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Community and Q&A

Re-roof and insulation question

tedtulare | Posted in General Questions on

I have a 22 year old home. It has a 25 year comp roof on it with 1/4″ Foamcore insulation under it. I need to re-roof and wanted more insulation (thinking about 3/4″ foam). Can I just run a 1″ x 4″ around the edge and put new drip edge and sheet over the old roofing with the foam and putt the new reflective comp roof over it? If so, what kind of foam do you recommend? Really appreciate your input. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    The short answer is no. You can't nail asphalt shingles through 3/4 inch of rigid foam. If you want to install a layer of rigid foam on your roof, you will need to install a new layer of OSB or plywood sheathing above the rigid foam before you install your new asphalt shingles.

    For more information on this issue, see How to Install Rigid Foam On Top of Roof Sheathing.

    A more in-depth answer -- advising you about the best way to insulate your roof assembly -- is impossible unless you provide us more information. If you want advice, we need to know your location or climate zone. We also need to know what is under the roof -- is it a vented attic or conditioned space of some kind? (In other words, is this a cathedral ceiling?)

    Finally, is there any other insulation in the roof assembly -- perhaps fiberglass batts? Or is your very thin layer of rigid foam all you've got?

  2. tedtulare | | #2

    I live in central CA in the middle of the San Joaquin Valley (very hot and dry in the summer). I have 2x6 rafters with 1/4" foam core below (under the sheet rock)as not to interfere with the ridge venting as I have a livable attic space the entire length of the house. It would not be practical to put bats in the 2x6 as it is already sheet rocked. My father built a cabin in the mountains with 2x6 tong and groove open-beam ceiling and they put 1 1/2" rigid insulation and nailed the shingles through the insulation (done to code). Why cant you do the same with 3/4" foam?

  3. GBA Editor
    Martin Holladay | | #3

    As far as I know, there aren't any asphalt shingle manufacturers that allow asphalt shingles to be nailed through rigid foam. They all require the singles to be installed over plywood, OSB, or solid board sheathing. If you doubt me, call up the manufacturer of the shingles that you plan to use.

    If your roof assembly has no insulation except 1/4 inch of rigid foam, you really need to install R-38 insulation on your roof. That is the minimum code requirement. If you follow my advice, you will dramatically reduce your energy bills.

    You can achieve R-38 with 10 inches of EPS, 8 inches of XPS, or 7 inches of polyiso. You can choose to install the insulation in multiple layers, followed by a second layer of roof sheathing; or you can install nailbase or SIPs that have the necessary R-value.

  4. iLikeDirt | | #4

    I used to live in an apartment like this in California that had a vented cathedral ceilings with shallow rafters, no insulation, and asphalt shingles. It was unbearable in the summer, and required 24/7 air conditioning when the maximum outside temperature was barely 80f due to solar gain. This type of roof assembly is criminally bad--literally. If it's time to re-roof, you should be thinking of adding on the order of 6+ inches of insulation like Martin suggests, not another 3/4". This may seem extreme, but it may pay for itself in a short period of time--even given your mild climate--due to decreased air conditioner usage. Another cool bit: you can use this new foam to extend your roof overhangs in conjunction with some new framing and the extended new roof deck over the foam, which will make a huge difference in your air conditioning load and comfort.

    One final optimization: don't put new shingles on. Go with a light-colored or bare metal roof. In your climate, the difference--even with thick insulation--will be noticeable and worth it. Also, you won't be needing to replace it again after another 22 years, which will raise the resale value of your house.

  5. tedtulare | | #5

    Maybe I didn't make it clear. I have 2x10 ceiling rafters / attic floor Joist filled to the top with blown-in cellulose insulation.
    I am trying to keep the heat from transferring from the roof into the attic space below.

  6. tedtulare | | #6

    I built my house so that the overhangs keep the sun off the walls in the summer. I would have to get a new wife before I can put a metal roof in the house and, since I have been married for 43 years, I do not have time to train a new one..

