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Community and Q&A

Can insulation “work against you” in a Cooling Climate?

homedesign | Posted in Energy Efficiency and Durability on

I noticed an unusual comment in the comment section of this page:
The comment is about 1/3 down the page (posted May 16, 2011 by Katrin Klingenberg)

In a cooling climate, the delta T is much smaller than in a heating climate, and due to the internal heat gains insulation actually starts to work against you at a certain point. I think anything up to R30ish is doable/defendable for all components, including roof and suspended floor. Beyond that it will only add to your cooling problem.

I understand how insulation can “work against you” in a building without air conditioning….
Or when the conditions outside are “better” than conditions inside.

The problem is that most Southerners DO Air Condition their buildings in order to be comfortable….
and for many of the hours during the Cooling Season the conditions outside are “WORSE” than conditions inside.

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  1. user-788447 | | #1

    Can there be a scenario where it is cooler outside but overly humid versus interior conditions where you wouldn't just open windows to allow internal heat gains to escape?
    I can't see how insulation can work against you in a thermal sense if you can open the windows to equalize interior and exterior temps unless you are trying not to introduce more humidity into the home.
    In hot/humid climates do y'all use ERVs to keep out the humidity during the summer?

  2. homedesign | | #2

    J Chesnut,
    YES, there are many times when ventilating or flushing could "work against us" even though it may be cooler Outside.

    Concerning ERVs....
    So far, I have only seen a handful of ERVs installed in a few high performance homes.
    I think an ERV would be useful as long as it included a "bypass" for times when Outside Conditions are "Better" than Inside conditions.

  3. GBA Editor
    Martin Holladay | | #3

    J Chesnut,
    An ERV can't "keep out the humidity during the summer."

    If you are ventilating, running the ERV can only lower the indoor humidity when it is dryer outdoors than indoors -- in which case an HRV or an ordinary fan would do the same thing.

    In most summer situations, the ERV raises the indoor humidity -- but not quite as much as the humidity would be raised if you were ventilating with an HRV or an ordinary fan.

  4. homedesign | | #4

    Since Martin has not debunked my observation ....
    I continue to believe that Katrin Klingenberg is Wrong about the merits of R-value in Cooling Climates.

    I think one of the most Affordable and Buildable strategies for Cooling Climates would be to build One Story homes with Vented Attics and Large Quantities(far greater than R-30) of Affordable Cellulose.

  5. user-659915 | | #5

    "In a cooling climate, the delta T is much smaller than in a heating climate,"

    Yes, this is generally true and it will certainly affect the economics of extra insulation.

    "and due to the internal heat gains insulation actually starts to work against you at a certain point."

    Emphatically no. This is flawed conceptual thinking, like calling for help but not too loud so as not to bother anybody. The extra insulation may not be particularly cost-effective but it will never increase your energy use for cooling as long as outside temperatures are higher than the interior. The A/C pumps the heat outside: the insulation stops it getting back in. If it's cool enough outside that you're not needing to use the A/C then standard natural ventilation options apply.

    The smart money of course is in minimizing internal heat gain in the first place. High-efficiency lighting and not too much of it. Cook outside to the extent possible. Properly insulated hot water storage - or an efficient on-demand unit. And if you pack the house with bodies for a dance party and the A/C just can't cope, that's when you open the darn windows and sweat it out.

  6. user-939142 | | #6

    they could also be referring to cost benefit

    the payback on the added insulation lengthens because a portion of the time it is providing no benefit or a much lesser benefit, and in some cases not allowing helpful passive gains like reflectivity on the roof during winter

  7. homedesign | | #7

    I believe the context of Katrin's statement is about extremely low energy enclosure design....
    As in meeting or approaching the Passivhaus standard.
    Of course "cost benefit" of each component or strategy should be weighed.
    Also all of the typical Cooling Climate Strategies should be employed first.

