanical ventilation system that brings in outside air. You need to add an HRV to avoid undoing all of your hard work by flushing your conditioned air outside and replacing it with unconditioned air that your furnace or air conditioner will have to condition.

    If you don't have ductwork that's in use year-round, you need to add dedicated ducting and air handling for the HRV…

    You see how this goes? Most buildings simply don't take well to energy retrofitting. They were designed in an earlier time, and they worked well for that time of lower energy prices and lower comfort expectations, but bringing them up to snuff in our modern world is a can of worms the size of the suitcase full of money you're going to have to spend to do it.

    I used to be really gung-ho about energy retrofitting. The more I learned, the more I became convinced that especially if your energy pig house is in a climate with both a heating and cooling season, anything short of turning it into a superinsulated, mechanically-ventilated passivehaus is mostly a waste of time and money. The rabbit hole is too deep. Without methodical planning, what you do to help in the heating season will hurt in the cooling season, and vice versa. You'll discover that you need to caulk everything, replace your combustion appliances, add mechanical ventilation doodads, wrap your house in all kinds of petrochemicals that smell bad and worsen the quality your indoor air, you name it. You'll take two steps forward and one step back. And there are a lot of things that are largely outside of your control, such as orientation, slab insulation, roof overhangs, and window sizing and placement.

    The costs, payback times, and carbon footprints of all these retrofits can border on the absurd, especially if you hire them out to pros. It might be worth it if you can do most of the work yourself and your current bills are insane because have an oil furnace and an ancient A/C. If not, you'd probably be better off saving the money to eventually build or buy a new, modern house with all of this stuff done right from the start.

  40. gusfhb | | #40


    As to running the AC 24/7

    Yes and no

    When you want AC during the day, do not shut it off at night. In my own home north of Boston, we tend to run the AC for weeks on end in the summer, but then when a nice dry warm spell comes, shut it off and open the windows. The weatherman is your friend. When walk out the door and say wow, it is gorgeous out check the weather to see what the next 3 days hold and act accordingly.

    I have hardly run the AC this year, a week or so in July and right now, but last year it was on almost all summer

    When you say high CO2 I respectfully suggest you are overthinking. I have no idea what my CO2 is but if the house gets stuffy [two kids and a big windy dog] yeah, we open the windows.

    I will say basically the same thing I said before, learn your house, it will take some time

    Consider a rule that the AC needs to run in 48 hour cycles as a thought process. On for 48 hours or off for 48 hours[rather than 12 as in the past] Modify as you deem necessary, it is your house..........

  41. Expert Member
    MALCOLM TAYLOR | | #41

    I wish people would stop posting costs for their projects or proposed solutions based on DIY to make them look more appealing. Sure things are cheaper if you don't include labor costs, but if you need to cut that out to make your case then what you are proposing really isn't a viable option.

  42. RZR | | #42

    Malcolm, our company (as do many) provide three basic levels of construction service.

    1. Design-build
    2. Design- build (some client DIY involvement)
    3. Consultation (100% DIY involvement)

    The 3rd pushes E&O liability, put people and the internet do it all the time. We put bids together and do jobs almost daily for clients that want to DIY and cut cost. Most builder's in the midwest leave basements unfinished so clients can DIY to save money. DIY is a very real viable option in today's economy. Where I am heading with my methods are low skill set high performing DIY, that is one of our markets, that is the market of the past when people had to barter labor (family and friends) to avoid going into large dept and large mortgages, that is what I am pushing into this generations mindset, stay out of debt, DIY. Not all have the time, but in labor exchange it can happen. Our attorney does not like it, the liability, but my insurance agent has the workers comp and E&O solution, "endorsements" :)

    All besides the point, with the proper low skill set low labor hour designs it is a false myth that adding mass is expensive, it's not once you understand how and where, compared to other solutions.

  43. jeffwatson | | #43

    [Nathaniel G]
    Nice way to describe it. Saving money on energy is always nice but a lot of this retrofitting stuff I seek out because I just want things that simply work efficiently.

