Helpful? 1

Installed R60 in using A/C more than before

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?

Asked by Jeff Watson
Posted Sat, 07/12/2014 - 16:12


24 Answers

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Helpful? 0

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?

Answered by Peter L
Posted Sat, 07/12/2014 - 17:43
Edited Sat, 07/12/2014 - 17:45.

Helpful? 0

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.

Answered by David Meiland
Posted Sat, 07/12/2014 - 18:40

Helpful? 0

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.

Answered by Jeff Watson
Posted Sat, 07/12/2014 - 18:59

Helpful? 0

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.

Answered by Graham Fisher
Posted Sat, 07/12/2014 - 19:27
Edited Sat, 07/12/2014 - 19:57.

Helpful? 0

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.

Answered by Jeff Watson
Posted Mon, 07/14/2014 - 09:11

Helpful? 0

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.

Answered by HD S
Posted Mon, 07/14/2014 - 09:57

Helpful? 0

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.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 10:08

Helpful? 0

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...

Answered by bob holodinsky
Posted Mon, 07/14/2014 - 12:07

Helpful? 0

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.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 12:37

Helpful? 0

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

Answered by Keith Gustafson
Posted Mon, 07/14/2014 - 21:18

Helpful? 1

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.

Answered by Jeff Watson
Posted Mon, 07/14/2014 - 21:34

Helpful? -1

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.

Answered by aj builder, Upstate NY Zone 6a
Posted Mon, 07/14/2014 - 21:36

Helpful? 0

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.

Answered by Kevin Dickson, MSME
Posted Tue, 07/15/2014 - 01:03
Edited Tue, 07/15/2014 - 01:22.

Helpful? 0

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.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/15/2014 - 05:58

Helpful? 0

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)

Answered by John Brooks
Posted Tue, 07/15/2014 - 09:21

Helpful? 0


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

Answered by Kevin Dickson, MSME
Posted Tue, 07/15/2014 - 15:08

Helpful? 0

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).

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/15/2014 - 15:23
Edited Tue, 07/15/2014 - 15:27.

Helpful? 0

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.

Answered by aj builder, Upstate NY Zone 6a
Posted Tue, 07/15/2014 - 17:35

Helpful? 0

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.

Answered by Dana Dorsett
Posted Wed, 07/16/2014 - 12:40

Helpful? 0

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.

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 07/16/2014 - 23:34

Helpful? 0

"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.

Answered by flitch plate
Posted Thu, 07/17/2014 - 00:39

Helpful? 1

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

Answered by Keith Gustafson
Posted Thu, 07/17/2014 - 07:57

Helpful? 0

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.

Answered by Dana Dorsett
Posted Thu, 07/17/2014 - 18:21

Helpful? 0

"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.

Answered by James Morgan
Posted Sat, 07/26/2014 - 14:29

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