    Katrin did not mention cost ...
    she said "beyond that (R-30) it will only add to your cooling problem"

    European Passivhaus recognizes that R-Value "works" in a cooling climate
    "Contrary to some publications, good thermal protection also helps to provide high thermal comfort in summer (particularly if applied in the roof) and to reduce temperature fluctuations."

  8. user-659915 | | #8

    @Bob - the first part of Katrin's comment could indeed relate to cost/benefit and my earlier post acknowledged this. Higher Delta-T = higher benefit of a given insulation cost and vice versa. The second part claims that adding insulation beyond a certain point could increase cooling load due to internal gains. This is nonsense. With all due respect, Katrin didn't think it through. I believe this is an understandable misconception due to the almost exclusively cold-weather focus of Passivhaus theory so far: as Passivhaus thinking penetrates further into the US south I believe it's inevitable this kind of fundamental error will be cleaned up. Props to John B for bringing this issue into the light.
    On the subject of cost/benefit of Passivhaus in the south: count me interested but not yet convinced. I'm hoping some time in the not distant future to see logged performance data from the small but growing number of Passivhaus buildings in mixed/cooling climates so that those of us who prefer to stay away from the bleeding edge can make a realistic assessment on behalf of our clients.

  9. homedesign | | #9

    Insulation AND airtightness can "work against you" in almost any climate.
    IT Depends...
    on the use,the surroundings, the season, the weather, the sun/night-sky and the time of day.

    I can see how the Cold Climatists ;-) may have developed a "misconception" about the cooling season.
    The Cooling Season in A Cold Climate is not the same as in a Hot or Mixed Climate.

    I think if you model a high performance house during the summer in some of the Cooler climates....
    the homes may actually perform better over the entire cooling season with LESS Insulation.

    I also think that during the Shoulder Seasons in the South....
    there will be "times" when LESS insulation = better performance.

    My thinking is that BECAUSE of Internal and Solar Gains we (the South) need to start cranking up the R-value dial in order to get our SOURCE Energy per household down closer to the North-Eastern levels

  10. user-659915 | | #10

    "I also think that during the Shoulder Seasons in the South....
    there will be "times" when LESS insulation = better performance."

    Please explain, John, in what circumstances "LESS insulation = better performance". I understand how in various climates the energy interchanges between a building enclosure and its exterior environment vary according to time of day and time of year. I understand how beneficial solar gain may be captured and made use of through proper fenestration. I understand how beneficial solar gain may be retained in the enclosure with thermal insulation and air-sealing to continue comfort conditions through the night. I understand how insulation and shading can keep unwanted solar gain at bay during a hot summer, how natural ventilation can be managed to remove unwanted heat from an interior that's warmer than ambient, and how in dry climates evaporative cooling can be made to achieve this trick even when ambient is slightly hotter. I can also understand that in certain (lucky) climates insulation of any amount is just plain irrelevant, and that in almost any climate there comes a point of diminishing returns where adding yet more insulation has negligible additional benefit. But I do not understand how in any climate, given proper solar orientation and layout, LESS insulation could conceivably result in BETTER thermal performance, any more than I would understand how building a leaky ship will keep it afloat better than a tight one.

  11. homedesign | | #11

    I hope I did not blurt something out without thinking it thru.
    Here is my thinking ...
    That the shoulder season in a hot or mixed climate could at times be "like" the cooling season in a cold climate.
    That is to say Hot, but Not-Too-Hot during the day and cool at night.

    When I first read Katrin's claim about "insulation working against you" in a cooling climate .. I thought WTF .. how can that be?

    I decided to download REM/Design and see if I could test her claim.
    I modeled a house with code levels of insulation for New Orleans..
    Then I Super Insulated the walls and ceiling...
    No surprise to me ... the cooling cost did not go went DOWN
    Super Insulation is NOT "working against you"..or at least not in New Orleans.

    I tried the same thing for Denver and Chicago..... and sure enough... according to REM/Design ...Super Insulation caused the Annual Cooling cost to go UP!
    I think the reason is related to Internal Gains and the fact that the Enclosure can not cool as easily at night.