  7. GBA Editor
    Martin Holladay | | #7

    In most cases, the temperature of an attic is irrelevant (especially if you have adequate insulation on the attic floor). Do you have any ducts in your attic?

  8. iLikeDirt | | #8

    Looks like you've discovered that the conventional wisdom of attic temperature being irrelevant to a ceiling with a well-insulated attic floor seems ill-suited for very hot climates and doesn't actually seem to result in as much comfort as you hoped. No doubt you will be told to add even more insulation. Go to 15", 20", 25"! It never seems to be enough. It may lower your AC bills but there will still be heat radiating down right onto your head, making you feel hotter than the air temperature would suggest.

    Simply put, your problem is roof solar gain. You need to prevent all that solar heat from getting into your attic. If your wife rejects reasonable options and cannot be convinced to listen to reason, then there are other ways you can try to make the best of it. White shingles will be better than darker shingles. And there's always barrel tile, which is popular in California and quite beautiful, not to mention seriously long-lived. You could install these tiles over radiant barrier material applied to the roof decking, and another radiant barrier sandwiched between the vertical and horizontal members of the purlins that the tiles will be fastened to. Detail the roof with space for hot air to escape at the ridge top and you've got yourself a solar chimney under the tiles but above the sheathing, and the sun will drive heat out of the roofing assembly rather than down into the attic. If your roof can't support the weight of real barrel tile, then you could see if metal shingles or stone-coated metal tiles would pass the wife test.

  9. GBA Editor
    Martin Holladay | | #9

    You are repeating an oft-repeated but discredited slander against conventional insulation materials. Insulation will not "radiate heat downward." A thick layer of insulation will separate a hot attic from the cooler space below, and will limit the flow of heat downward. The thicker the insulation, the better the insulation is at slowing the flow of heat.

    The bottom layer of the insulation will be at the same temperature as the drywall ceiling. The uppermost layer of insulation will be at the attic air temperature. There will be a temperature gradient through the insulation layer, and the gradient will range from the attic temperature to the drywall temperature.

  10. iLikeDirt | | #10

    I think you misunderstood me, Martin. it's not the insulation that radiates heat downward (we both know that insulation resists all three form of heat transfer through it), it's the drywall under it that radiates heat onto your head. Insulation does not actually stop or reduce heat load, it only slows it down, in proportion to its thickness and R-value. Let's ask: why is there a temperature gradient through the insulation? Only because of the air conditioner cooling down the drywall below it. But drywall has a low heat capacity, so it will not stay at 75f for long. While the attic is hot, the heat continuously passing down through the insulation--represented by the temperature gradient through it--will quickly heat the drywall back up to 76f, 77f, 78f, etc, until the air temperature of the room rises to the point where it kicks the AC back on and some of the cooled air does heat exchange with the drywall in the ceiling, cooling it down again.

    Lowering the attic temperature will lower the delta T through the insulation and slow heat transfer. Surely it makes sense that a delta-T of 75 (150f attic, 75f interior) will drive heat faster than a delta-T of 40 (115f attic, 75f interior). With high insulation, halving the BTU heat gain through the ceiling may seem trivial, but the felt effects are much greater due to the diminished heat gain in the drywall itself, which is what is radiating heat down into the room. Less heat gain in the drywall means less radiant heat emitted into the room, and at a lower rate. This is important because your head is one of the most sensitive parts of your body to heat, just like how warm feet have a disproportionately positive effect on your comfort in cold temperatures. Reducing the heat gain of the ceiling drywall is critical for comfort, and every little bit helps. This phenomenon may not be apparent in Vermont where you don't need AC at all unless your house was terribly designed and built, but I assure you it is easily felt in just about every building here in the southwest.

  11. GBA Editor
    Martin Holladay | | #11

    Utter nonsense. If the air conditioner is set to 75 degrees, then the indoor air temperature will be 75 degrees, and so will the ceiling drywall. If the air conditioner can't maintain the indoor air temperature at 75 degrees, then the air conditioner is undersized.