    Just like you said, you do one thing, then that leads to another. I think the issue is primarily that homeowners don't have knowledge of all steps that must be taken at the very beginning, and we kind of only learn thing as the issues arise. Of course if we learn everything up front, we might just say forget it altogether...which in turn would hurt industry or guys in the business of say attic insulation.

    I am a new homeowner, have never lived in house, but I try to learn as much as possible. The way we used A/C was just to get the temperature down & we would then turn it off; it'd run for a single cycle. I now know that the A/C reduces the temperature of the air so that the living space can then transfer its heat to the cool air. This process needs to continue because the heat can't flow to the exterior as easily as it did with less insulation, hence the need to let the A/C remove that heat (from the inside), even when it's colder outside. I also learned the humidity in my house may be too high as's about 60-65% RH and 65-75% in the basement...learned that I'm supposed to aim for under 50%.

    Thanks for the advice as well. I have another thread about my CO2 problem, but after the R60 insulation, seems opening the windows hardly does anything without also running fans (and it still hardly does anything).

    True, costs are more appealing if there's some (hidden?) DIY work involved. At the same time, I feel DIY encourages seeking additional knowledge vs. if you just paid someone to do something (unless this person is very forthcoming about knowledge transfer...with what I've seen so far with people who've worked on my home, they don't like to share too much about their trade). Can't blame them though; if my job was in the trades, I'd probably do the same...primarily for the reason Terry mentioned - liability. For a thorough individual seeking knowledge, it sucks, but unfortunately that group of people is probably the minority, so it's best just not to say anything!

  44. MICHAEL CHANDLER | | #44

    I think one of the issues with blowing attic cellulose insulation is it installers tend to aim high so that the cellulose will fall down gently on the floor of the attic in an even blanket. This means that they're aiming their tube at the upper end of the baffles and the ability of the baffle to carry air means it's a natural thing for the cellulose to go into the baffle and clog it up. My suspicion is that this has dramatically reduced the air flushing potential of the area above the insulation in your attic so that attic is holding on to heat and not cooling the roof above it in the evening. A simple experiment would be to ask a friend to walk around your house with a leaf blower blowing directly into the soffit vents and monitor the air movement in the attic. If the baffles are clear you would see minor plumes of dust shooting out of the tops of them. If clogged you might see major plumes if the blower can get enough pressure to open them up.

  45. iLikeDirt | | #45

    This has been bugging me for a while so I decided to undertake a bit of an academic study of thermodynamics, and now I feel like I may have a better grasp of what’s going on vis-a-vis the disappointing cooling performance of superinsulated attics.

    The two measurements we need here are volumetric heat capacity: how much heat a material is capable of storing in itself; and thermal diffusivity: how fast heat is able to transfer through a material.

    When we look at insulation, we typically look at its R-value; its total ability to slow down heat transfer. This is important, but it doesn’t give us anywhere near the whole picture. Insulation does indeed slow down heat transfer, but because insulation has mass, as heat moves through the insulation (slowly), it is stored in the material itself. This is a critical point that I have almost never seen discussed. As we’ll see, for heating performance, this is desirable, but it is bad for cooling performance, especially in an exposed application like an attic.

    Consider the case of an uninsulated 1,200 square foot attic during the summer or in a hot climate. Heat transfers very quickly through the air to the ceiling drywall; air has a high thermal diffusivity value of 21.33 mm2/s. But air also has virtually no heat storage capacity; its volumetric heat capacity is 1.205 kJ/m³K. As a result, the summer performance is what we would expect: the ceiling gets very hot during the day, but it also cools down at night.

    Now let’s superinsulate that attic and dump 24 inches of cellulose on top of the ceiling. Cellulose transfers heat very slowly (that’s the point); its thermal diffusivity is 0.811 mm2/s, or 26 times slower than air. But the cellulose is more massive too, with a higher heat storage capacity. This 1,200 square foot attic now has 5,520 pounds of material in it (2.3 lb/ft³ 2 feet deep * 1,200 square feet). And the volumetric heat capacity of cellulose is 53 kJ/m³K -- 44 times as much heat storage capacity as air!