    I realize that REM/Design is not as sophisticated as PHPP... so I asked Mike Eliason if PHPP actually showed an increase in Cooling Cost using cranked up R-values in New Orleans.
    Mike admitted that he could not support Katrin's Claim with PHPP.

  12. user-659915 | | #12

    "I think the reason is related to Internal Gains and the fact that the Enclosure can not cool as easily at night."
    Or more likely, the reason is a faulty algorithm. I don't see where the theory would support the calculated results you noted. Extra insulation properly located would reduce heat gains during the day, and see no reason that it would it reduce heat loss at night, given adequate provision for ventilation. Is there something in the theory that I'm missing? More to the point, can anyone produce reliable real-world field data showing increased insulation resulting in increased energy use?

  13. homedesign | | #13

    I am not advocating to reduce insulation... I am advocating the opposite.
    I do not believe that increased insulation will result in increased energy use over an entire year.
    I am only saying that I can imagine time intervals when increased insulation is counter productive.
    There are many times in humid climates when ventilation flushing is not desireable.

  14. user-788447 | | #14

    What's your approach to subslab insulation in your climate. Over the course of a year does the cooling effect of no insulation outweigh the penalty in winter? Do you need subslab insulation to avoid condensation during humid summer days?

  15. Mike Eliason | | #15


    i didn't admit i couldn't support kat's claims - because we have seen the same thing, though it is rare - at least how we plan for those regions. in our models, it seems to be more a function of design that results in this. i think what causes this, and if i can find time i will verify, is that internal gains + solar gains are basically trapped inside the envelope, thereby increasing cooling loads.

    we noticed this specifically when we didn't have glazing nearly completely shaded - that once you are at the threshold of passivhaus, that increasing the amount of insulation can be counterproductive (but again, not always, depends on design). also, even with the utilization of low-SHGC glazing, there can be significant solar gain on the north side in summer.

    on the passivhaus we modeled for the new orleans competition, our peak cooling load was around 1/2 of the peak load of a one-ton mini-split - so the increase in cooling loads could be moot.

  16. user-939142 | | #16

    the slower cooling at night may be valid on a fixed building

    but in the real world, the homeowners open the windows and solve that problem in about 20 minutes for free, something that can't be modeled. the always humid climates could make some days problematic

  17. homedesign | | #17

    Mike Eliason,
    Sorry if I made the wrong conclusion from your comment #23
    I thought you were saying that you had seen the "phenomenom" during training but could not duplicate it with a New Orleans PHPP model.
    Are you saying that PHPP DID show an increase in annual cooling cost with increased wall/ceiling insulation in New Orleans?

  18. homedesign | | #18

    Bob Coleman,
    I sort of agree with your comment #16, except I would say that night flushing is problematic for MANY (not just some) days/nights in a Humid climate.

  19. homedesign | | #19

    J Chesnut @ comment #14,
    I am living near Dallas...right on the border of Hot Humid and Mixed Humid.
    I have never seen anyone using slab edge or under slab insulation in my region.
    I am very interested in exploring slab edge insulation and under slab insulation for the perimeter of the slab... but not the interior of the slab.
    Armando and I were discussing Slab insulation some on this thread
    I think there would be an annual advantage(in my climate) to insulate under the slab at the perimeter but not insulate the interior of the slab.
    I have not seen a problem with slab condensation ... but it could be a problem if humidity is not controlled.

  20. gusfhb | | #20

    Perhaps I am being obviousman, but if your cooling loads go up due to internal heat loads, it is time to look a those internal heat loads, not the level of insulation. In heating climates all of these things are your friend. With so many living in cooling zones, maybe some enterprising company will develop appliances with external vents[ovens dishwashers] and fridges with separate compressors.

    Perhaps it may be time to consider that when you are down to 5k btu cooling loads for a whole house, you are down in the noise when trying to calculate improvements.

    Perhaps this is a spot where the human brain takes over for the spreadsheet,a signal, if you will that you are approaching the point of zero gain when you are pondering putting the fridge in the driveway to lower your ac bill........