    Assuming that the air conditioner has been sized properly, and the indoor air temperature is at 75 degrees -- or wherever the homeowner likes to set the thermostat -- then the ceiling drywall, as I said, will be at the room temperature.

    If your attic is warmer than 75 degrees, then what you want is a thick layer of insulation to separate your cool drywall from your hot attic. The thicker the insulation, the better.

    Of course it is true that insulation doesn't stop heat flow -- all it does is slow it down. That's a fact of physics. The fact is true in winter as well as summer. But it's much more comfortable to live in an insulated house than an uninsulated house, because it's pretty easy to install enough insulation to slow down the flow of heat so much that the house is considerably more comfortable than outdoors, and energy bills are relatively low.

  12. tedtulare | | #12

    A little over my head on the tech talk but what do you think about me striping my roof with vertical 2x4's over the old rafters laying flat and installing new OSB with the 3/4" foam attached (foam down), putting a bug screen at the bottom and installing new reflective comp and re-installing the ridge vent over the top of all of it?

  13. GBA Editor
    Martin Holladay | | #13

    Adding rigid foam above your roof sheathing will make your attic a little cooler. But you still haven't told us why you want to lower the temperature of your attic. Do you have ducts in your attic? If so, are the ducts insulated? Is there an air handler in your attic?

  14. iLikeDirt | | #14

    I disagree, Martin. Like I said, the ceiling drywall (and the wall drywall, and the rest of the materials within the insulated envelope) will only be 75f because of heat exchange with cooled air from the air conditioner. Once the air conditioner turns off and the average air temperature equalizes, having absorbed heat from those materials, they are are going to heat up again from heat gain through the building envelope. The thermostat doesn't sense radiant heat, it senses air temperature. Radiant heat emitted from solid objects that have gained heat is a big sources of felt discomfort. It is quite possible to feel uncomfortable due to radiant heat from objects at 79f when the air temperature is 75f and the humidity is 30%. I respectfully disagree that this is utter nonsense, as I feel this very phenomenon every day during the scorchingly hot summer where my attic gets to be 150f, as perceived by my body ("it feels hot!") and verified with my infrared camera ("The ceiling is hotter than the air temperature!").

    Bringing up an uninsulated house is a strawman argument: I as well as everyone here will fully agree that insulation makes a house more comfortable. But ending the conversation there leaves out substantial nuance and depth, which IMHO happens to be extremely important in the very hot and sunny climates that Ted and I live in. The intensity of the sun and heat here means that to get to the point of comfort with insulation and air conditioning alone (i.e. ignoring entirely the subtle and not-so-subtle effects of mass, radiant heat, and solar gain) requires that you get to like R-70 or more.

    Ted, maybe it would be good to clarify why you want to do this. Do you feel that there is too much heat gain from the ceiling? What is your reason for wanting to lower the temperature in your attic?

  15. tedtulare | | #15

    I want (need) to lower the temperature of the attic. I am needing to re-roof the house and feel the only possible option is to reduce the heat transferring through the roof.

  16. iLikeDirt | | #16

    But WHY do you want to lower the temperature in the attic? Is your house uncomfortable when it's hot and sunny?

  17. GBA Editor
    Martin Holladay | | #17

    If your ceiling is warmer than your indoor air temperature, and is radiating heat downward and making you uncomfortable, there is an excellent chance that (a) your attic is hot, and (b) your insulation layer is too thin.

    Of course, if you don't have an air conditioner, the temperature indoors will be just as hot as the temperature outdoors -- sometimes hotter. (There are exceptions, of course. If it cools off at night, skillful use of a whole-house fan can lower indoor temperatures.)

    Anyway, you really don't want a hot ceiling. The way to keep your ceiling cool is (a) to run your air conditioner, and (b) install a thick layer of insulation on your attic floor to limit the flow of heat from your hot attic to your cool ceiling.