    This thick insulation will certainly slow the flow of heat radiating down from the roof sheathing (that’s the point of having it), but in slowing the heat down, it is also storing the heat in its own mass, and the bigger the mass, the more stored heat! As a result, the peak cooling load during the day is indeed reduced because the time that the heat takes to penetrate the insulation is increased, but the heat flow is also time-shifted; the stored heat in the insulation continues trying to enter the house even after the sun goes down. Why? Because a typical under-ventilated and shingled attic remains hot for several hours after the sun has gone down, so there is still a delta-T through the insulation, and all this heat continues to transfer through the insulation and charge it with even more heat. Because of the temperature gradient through the insulation, heat stored in the top and middle parts of the cellulose will continue to transfer down towards the ceiling drywall.

    Even once the air in the attic itself has cooled off, there’s still stored heat in the insulation. The heat in the top layer of the insulation will soon get into thermal equilibrium with the attic air, but the temperature of the middle layer of insulation will still be much higher (remember there’s 24” of cellulose in this example!). And some of that heat in the middle is going to continue transferring down towards the ceiling. The ceiling drywall will not cool off because it will still be experiencing a delta-T; not with the attic, but with the hot insulation itself! And soon the sun will be out, heating the roof and the attic again, causing the same process to repeat. Increasing the insulation thickness will not address this problem as it simply provides more mass in which to store heat even as it further increases the time that the heat takes to flow through it.

    So what is the net result of all of this insulation? The ceiling stays cooler during the day than if it was uninsulated, but it never cools off at night. It experiences small fluctuations in temperature around a point that is higher than the indoor temperature; it is constantly adding small amounts of heat into the house, never pulling any of it out. The house stays cooler during the day, but it’s warmer at night. Air conditioning use is time-shifted to the night, but total runtime may not be reduced by as much as expected. And if it is a very large air conditioner, it will be ill-suited to the new pattern of a small-to-medium constant heat influx rather than the old pattern of a large multi-hour heat influx, followed by a heat outflow at night.

    Critically, the more insulation you have, the more it is going to take to pull heat OUT of it in the form of lower attic temperatures and longer periods of time where this is the case. This is why adding more insulation to an attic that gets hot doesn’t seem to help very much; this extra mass requires longer periods of lower attic temperatures to cool off the insulation than before. Unless you provide that, you're allowing the insulation to near-continuously absorb heat during the cooling season.

    The solution to this problem, contrary to what I have frequently seen written on the subject, is not to futilely continue piling on more insulation, but to reduce the actual temperature of the attic itself, both during the day and during the night. That way, the heat flow will not only be lower, but there will be less heat stored in the insulation, and finally--critically--the direction of heat transfer can reverse during the night, which will actually pull heat OUT of the insulation, “recharging” it for the coming day, and potentially even pulling some heat out of the ceiling drywall too.

    There are a couple of complementary ways that it seems feasible to do this:

    1. Increase ventilation to cool down the attic faster. The outside air temperature is almost certainly bound to be lower than the attic temperature, both during the day and at night. Replacing hot attic air with cooler outside air will reduce the heat transferred to the insulation, lowering the heat flow. And in climates with high diurnal temperature swings, as soon as the ambient attic temperature falls below the interior temperature (quite possible during the shoulder seasons and even during the summer where I live), you get the awesome effect of the heat flow reversing direction, and the stored heat starts being pulled upwards out of the insulation and into the escaping air of the attic rather than down through the ceiling drywall.

    2. Reduce the heat stored in the roofing material to reduce heat flow into the attic faster. With dark heavy shingles on the roof, they’re going to soak up a ton of heat during the day, and this mass will take many hours to cool off once the sun goes down. Even at 1 AM, the roof sheathing is still going to be hot and radiating heat down onto the attic floor insulation. With a lighter weight roof that has less heat storage capacity, there will be less mass to store heat, and the roofing and sheathing will cool off faster, reducing the heat flow into the attic more quickly. There is also a case here for eliminating the roof sheathing entirely to reduce the weight and turn the underside of a bare metal roof into an automatic radiant barrier, as long as there’s enough racking resistance added instead.

    3. Use a radiant barrier and/or reflective roofing to actually reduce the amount of heat that enters the attic in the first place; lower attic temperature means a lower delta-T through the insulation, which means slower heat transfer, less stored heat, and faster cooling-off.