  21. homedesign | | #21

    I agree that internal loads are "your friend" not my friend (except during the winter)
    Internal loads are not our only load... they only contribute.

    Maybe it is silly for a residence ....but I have heard that Daikin (Japanese mini-split co) also builds split unit refrigerators for grocery and convenience stores

  22. GBA Editor
    Martin Holladay | | #22

    Most supermarket refrigerators or walk-in refrigerators for schools and commercial buildings have outdoor condensers.

  23. homedesign | | #23

    I agree that it is currently challenging to "cost " justify building R-30 and greater wall assemblies for Hot & Mixed about in 2030?
    In the meantime....
    I see nothing wrong with an airtight/Greater than R-30 Ceiling with a vented attic
    Does Greater than R-30 ceiling insulation "work against ANYONE"?

  24. homedesign | | #24

    thanks Martin

  25. gusfhb | | #25

    Since high r value roofs are usually much cheaper than the corresponding wall would be, I can't see how even higher values wouldn't pay off in a place like arizona or nevada. I will bet sun loading in the south raises the 'real' Delta T higher[IOW the roof surface is 140]

    I was thinking of commercial coolers, but they probably are a bit overkill.

    I bet there is a refrigerator tech/green builder down there that has replumbed/rewired his kenmore to have an outside compressor.

    heat pumps heat pumps everywhere....ok where is the guy designing the VRV mini split/refrigerator/hwh/hair dryer

  26. homedesign | | #26

    I think a lot of the Passivhaus Designers and Building America Designers ...
    and well heck...almost All of the High Performance Home Designers
    (except for Riversong ,Chlupp & a few others) are hung up on Vaulted Ceilings, Compact Roofs and Unvented Attics.

    It's No wonder that high performance homes are Not-So-Affordable

  27. Expert Member
    ARMANDO COBO | | #27

    I’ve found here in north TX that R26 walls and R38 roofs are cost effective, the issue is to learn to install some of the insulation inside and some on the outside. And yes, if you do WUFI or ASHRAE Fundamental’s analysis on walls or roofs here in north TX, you can easily see that a 2x4 wall with any insulation correctly installed BUT no outside insulation performs better than a 2x6 wall with the same insulation installed correctly; and the farther up north you go the more of an issue it is.
    Vaulted ceilings are fine as long as they have a way to be ventilated by a fan or a return air up high; and there are thousands of affordable housing projects, Habitat for Humanity homes and starter homes around the country that showcase how very affordable they can be.

  28. albertrooks | | #28

    What a great thread.

    I can barely get my arms around a heating climate, let alone a cooling climate... But never the less...

    The point of the envelop is largely to create a separate climate from the exterior. It appears to me that in the end, you desire to isolate and control the interior. The isolation happens by "breaking" the external effects of the exterior upon the interior. Once done, controls are added to influence interior conditions. The "controls" would be most efficient when they are simply influencing the interior and not opposing exterior conditions.

    The above is nicely over simplified, doesn't take into account that it's actually really hard to build a thermos that's big enough to live and, that has a great view of the bay...

    If the envelop is good enough in a cooling climate, essentially at stasis, then wouldn't the next target be isolating and dealing with the internal heat gains?

    I actually though the comment by "obvious man" aka Keith Gustavson was a bit brilliant, and come to think about it, a bit "duh!". It really does pay to start looking at individual components like compressors and DHW heaters. Not to get all "magic boxy", but starting with simply moving the condensers outside and then, eventually looping them into one unit is a pretty interesting idea.

    I wouldn't toss that one aside. Bigger things were started on lesser ideas. It'd be nice if someone knew a guy who was pretty good at refrigeration and had some time to develop an idea. It would sure be nice if the next generation of innovative products were developed and built in the US.