  18. exeric | | #18

    I guess I have to bring up the obvious. If one wants to keep the attic cooler than it now is (for whatever reason) and one doesn't live in the attic the answer is obvious: Staple a perforated radiant barrier up there. It will seriously lower the temperature, though not near enough to live up there. Ted would have to put up some 1x's that span across the rafters first to do that since it sounds like there's sheet rock up there now. Radiant barriers need an air space. One certainly wouldn't need that many of those stringers and one could allow significant drooping between strings for it to be very effective.

    There seems to be a logical contradiction in Ted's question. He says there is "livable space" in the attic. Which suggests the intention to insulate the roof to "livable" standards. Yet he is talking about a very small change in the insulation value directly on the roof that obviously wouldn't attain a livable environment up there. A radiant barrier isn't going to provide a livable standard up there. But if he is serious about cooling the attic without providing that standard then a radiant barrier is the answer. (And dirt cheap too!)

  19. tedtulare | | #19

    My attic (while not living up there) is a usable storage area and is fully enclosed wall and ceiling but gets really hot in the summer. I do spend some time up their with hobby stuff. I have 2x4 walls about 5' high with R11 bats and 1/4" foam core insulation and 1/2" sheet rock over that. The ceiling has 2x6 rafters with R11 bats, 1/4" foamcore and 1/2" sheet rock on the finish side There is a small air gap that allows hot air between the sheet rock and the bottom of the roof sheeting and a ridge vent at the top but, despite all of this it still gets n=hotter than we would like it to be. There is no AC in the attic but only fans. Really need to stop the heat from the roof from penetrating through the existing roof sheeting as mush as practically possible. The problem is not in the house as much as the attic.

  20. iLikeDirt | | #20

    My attic has R-50 mineral wool, cellulose, and fiberglass (a mix of the three in different layers), with a total depth of about 15 inches. It has red shingles. Due to the low pitch of my roof and the fact that it was not framed with raised-heel trusses (because they didn't exist in back then, I think), if I were to add more, it would still be only R-50 or so around the eaves. The attic temperature during the middle of the day climbs to 150f.

    I suppose I come at this from the perspective of trying to minimize the usage of mechanical equipment. In my mind, a house--especially a high-performance house with great insulation, verified airtightness, good control layers, etc--that relies on an air conditioner to be comfortable in the summer is a failure. We should be designing and retrofitting houses for maximized passive comfort, which has the same effect of lowering utility bills by reducing equipment runtime than super insulation does. A house that is comfortable without AC most of the time is a house that may even have lower utility bills than a house with such thick insulation that it requires AC all the time but it only has to run for an hour or two a day.

    Perhaps some background may help explain my perspective: Over the past few years I've tried my best to implement as much of what I've read here in my own house as I can: I've increased the airtightness (17 ACH50 -> 6.8 ACH50 so far), I've replaced the windows with Low-U-value Low-E ones, I've added 1.5" of rigid foam around the entire wall section of the building envelope, and I've tripled the attic insulation from about R-15 to R-50. For my efforts, I have not gotten much if any increased comfort, and my utility bills barely budged during the summer because I use a swamp cooler, and are lowered only a frustratingly small amount during the winter because I don't use much heat anyway. And, most disappointingly, the strategy of nighttime cooling has severely diminished in effectiveness because the house loses much less heat at night, so to make it work at all, I need to open all the windows and run my whole-house fan, which represents an increase in energy usage, since I didn't have to do this before in order for the house to cool down adequately at night, and it's a minor security risk. I feel like I put a sweater on my house and in the summer, it's made it hotter, not cooler, and I've been left thinking I was sold a bag of goods that didn't really apply to my climate, at high cost to myself, and I've been trying to figure out what I did wrong and how I can plan to do a better job when I build my next house.

  21. GBA Editor
    Martin Holladay | | #21

    If you occasionally hang out in your attic, it makes sense to insulate your roof slope. Ideally, you would meet minimum code requirements when you install this insulation -- but if you can't afford R-38 insulation, whatever you can afford is better than nothing.