    4. In a climate with no heating requirements, don’t use a large amount of insulation; Past the first couple of inches, it doesn’t really help you very much because it stores more heat. Instead focus on keeping the heat out of the attic in the first place. If you live in southern Florida, you want a ventilated metal roof, radiant barrier sheathing (if any sheathing at all), and strong attic ventilation. Coincidentally this is how they build roofs in hot climates anyway...

    Finally, an interesting point is that in the winter, all of this heat storage in insulation is highly desirable, as it helps your house get through the night without cooling off too much. For example, after the sun goes down, temperatures drop and the attic air gets even colder, and it pulls heat out of the insulation on the floor faster than it was being pulled during the day. The more insulation you have there, the more heat that will be stored in the insulation and the slower this will happen. So the insulation right above the drywall will be at maybe 75 degrees, and it will take a very long time for the heat in that bottom layer to get sucked out by the cooling-off of the layers above it. As a result, your ceiling drywall will not really cool off during the night, which is exactly what you want in winter. But it’s the opposite of what you want in summer!

    All of this makes sense to me but of course I could be completely wrong; I’m only a layman who has studied building science and thermodynamics in an amateur manner.

  46. Brian Knight | | #46

    Felt a need to comment on my anecdotal situation, mainly because another forum points to this thread as an example that airtight and insulated isnt the best approach for improving comfort and saving energy.

    Nathaniel, I agree increasing attic ventilation could help, but probably not doing so mechanically. I really liked Michael's comments on something to be aware of. I dont think any negative drawbacks of increased mass of asphalt compared to metal would be measurable. Switching to a light colored finish on the other hand seems to have measurable benefits. I disagree with insulation not being helpful in cooling climates. Sure, not as helpful as heating climates but insulation mainly slows heat transfer, not stores it. I think high amounts of roof insulation is highly desirable in cooling climates to slow heat transfer from the most sun-exposed area of a home.

    In my home which is a small, simple stick-built home from the 20s, Ive used night-time flushing to avoid alot of AC usage. After 4-5 years of living here, I finally poked my head into the attic scuttle hole and discovered, to no surprise, that I had very little insulation on the attic floor, barely R11 for the most part. I air-sealed as much as possible and blew in cellulose to about R50.

    Not only have I cut my heating oil use by more than half, Ive drastically reduced the amount of time I have to run my AC. In other words, air-sealing and insulation has made me MUCH more comfortable and saved me alot of money on dirty energy costs. Air-sealing and increasing insulation at the attic floor is typically one of the best comfort and home energy investments one can make. Individual results may vary. I hope Jeff gives an update after living with his weatherization work after another summer.

  47. Expert Member
    Dana Dorsett | | #47

    (Without checking the math or assumptions...) A volumetric heat capacity only 44x that of air means that the thermal mass of the attic insulation is pretty negligible, given just how low the heat capacity of air is. The thermal mass of R50 low density cellulose is barely higher than a half-inch of additional ceiling gypsum.

    The mass effects of deep cellulose introduces time lag to the peak load experienced at the interior, but the logarithmic time constant of that delay is less than 100 minutes, nothing like the delays seen with insulated concrete assemblies (where the constant is measured in hours.) It decreases both the peak and average load, and is still the solution, not the problem. The cellulose itself simply does not have sufficient thermal mass to keep the ceiling from cooling off over night, and it's cooling off from a lower starting point.

    The collective thermal mass of everything inside the conditioned space side the insulation has a FAR greater influence on the heating/cooling time constant of the house. With R50 insulation in the attic the heat gains through the roof are going to be a tiny fraction of the window gains, even less than the gains of the NORTH facing windows on a typical house, even if the attic hits 130F during the peak part of day. That's why even though radiant barriers at the rafters will reduce the peak attic temps in a ventilated attic, the energy use benefits of radiant barriers when there is R50 on the floor is in the statistical noise (less than the estimation error of air infiltration rates in most load calculations), and not cost effective.