  29. user-659915 | | #29

    "And yes, if you do WUFI or ASHRAE Fundamental’s analysis on walls or roofs here in north TX, you can easily see that a 2x4 wall with any insulation correctly installed BUT no outside insulation performs better than a 2x6 wall with the same insulation installed correctly; and the farther up north you go the more of an issue it is."
    I've looked at this sentence three times but I still don't think I'm understanding it. An insulated 2 x 4 wall performs better than a similarly insulated 2 x 6 wall? And what do you mean by 'no outside insulation'?

    John B:
    I think if night time flushing is ineffective then I suspect you might have a too much thermal mass problem rather than a too much insulation problem.

  30. homedesign | | #30

    I can't decipher what Armando was saying either

    I am not suggesting to reduce insulation
    I was just trying to noodle out why Katrin and or others may believe insulation can work against you during the cooling season.

    Mike E,
    Were you able to get PHPP to show "the phenomenon" in N'arlens

  31. homedesign | | #31

    Kudos to you for promoting (double-code)R-26 walls for N.Texas
    Why not the less expensive (double-code) Ceilings too?

  32. gusfhb | | #32

    nighttime flushing would be ineffective when the nighttime is above the acceptable temp, or if humidity is a factor. It seems to me one or both is an issue a lot of the time in a lot of the southern climate.

    with any humidity involved, once you live with ac, you have to live with ac, opening the windows is frequently more expensive than leaving them shut[if you are going to turn it on again tomorrow]

    I kinda figured something was backwards in Armando's post, or I was lost........

  33. homedesign | | #33

    I think you are understanding the Southern Climate.

  34. GBA Editor
    Martin Holladay | | #34

    As someone who grew up in a hot climate without air conditioning, I'm curious: how many people in Texas live without air conditioning? It used to be common to live that way.

    Clearly, for those who live without air conditioning, and whose homes still have internal gains and solar heat gain through windows, insulation would often work against you during the summer, unless nighttime temperatures are dependably low enough to allow a nighttime flushing strategy.

  35. homedesign | | #35

    Martin, When you say hot climate... where?
    was it also a humid climate?
    Was it often 88 degrees after 10pm?

    My father grew up without indoor plumbing and he walked many miles to school.
    To answer your question... I do not know of anyone (near Dallas) who is living without air conditioning. Even the lowest income families at the very least have window units.

  36. homedesign | | #36


    Clearly, for those who live without air conditioning, and whose homes still have internal gains and solar heat gain through windows,

    Clearly,these people would be dead and living in homes without Windows.

  37. GBA Editor
    Martin Holladay | | #37

    Obviously all code-compliant homes have windows, which is why I wrote that all people living in the South (like people elsewhere in the country) "still have internal gains and solar heat gain through windows." I'm not quite sure what your point is about living in homes without windows.

    I grew up in Beirut, Lebanon, which has hot, humid summers. During the 1960s, almost no one had residential air conditioning in Beirut. However, the movie theaters all had air conditioning.

  38. homedesign | | #38

    Sorry for the bad joke.

    I think it is safe to say that Air Conditioning is a "given" in my climate/culture.
    So I think Insulation is a "given"

  39. GBA Editor
    Martin Holladay | | #39

    Looks like the weather will be more pleasant this week in Beirut than in Dallas.

  40. Expert Member
    ARMANDO COBO | | #40


    An insulated 2 x 4 wall performs better than a similarly insulated 2 x 6 WITH NO OUTSIDE INSULATION

    Outside insulation means rigid foam on the outside of the wall and roof sheathing. Exterior insulation warms the framed cavity and raises the interior surface of insulation above the dew point. Most years in DFW you will not have that problem, but earlier this year where temperatures were abnormally cold, there was a small chance for condensation, depending on indoor temperature and humidity; in fact, we inspected 3 homes with such problems last winter in Dallas. It becomes more of an issue in CZ4 and higher.

    I do design homes with vented attics as well, with healed trusses and R38-42 cellulose or BI fiberglass. It’s a matter of where the ducts are installed. All HVAC equipment are in closets in the conditioned space, but the duct system sometimes goes in the attic and sometimes goes on a furred down ceiling, but always in a conditioned space. Also it depends on the roof style; with high pitched roofs and no rooms in the attic space, makes no sense to conditioned such a great volume of air, therefore a ventilated attic is better.