    Reflective roofing or white roofing is better than dark-colored roofing. A radiant barrier is a good idea if you can install one that faces an air space. A ventilation channel can't hurt. Of all of these strategies, good old-fashioned insulation (the stuff with an R-value) is usually the cheapest approach, however.

  22. exeric | | #22

    "Of all of these strategies, good old-fashioned insulation (the stuff with an R-value) is usually the cheapest approach, however."

    Martin, I seldom disagree with you but I do here. I think you're seriously showing your Vermont roots here. Not that that is a bad thing. For what Ted is trying to do, whether one agrees with it or not, good old fashioned insulation is definitely not the cheapest approach.

  23. GBA Editor
    Martin Holladay | | #23

    I listed the options. Of course, choosing a light-colored roof is generally possible at no upcharge, so that approach rises to the top of the list. Radiant barriers can be cost-effective when installed by a homeowner, but almost never make sense when the installation has to be paid for.

    Everything costs money, including insulation. But you generally get more bang for your buck with insulation than a radiant barrier.

  24. exeric | | #24

    Martin, you make a very good point but when one applies it one has to be consistent with previous arguments one makes when applying those axioms. For instance I could argue, though I don't, that a ground source heat pump gives you more bang for your buck because it has the highest theoretical COP. But we both know it's overkill. Overkill is not bang for the buck. I could even argue that in Nate's case where the insulation seems to keep the heat "in" at night could be a case in point.

    I would argue that in retrospect, and of course after the fact, probably the first incremental change to keep heat from getting into house in the first place is some kind of radiant barrier or light colored roofing. After that one would work step by step to see what level of insulation is required to keep this lesser level of heat infiltration managed. Especially if at night you live in a local such as California or the southwest where it is cool and non-humid. This kind of thinking all applies to an arid cooling climate which is just different from the heating climate you live in. Insulation may not be the biggest bang for the buck here. But I'm certainly a believer in insulation despite that and am in the process of putting in insulation in the wall and above ceiling. But that is "after" doing the preliminaries of trying to keep heat from getting in in summer with radiant barrier techniques.

  25. GBA Editor
    Martin Holladay | | #25

    Even in hot dry climates (for example, Nevada), radiant barriers don't make economic sense. You can install one for other reasons if you want -- for example, to make puttering in your attic more comfortable -- but retrofitting a radiant barrier costs a lot and yields little energy savings.

    Here's what I have written on the subject:

    In 2001, researchers from the National Association of Home Builders (NAHB) Research Center evaluated several energy-retrofit measures in a 1,270-square-foot ranch house in Henderson, Nevada. The researchers focused on measures appropriate for hot climates.

    At a cost of $650, the researchers installed a radiant barrier on the underside of the attic rafters. The energy savings attributable to the radiant barrier were calculated at $11 per year, meaning that the simple payback period for the radiant barrier was 59 years.

    After I wrote a report on the research for Energy Design Update, several radiant-barrier dealers wrote letters complaining that the numbers couldn’t be right. However, Danny Parker, a senior researcher at the Florida Solar Energy Center, came to my defense. Comparing the NAHB researchers’ findings with the results of his own Florida research, Parker wrote, “I looked over the NAHB report and think it’s pretty good. … In the section on economics in our detailed comparison with ceiling insulation, we show a radiant barrier system to be more expensive than added ceiling insulation. And our retrofit experience with radiant barriers shows them to be very expensive to put in (high labor costs).”

  26. exeric | | #26

    Well, I put the radiant barrier up in my own house. It wasn't hard, though it was tedious. Like I say, at least in my case it was dirt cheap and I also do not believe those results. I've generally found you to be very fair and you know I've come to your defense often when you've been attacked unfairly. But on this particular issue I just think you have a bias out of proportion to the facts. I have never said or implied that a radiant barrier was a panacea for a lack of insulation. You know that.