    Radiant barriers will have a measurable economic benefit if the ducts & air handler are in the attic, above the insulation, but even then it's pretty small if the ducts & air handler are both air tight and insulated. See the "Insulated Ducts" and "No Ducts" portions of table on p.5 of this document (and note, the code-minimums referenced in that document are at IRC 2009 levels, which are lower than R49 for climate zones 5 & lower.

    The effects of ventilation vs. no ventilation are similarly small, though ventilation is important for moisture-purging in heating dominated climates. In humid cooling dominated climates ventilating an attic with the ducts & air handler in the attic (even insulated & air sealed ducts & air handler) results in higher moisture content in the structural wood, since the parasitic cooling losses of the system reduces the attic temps, which leaves it near the outdoor dew point overnight due to the nighttime further radiational cooling of the roof deck. In those configurations venting the attic is a higher risk of moisture issue than sealing the attic, even if the insulation is at the attic floor. If the attic is inside the pressure boundary of the house the dew point of the attic air tracks that of the conditioned space air, which is lower than the overnight outdoor dew point in those climates. (Local codes will often still require attic venting, just as they often require foundation crawlspace venting in humid climates, even though it's counter to what current building science demonstrates.)

  48. iLikeDirt | | #48

    Thanks Dana. I have a couple of questions for you, since your level of knowledge appears to be worlds above mine.

    You said: "The mass effects of deep cellulose introduces time lag to the peak load experienced at the interior, but the logarithmic time constant of that delay is less than 100 minutes, nothing like the delays seen with insulated concrete assemblies (where the constant is measured in hours.)"

    Could you share how this is calculated? I've calculated the thermal diffusivity of these materials (in a manner than I at least think is correct) and found that there was not a huge difference between the values for insulation materials and mass materials, which surprised me.

    I can understand your points, but surely lowering the delta-T is always impactful, right? I mean, that's what we're doing when we turn down our thermostats in the winter. Even changing the delta-T by 3 or 4 degrees makes a difference. So shouldn't it make a correspondingly larger difference if a radiant barrier can reduce the temperature of an attic by 10 degrees or more? Doesn't that correspond to a lower delta-T through the insulation?

    Same with ventilation. Intuitively, facilitating a process of smoothly and rapidly replacing 130 degree air with 90 degree air would seem to reduce the delta-T through the insulation by 40 degrees. Can you explain where I'm going wrong with this train of thought?

    Your points about moisture are well-taken. I happen to live in the high desert where summer moisture is non-existent so extra ventilation wouldn't seem to carry any drawbacks given a well-sealed attic floor.

  49. GBA Editor
    Martin Holladay | | #49

    Like you, I'm interested in hearing Dana's response. In the meantime, here's my perspective: thermostat setbacks save energy in a house with average insulation details. However, as the building envelope gets better insulated, the effect of the thermostat setback is less and less.

    If a house has a lot of R-4 windows and R-11 walls, thermostat setbacks save considerable amounts of energy. But if an attic has R-60 insulation, it doesn't really matter what temperature the attic is.

    The difference in the rate of heat flow between the two scenarios -- a 130 degree attic to a 72 degree interior versus a 90 degree attic to a 72 degree interior -- is measurable but very small, because the R-60 insulation layer between these two environments represents the best insulation barrier in the entire house.

  50. GBA Editor
    Martin Holladay | | #50

    If you can install an attic fan that dependably and quickly lowers your attic temperature from 138 degrees to 108 degrees, without depressurizing your house, and does it on enough days per year to save enough BTUs to justify the installation cost of the fan and the required controls, and to justify the electricity required to run the fan, you will be breaking new ground.

    Many people have tried to do what you seek to do -- but every researcher who has looked at the work of those who try to do this has concluded that the energy saved is not enough to justify the cost of the required equipment and the energy needed to run it.

    In most cases, these attic fans actually suck conditioned air out of the house, raising energy bills -- and in some cases cause water heaters to backdraft.

  51. iLikeDirt | | #51

    I made a manual J spreadsheet for my own house and I have an R-50 attic. To put some specific numbers out there, when I plug in a 138 degree attic temperature (design temperature + 40), the cooling load is 1,512 BTUs. With a 108 degree attic (design temperature + 10), the load drops to 792 BTUs. To get down to that level with more attic floor insulation, I would need to go up to R-95 which would actually not be possible around the eaves due to the low pitch of the roof.