  41. gusfhb | | #41

    Party cloudy with a chance of grenades towards nightfall..................

    I think with the windows 'open' the chances of a well insulated house rising above outside temp are slim.

  42. Expert Member
    ARMANDO COBO | | #42

    I can relate to Martin growing up in Beirut. I grew up in South America w/o AC systems in houses; only in some commercial bldgs and movie theaters, but temperatures where never 105°F and +70% RH like they are in DFW.... ARGHHHHHH.......

  43. Expert Member
    ARMANDO COBO | | #43

    Let me give you folks some idea of MJ loads here in DFW. I just finished designing a 4,932 sf house with R24 walls and R30 roof, 1/2” rigid foam R3 outside insulation and R21 cellulose on the walls, and 1” on top of the roof decking and 7” OCF under the decking (conditioned attic). Stucco cladding. 15% glazing with Low-E, argon, clad windows. 2-2t 16 SEER AC & 2-40kbtuh 96AFUE multi-speed furnaces, make-up air and expected less than 1ACH50.
    Here are some interesting loads.
    1st floor heating: Floors 48%, Glazing 19%, Walls 18%, Ventilation 12%, Ceilings 2%. Total 35Kbtuh.
    1st floor cooling: Glazing 45%, Walls 20%, Internal Gains 19%, Ventilation 5%, Ceiling 3%. Total 18Kbtuh.
    2nd floor heating: Walls 27%, Glazing 25%, Ceiling 20%, Ventilation 12%, Ceilings 20%. Total 23Kbtuh.
    2nd floor cooling: Glazing 36%, Ceiling 17%, Walls 16%, Internal Gains 16%, Ventilation 8%, Total 19Kbtuh.
    With a post tension & pier and beam foundation, you can find anyone to insulate the foundation. I’ve been trying to talk engineers and builders into, but no cigar. I guess we just wait till the codes makes us do it.

  44. user-723121 | | #44

    John Brooks,

    You have stirred things up again, good job. I think using more attic insulation in a cooling climate is a great idea, why stop at R-38 or R-50 for that matter? In Minneapolis yesterday the high temperature was 85F, my attic temperature right on top of the insulation was 110F. The inside temperature was 78, the delta T for the walls was 7 and for the ceiling was 32. The roof system is an 8/12 pitch, truss rafters, vented at the eave and ridge with blown fiberglass on top of the 5/8" drywall flat ceiling. I'm sure the attic temperature near the roof sheathing was much higher, I will try and check this out on a similar sunny day.

    With Dallas temperatures running in the low 100's, the attic delta T must be incredible. Cathedralized roof insulation systems would not benefit from the air/venting buffer the way a typical roof truss and blown insulation would. I would say the majority of the Dallas cooling load is coming through the ceiling, the drywall acting as a giant radiant panel. Just my observation from the cool far north.

    Attic temperature update. Outside temp. is 84F, attic temp. 1' from the ridge is 127F, this is 8' above the blown insulation. Full sun today just like yesterday.

  45. user-723121 | | #45


    What is the expected cooling load in kWh for a normal season? How many cooling degree days for the location?

  46. homedesign | | #46

    Armando, I noticed that you were concerned about Global Warming Impact for the wall cavity insulation on this thread
    Why aren't you concerned about the roof?
    I agree with Brett Moyer.... why not use less or no foam?

  47. Expert Member
    ARMANDO COBO | | #47

    Why are quotes showing so small and hard to read??? I'm trying again...

    "In KC, you need 1 1/2” min. (2” is better) rigid foam on the outside of the wall assembly and 5.5” of cellulose (better) or open-cell foam. If you are going to do a ventilated attic, then you need R38 BI cellulose or BI fiberglass over the ceiling and with healed trusses or stick frame roof over a 12” pony (knee) wall. If you are going to do an unvented attic, you need 8” of open cell under the roof decking and 2” of rigid insulation on top of the roof decking."
    As you can see John, I said different options.