    Perhaps the fact that there are people who "have" conflated those things makes you see red whenever radiant barriers are brought up. I'm on your side but am just trying to correct a bias you seem to have that perhaps was brought on by people in the past who oversold radiant barriers. I'm not them.

  27. GBA Editor
    Martin Holladay | | #27

    Radiant barriers work, as I have often stated. The best application is probably radiant-barrier roof sheathing in hot climates, especially if the house has ducts in the attic. (Even better: don't put ducts in an attic.) In new construction, radiant barrier roof sheathing in hot climates is a cost-effective measure.

    Radiant barrier retrofits in existing attics are also effective -- they just aren't cost-effective. Other measures make more sense. But if you have the time to install a radiant barrier in your attic, the measure definitely works.

  28. Expert Member
    Dana Dorsett | | #28

    The amusing collection of armchair theories aside, under CA Title 24 replacement roofing has to meet a specific solar reflective index (SRI) for the particular roof slope and CA climate zone. It's mandatory for roofs of 2:12 or lower. If this is a DIY and you're not pulling a permit you won't have to face down the inspector on this, but it's worth knowing the regs.

    If the owner wants to use lower SRI roofing than code they are required to install a radiant barrier under the roof deck, or install sufficient bulk insulation to compensate (which requires an engineer to sign off on it.)

    Whether you believe in science or measuring stuff or not, the folks at Oak Ridge National Labs who actually measure & model energy use (and vet their models with in-situ measurements) have a handy primer on radiant barriers:

    If you look at page 6, as long as you're at IRC 2009 levels of attic insulation and don't have ducts in the attic, the energy use dollar savings of adding a radiant barrier struggles to hit $20 in most locations. (That's annual energy use, not peak load mitigation.)

    But using a higher SRI shingle has comparable or better results, and has no cost-adder if done only when the roofing needs to be replaced.

    But attic-ka-shmatic- Ted's problem is that it's living space with a cathedralized ceiling, not an empty attic, with effectively NO insulation:

    "I have 2x6 rafters with 1/4" foam core below (under the sheet rock)as not to interfere with the ridge venting as I have a livable attic space the entire length of the house."

    Sub code but way better than what you have:

    In relatively dry US climate zone 3B climate, dense-packing the ceilings with cellulose would be fairly safe (bringing the whole-assembly R-value up from about R2 to about R15). The hygric buffering of the cellulose should be protective enough from winter moisture drives. But it would be absolutely safe if you stripped the old felt & shingles, replacing the old felt with a layer of #15 felt, then established a vent space above formed by 2x furring through screwed to the rafters, with a new nailer deck above that for the new #30 felt & shingles. The insulation blowing holes can be drilled through the structural roof deck then plugged prior to the felt, avoiding cosmetically marring of the finished interior.

    What's still WAY sub-code, but way better than what you have. If that's the way you go it's worth buying the roof decking that comes with the aluminized back side for the nailer deck, as well as installing a high SRI shingle for a hint more performance. The #15 felt on the structural roof facing the vented cavity provides a sufficient drying path for winter moisture drives in that climate, and the aluminized decking & high SRI shingle provides relief from peak solar gains.

    Meeting code:

    To meet IRC 2012 code minimums for zone 3B (R38 min, or U0.030 max) you'd need to blow insulation into the rafter bays (1.8lb fiberglass would deliver ~R22 center-cavity, 3lb cellulose would deliver ~ R20) and add 3" (R18) of rigid polyiso or 4" of EPS (~R16) above the roof deck. If you went with 1.8lb fiberglass rather than cellulose you can probably get away with 2" of polyiso or 3" of EPS (~R12), which is well-shy of R38 center cavity, but unless you have the highest framing fraction roof in CA it would come pretty close to making it on a U-factor basis.

    The new nailer deck holding down the foam must be mechanically fastened to the rafters. With 4" foam and a 1/2" nailer deck tight to the foam (seams staggered with those of the foam) and the original 3/4" structural deck you'll need 6" pancake head timber screws 24" o.c. to get sufficient bite into the rafters, which is still DIY-able, but toward the awkard side. It's a lot easier with 2-3" foam 4-5" screws.