    For comparison, the whole-house cooling load right now is about 14,500 BTUs.

  52. Tim C | | #52

    Removing 600 BTU with a EER 12 air conditioner takes 50 watt hours of electricity. Attic temperatures are only elevated for about 12 hours of the day, and by a lot less than 40 degrees for most of that (or all of that, if you've got ridge venting). So you're looking at saving maybe 300 watts in conditioning energy on your 99% design load. At my electricity prices, that would be 3 1/2 cents a day.

    Assuming a $60 5-watt solar attic fan could entirely eliminate the delta-t (which it can't) and you have 150 peak cooling days a year (I don't want to live there), you're looking at a simple payback of over a decade.

  53. Expert Member
    Dana Dorsett | | #53

    Nathaniel- you're really over thinking this. The 720 BTU/hr difference in calculated loads between the 138F attic (1512 BTU/hr ) and 108F attic (792 BTU/hr) is approximately the cooling load of two conscious adult humans sitting at the kitchen table. How impactful is that, really?

    Humans put out about 225-250 BTU/hr sensible heat per person just sitting around, plus another 100 BTU/hr of sweating & exhaled moisture when conscious doing anything as strenuous as eating a PB & J. It bumps up another 50-60BTU/hr if they're standing while eating, higher still if they're more active, say playing Dance Dance Revolution on the Wii. To do a Manual-J correctly you add in those occupancy & activity based cooling loads, as well as any electrical plug loads- the difference the number of occupants and what those occupants are actually doing are a bigger factor than the attic temp factors cited.

    And, the sensible cooling load from the ceiling is that magnitude only during the peak attic temp hours. Nobody's attic averages 138F over a day ('ceptin' maybe those folks in hell, or Texas, which is well known to be even hotter. :-) ) A pretty-good mini-split can run a COP of about 3 running full blast even when it's 110F outside. That's with 720 BTU/hr of "additional" peak load, that about 70 watts of additional peak power. The same pretty good mini-split runs a COP of about 8-9 when idling along when it's 85F outside, in which case 720 BTU/hr represents 23-26 watts of additional power. If your attic ventilation fan consumes more than 25 watts it's probably burning more power than it's saving. Even if the fan took zero grid power- a solar powered attic ventilator with it's own PV panels or something, it takes quite a bit of 138 F attic time to add up to $10 of "extra" power use, and it's not clear if a self-powered solar ventilator pays for itself within it's anticipated lifecycle when there is R50 on the attic floor, or whether applying the upfront ventilator cost to modestly upsizing a grid-tied PV array to cover the "extra" power use wouldn't be a better investment.

    There is no simple calculations for determining the thermal lag time at a given ramp rate for the attic temperature. It's fairly straightforward for step functions in temp (infinitely fast attic temperature change) which has a characteristic logarithmic time constant similar to R/C electronic filter circuit. (The distributed R- and thermal mass can be models the same as the limit infinite series of tiny resistors and capacitors- a partial differential equation.) You can use calculus methodologies to come up with reasonable models of the phase delay and magnitude of sinusoidal temperature or other temperature profile changes over time in the attic (real attics probably don't experience exactly sinusoidal temperature changes over a 24 hour period.) But it's really in the "who cares" territory for what that does to the peak and average cooling loads, given just how miniscule the thermal mass of the cellulose is relative to the thermal mass of the stuff fully inside the thermal envelope of the house. It's calculable, but it's in the measurement noise of a real house. If you really care to chase third and fourth order effects and model it with greater precision, knock yourself out:

    The mass / heat diffusivity effects on peak & average cooling & heating loads are at least measurable in R-C-R assemblies, where that C is concrete (such as a 6" wall built with insulated concrete forms), but that's more than an order of magnitude greater thermal mass than we are talking about with 2lbs per square foot of cellulose. The settled density of open blown cellulose is about 1.5lbs per cubic foot (not the 2.3lbs cited), sometimes less, and at R3.5/inch you're really talking ~1.8lbs per square foot for R50, not the 2.3lbs mentioned. But OK split the difference, call it 2lbs. The thermal capacity of 2lbs /ft^2 of cellulose is about 0.66 BTU per degree-F per square foot of ceiling, whereas 6" concrete is ~75lbs per square foot, with a thermal capacity of about 13.5 BTU/degree-F, or about 20x the thermal mass of R50-60 cellulose. While measurably shaving the peaks, those effects of ICF still barely makes double-digit dents in annual energy use. With the much smaller thermal mass of R50 cellulose it's a bit silly to be discussing how much it's thermal mass affects the peak or average loads- in practical terms it's ALL about the R. Even if you assumed the average temp of the cellulose splits the temperature between 138F and the room temp during the peak (which it doesn't due to the thermal storage lag), assuming a 78F ceiling at peak load that's about 30F x 0.66 BTU= 13.2 BTU per square foot of peak stored heat in the ceiling insulation, which begins returning to the attic as soon as the attic temp drops below the temp of the top layers of cellulose. If the attic drops to under 95F at night something like half of that stored heat leaves via the attic. But it's still too small to be relevant.

    Martin: The temperature setbacks only save money on a low-R high-gain house only if the AC is a bang/bang 1-speed system. With modulating systems there is about 3:1 efficiency delta at moderate outdoor temps (say mid-80s F) between highest modulation & lowest modulation. (See: See Figure 14, p.27: ). Keeping the house at constant temp would be both more comfortable, and it will use less overall cooling energy. (And importantly from a grid distribution & generation infrastructure perspective, it has dramatically less PEAK power draw.) Where a bang/bang 1-speed might save ~10% on energy use with setbacks, to be able to cool off the low-R high-gain house in a reasonable amount of time it has to be able to handle both the peak cooling load PLUS the amount of heat being radiated into the space by the over-temp thermal mass of all the stuff indoors, and the average radiation temp of that radiating stuff makes it less comfortable for the humans at any air temperature. With an R1 low mass tent or an uninsulated house fashioned from cargo containers setbacks would still save energy, but even with a fairly crummy framed house it wouldn't.

    The setback/energy use argument has long been used to rationalize 2-3x oversizing, but even there those energy savings come with a real comfort impact. With right sized or modestly oversized modulating mini-splits the energy savings of "set and forget" is WELL into double-digits most of the energy is being pumped out at 1.5-3x the efficiency compared to it's efficiency running at full speed to cool off the superheated house. Even though it's considerably more net energy being pumped out in the crummy house case, the dramatically higher average efficiency at part load means there is still less total cooling energy used.

  54. iLikeDirt | | #54

    All right, thanks Dana! That all makes sense. Looks like I know just enough to mislead myself in plausible ways!

  55. GBA Editor
    Martin Holladay | | #55

    OK, you're right. When I wrote my comment on thermostat setbacks, I was thinking of heating season savings rather than summer savings -- so my comment didn't make much sense in this context. (I wrote, "If a house has a lot of R-4 windows and R-11 walls, thermostat setbacks save considerable amounts of energy. But if an attic has R-60 insulation, it doesn't really matter what temperature the attic is.")

    So thanks for setting me straight about air conditioner setbacks ("The temperature setbacks only save money on a low-R high-gain house only if the AC is a bang/bang 1-speed system.") Your clarification is helpful.

    That said, I think we are both on the same side of the fence: We're both saying that R-60 insulation is so thick that a homeowner doesn't have to worry about inward heat flux from a hot attic. This is a trivial amount of heat.

  56. freeenergy | | #56

    DUH, this is easy (impressed by the over-analysis...) if you're seeking outside air temps, move them has nothing to do with a well insulated attic.

    Go to Walmart and buy a $17.00 box fan, open window, and place it there. THE definitive way would be to shoot the drywall ceiling with an IR gun before and after....can you say,"R-60 is better, by a LOT!!"

    It's the air spaces within the insulation that are effective, NOT any thermal mass....the more thermal mass, the HIGHER the conductivity......if you're in the decision process, use only blown fiberglass @R-60. I've used the cellulose, and have seen significant settling (less air spaces...)

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