  48. Expert Member
    ARMANDO COBO | | #48

    Dallas CDD, depends on who’s doing the reporting is around 2800, but I've seen 3500, I don’t know what the engineer used. Also I don’t know what the year’s cooling load supposed to be in Dallas, but whatever it is, this house should use half; it has a design score of HERS 50.

    My job as designer is to recommend best practices for what the client wants, I don’t pull a gun to their heads and make them choose a particular product, design or size; and here is what I said:

    In KC, you need 1 1/2” min. (2” is better) rigid foam on the outside of the wall assembly and 5.5” of cellulose (better) or open-cell foam. If you are going to do a ventilated attic, then you need R38 BI cellulose or BI fiberglass over the ceiling and with healed trusses or stick frame roof over a 12” pony (knee) wall. If you are going to do an unvented attic, you need 8” of open cell under the roof decking and 2” of rigid insulation on top of the roof decking.

    FYI, I discussed with the client to do a conditioned crawl space and ventilated attic as an option, as well as a furrdown on the second floor ceilings for the duct system. He chosed a conditioned attic.

  49. homedesign | | #49

    Thanks Armando,
    I can relate
    I try to suggest things to Clients and Builders ... and in the end they do as they please

    Armando:"it has a design score of HERS 50"
    ah-ha a perfect HalfassivHaus !

  50. user-659915 | | #50

    "Clearly, for those who live without air conditioning, and whose homes still have internal gains and solar heat gain through windows, insulation would often work against you during the summer, unless nighttime temperatures are dependably low enough to allow a nighttime flushing strategy."

    Clearly, how? In the south, high levels of insulation (together with proper shading etc, natch) are the main protection against daytime heat gain, and in no circumstances could accumulated heat leak OUT through an under-insulated enclosure if night-time temperatures are not low enough for night-time flushing by ventilation. If one were foolish enough to have large unshaded western windows and huge amounts of thermal mass in addition to a highly insulated enclosure, yes, one would have created a very large solar oven. But it would make no sense whatsoever to blame the insulation for the resulting hot box.

    By the way, I've always seen it written that a truss has an elevated 'heel', not 'heal' as per Armando's post.

  51. Expert Member
    ARMANDO COBO | | #51

    James, It was a typo... it is heel or energy truss.

  52. user-723121 | | #52

    I'm wondering how many cooling load calculations are using (normal) ambient temperatures for the ceiling portion? Conditioned attics with blistering hot roofing are going to have a very high delta T. On the reverse, in a heating climate, a conventional truss roof with blown insulation where the roof and attic space is warmed by the sun will have a lower delta T than at ambient.

    I know some of the energy simulation tools ask about roof color but do they really account for the dynamics of solar interaction with the building, windows and solar gain aside?

  53. homedesign | | #53

    Doug, I have had a similar "wonder" about some of the Energy modeling programs.
    I recently realized that REM/Design is not-so-sophisticated about wall orientation ..etc.

    My understanding so far(I am a newbie)... is that REM/Design and the related HERS index can NOT distinguish between Insulation at an Attic Floor and Insulation at a Roof Deck.
    ......2 very different animals

  54. user-723121 | | #54


    My non-scientific finding is at solar noon, in Minneapolis,with a day of full morning sun and ambient at 85F, the roof deck temperature is 127F and the attic floor temp. is 110F. Timberline roofing is black and the 8/12 pitch roof faces due south + or - 10 degrees.

    If ceiling heat loss/gain calculations are made using normal ambient temperature there is going to be some undersizing of HVAC systems and a poor accounting of cost/benefit for additional insualtion. Roof design also comes into play as you have stated, it appears to me a vented roof with an insulated attic floor has an advantage over the insulated roof deck. There is also the additional volume and envelope surface area that comes with a conditioned attic.