    For IRC code minimums reference:

  29. BobHr | | #29


    8 or so years ago I did some research on insulation and radiant barriers. The best information was a study by the Florida Solar Engery Center. To sum it up the homes with the least amount of insulation, below and R 10, got benefit from a radiant barrier. As the amount of insulation increased the benefit declined. It only took an R38 for the conclusion to be reached adding a radiant barrier would not have a measurable impact on energy saving and would not be cost effective.

    R38 gave you benefit 24/7 365. Beef up the R value and spend the extra money in a cost effective manner that is best for that particular home.

    You could build a shelter over the ac to shield it from the sun

  30. BobHr | | #30

    That last thing was a joke

  31. iLikeDirt | | #31

    Robert, I recently increased my attic insulation from about R-15 to R-50. So far it has made no perceptible positive difference during this cooling season. The only thing I have noticed at all is that nighttime cooling is now less effective, and I need to run the whole-house fan at night to cool the house down, whereas before I didn't need to do this. I was hoping to be able to wait longer in the season to turn on my swamp cooler (no AC), and turn it on later in the day and turn it off earlier, but so far my house feels like it heats up during a hot summer day as least nearly as fast as it did before, and now I actually need to run it longer into the night because the house cools down so much more slowly after the sun goes down. On mild days (high of 90f, some clouds), I can usually wait until noon or so to turn it on, but on really hot ones (high of 107f, blue sky) I more or less need to keep it on 24/7, even at night, because otherwise the house never cools down on its own the way it used to. I have not needed to do this in the past. I'm very disappointed; the money I spent on the increased attic insulation feels like it was not only totally wasted, but that I actually paid to make my house less comfortable and raise my use of mechanical cooling equipment.

    The work was done in the spring so I haven't gone through a heating season with it, and I'm hoping it proves effective at that point.

  32. iLikeDirt | | #32

    I apologize for hijacking this thread. I do think the subject is important, but perhaps a GBA editor should move many of the posts in this thread into a new one so we can concentrate on helping Ted here.

  33. GBA Editor
    Martin Holladay | | #33

    Where do you live? Do you need space heating during the winter? If so, I imagine that your insulation has lowered your heating bills.

  34. iLikeDirt | | #34

    The Albuquerque area. It snows in the winter. I hope the extra attic insulation will make a difference, but my worst fear is that it won't, because I will have cut my house off from most of the free solar heat entering through the previously poorly-insulated attic throughout the day. I recently added a parameter for solar heat gain to the building envelope itself in my DIY manual-J-ish spreadsheet (using air temperature alone for heat gain and loss calculations is inadequate in an intensely sunny climate) and found, alarmingly, that the attic insulation work will theoretically increase the winter heating load by a small amount. It's quite possible that my calculations are incorrect, and I hope they are, or else I will have spent nearly $3k to make my house less comfortable and more dependent in mechanical conditioning in both summer and winter--the exact thing I was trying to avoid.

  35. GBA Editor
    Martin Holladay | | #35

    I hope that you end up satisfied with the performance of your house this winter. If your hunch is correct, and your insulation improvements end up raising your energy bills, summer and winter, then your house will be unique, and will defy the laws of physics. Your house will certainly be worthy of further study if it proves to be unique.

  36. Dana1 | | #36

    If Nate's place is overheating during the day and had previously relied upon a lossy attic to do "free" cooling at night (the way he expects to get "free" heating during the day in winter) it's likely that the window gains are on the high side. That may hurt during the cooling season, but as long as there is at least one low-E coating it will be a net-gain in winter too.

    Using the roofing (and low-R attic) as a unglazed solar collector is never a recipe for lower annual utility costs.

  37. GBA Editor
    Martin Holladay | | #37

    You're right about the possibility of high solar gains through windows. Another possible factor is high internal gains from people, dogs, televisions, computers, refrigerators, and lighting.

    During the summer, it's a good idea to reduce the number of people and dogs indoors, and to reduce the use of televisions, computers, refrigerators, and lighting.

    During the winter, on the other hand, it's a good idea to maximize the number of people and dogs indoors. Using televisions, computers, refrigerators, and lighting during the winter raises your electric bill but helps keep your warm.

  38. iLikeDirt | | #38

    More info: the house is north-south oriented, with one particularly large west-facing window. All windows have been replaced with vinyl double pane units that are U-0.26, SHGC 0.2. High internal gains are unlikely; there are two adults, one small child, and one medium sized dog. No television, all LED lights that are all off during the day, and an Energy Star refrigerator rated to use only 383 kWh per year. There are no desktop computers, only two low-power laptops, of which only one is usually in use. No real electronic gadgetry to speak of. Our monthly kWh usage ranges from 110 to 190. The gas range is probably the biggest source of internal heat gain; the oven is very poorly insulated and when it's on, waves of heat come off it. I need to do something about that.

    I will mention that I've already seen a small positive gain in my gas bill (around $25/mo savings) during this past winter from the new windows and windows and wall insulation work (done last November and December, respectively). But with both the wall and attic insulation work done (done late this March), this summer has been less comfortable and my electricity bill has risen overall, especially during the hottest months. Here are my monthly kWh usage figures from this year (post extra insulation) vs the same month in the prior year:

    April: -2 kWh
    May: -8 kWh
    June: +46 kWh
    July: +32 kWh

  39. Expert Member
    Dana Dorsett | | #39

    A monthly bill from year to year is completely meaningless. Look at the exact meter reading dates in the comparative months, and then the cooling degree days it covered using a nearby weather station on

    A variance of less than 50kwh could easily be the exact number of hours covered by a billing period, changes in occupancy, or a newly developed propensity for binge-watching Korean soap operas on Netlflix on weekends on the laptop.

    Take your worst month of perceived power use, June up 46 kwh for the month over the prior year. Assuming a billing period of 30 days/720 hours, that's an average increased load of 46,000wh/720 hrs= 64 watts. Another way to look at it is it's about 1.53 kwh per day.

    My family of 3 sees year on year variances on comparable billing periods that big or bigger (in both directions) even without changes to the building envelope, and really big ones related to having house guests or taking vacation time.

  40. iLikeDirt | | #40

    According to degreedays, last summer was actually hotter; between 10 and 100 CDDs more vs this summer.

    What method would you recommend? I can tell you my feelings (feels worse) and my utility bills (using more electricity during a slightly cooler summer); what else should I use?

  41. tedtulare | | #41

    What do you think of running a 2x4 flat on the perimeter and putting 1 1/2" rigid foam and sheeting over it? My theory is to stop the heat from entering into the attic space. ff I do this, with the existing R30 in the ceiling, will
    this get me up top code if I include the reflective comp shingles? FYI, my roof is 6" in 12" pitch.

  42. GBA Editor
    Martin Holladay | | #42

    Building codes do not allow you to put half of your R-value at the roof line, and half of your R-value on the attic floor, and to add the R-values together.

    The code wants you to choose: either insulate at the roof line, or insulate on the attic floor.

    That said, your approach will probably achieve your goals. To get the full benefit of the new rigid foam, you need to seal up any soffit and ridge vents, and to install insulation in the gable walls of your attic (if there are any gable walls in your attic).

  43. tedtulare | | #43

    Kind of in a quandary then. No real way to beef up the insulation in the attic or, vaulted ceiling. The only practical way I can see to add insulation is on the roof and with the reflective shingles. I am sure adding the foam and reflective shingles will do what I want it to do but, you are saying it will not meet code that way (if I understand correctly). I need a new roof so, I guess I have only one option (do it without a permit)?

  44. GBA Editor
    Martin Holladay | | #44

    In many areas of the country, routine repairs like re-roofing don't require the homeowner to bring the house up to code compliance. Talk to your local code official.

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