  55. user-659915 | | #55

    "Roof design also comes into play as you have stated, it appears to me a vented roof with an insulated attic floor has an advantage over the insulated roof deck. There is also the additional volume and envelope surface area that comes with a conditioned attic."

    An obvious point which is too often simply ignored. At any pitch over the very lowest the additional area is substantial. The contractor for a small home I recently designed wanted to use spray foam at the roof deck. The home has a 7/12 gable roof running east/west. Setting aside for a moment the significant difference between the Delta-T at the roof deck and the Delta-T at the ceiling, these are the relevant area calculations:

    Plan A: Cellulose at ceiling joists, area same as floor plan, 987 sf @ R40.

    Plan B: Icynene at roof deck = roof plane area +gables + knee wall at inset porch, 1,612 sf @ R20.

    My client, an intelligent person who can do the math and who is not easily convinced that less insulation is better, is therefore happy to go with the cellulose.

  56. homedesign | | #56

    All Righty Study Group
    We are making progress!

    James, I am jealous... where do you find these "intelligent clients"?

    Doug, I have asked my "wonder" about REM/Design and Ceiling Delta-T over at a RESNET discussion group

    Mike Eliason (or other PHPP jockeys), how about PHPP?
    Does PHPP distinguish between Ceiling Assemblies exposed to an attic vs exposed to "the sky"?
    (I am guessing that it does)

    I think the HERS index dial is stuck somewhere around 50

    Why does the R-value dial go beyond 30 only in the Cold Climates?

    Help me out here ;-)

  57. user-659915 | | #57

    John, there's a great deal of intelligence in most people. You just have to figure out a way to connect with it.

  58. homedesign | | #58

    I was thinking about the size of the much as the choice of assembly.
    sounds like a reasonable size home
    I do get a few smaller homes to design... most are not-so-small

  59. user-723121 | | #59

    I would encourage builders, designers and others working with energy simulation tools to create a "reference house" for their location. The reference house would be typical to new homes built in the area, keep it simple such as a 2 story with 1,000 square feet on each floor. By using a round number for home sizing, efficiency is easily assessed on a sq. ft. basis.

    The reference house would be built to code for a given location with typical materials, glazing, wall and roof design. R- values are to code and I would use equal glazing on all elevations for simplicity sake.

    By running a simulation on the reference home, heating and cooling loads can be established for a building built to code in your location. This will serve as a baseline for comparing options like thicker walls, additional ceiling insulation, window upgrades and glazing placement. Payback for the various upgrades can be made on an individual basis by plugging in material and energy costs.

    Once the reference house is created you can run various ACH50 scenarios to see what reduced infiltration rates will mean to the bottom line. With some tinkering of building design, R-values, and building more airtight you will find a sweetspot and your standard construction will far outperform conventional built structures.

  60. Mike Eliason | | #60


    on our first pass w/ the new orleans project, with a really compact building, low-SHGC glazing and before adding shading, we did notice a threshold where the internal gains + solar gains ticked up but i don't recall how much. when we tweaked the design and increased the surface area while nearly completely shading the windows in summer and shoulders, that issue doesn't exist - so again, it is possible, but i think it's very climate and design driven than anything. only our floor assembly was above R-23, and that was to reduce thermal bridging. had it been a slab on grade, R-10 (fully insulated) would have been enough.

    regarding insulation exposed to attic v. elements, somewhat. PHPP has an input for exposed or screened surface films (of insulation) - and if you wanted to take into account the attic, it might be possible to model. generally everything is modeled from insulation to insulation. if you want to go nuts and take into account the absorptivity/emissivity of exterior assemblies - you can input that as well. generally, there is enough redundancy in the model that i don't believe you need to take that into account for most places - though i think they did at the oesterreichhaus in whistler, which is one of the reasons an all-black facade was used.

    there is also a tab to assign a 'temperature zone' to different assemblies. the version of PHPP i have is the I-P version (which is an overlay of the S-I version), and so much of the formulae are hidden in the background, I believe there are various factors for these, but don't recall offhand.

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