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Typical 4-br type 80s-ish build suburban house in the south: downstairs ac is super-powerful; upstairs ac is fair. Hard/impossible to keep the upstairs below about 75 say in summer.

The problem is the attic gets INCREDIBLY hot.

I installed a https://www.amazon.com/dp/B01E0FU6IG which was a huge help. I put a few massive fans up in the attic to blow air out the large vent up there - again, it helps.

The attic does have loose insulation around the floor-beams. But the attic is just SO hot it doesn't help.

My question, if I have spray foam done on the underside of the roof, in fact ...

will that cool the attic?

The attic has a 30" circular opening to the outside (obviously with wire mesh etc), indeed my fans up there I point at that.

So it wouldn't be a "sealed attic" approach, but rather, the spray-on layer would (I hope!) stop heat off the under-roof from baking the attic.

The attic has a central area which is 5' height but then unfortunately two wings which are hands-and-knees access height. My guess is these two wings are the bad design that cause the insane heat in this attic.

Would spray foam reduce the heat in the attic?

(Yes, unfortunately the roof is dark colored!)

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  • 1
    Is there only the one opening/vent or do you have vents at/near the bottom of the attic also? Usually do not want insulation on the bottom of the roof deck, but better ventilation. More insulation on the attic floor is usually good, keeps the heat from the living space.
    – crip659
    May 12, 2023 at 11:50
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    Got pics of the attic, roof, and eaves (from below)?
    – Huesmann
    May 12, 2023 at 16:21
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    Not really an answer, but on the spray-foam thing, this blocks airflow to your roof wood, which causes the wood to rot very quickly. You'll need an entire new roof within a few years. And you'll be unable to sell the house, because every surveyor will recommend that buyers avoid it.
    – Graham
    May 14, 2023 at 8:47
  • "my fans up there I point at that" The wording of this seems to indicate that you have fans in the attic that are some distance away and are just pointed in the direction of your 30" opening. If you really want good draw and airflow, the fans should be mounted directly on your opening.
    – Glen Yates
    May 15, 2023 at 14:13
  • @GlenYates - indeed I have a "small" (say 20") literally sitting over the large opening; the other massive fan (I can hear it now, it's 90° here today, so it's "all fans all day") I have tried different positions, and as I mention (interestingly) it doesn't matter where Enormous Fan is. I can have E.F. blowing air in to either of the "dead" wings, or sucking "out from" either wing, today I'm trying it basically pointing "straight up" in the middle (so, one would guess), it's just blowing the hell out of hot air trapped Up There, ultimately some going out the out hole...) Fascinating, really!
    – Fattie
    May 15, 2023 at 18:10

10 Answers 10

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Spraying the roof deck would create an insulated box. It's difficult to predict how that would affect average interior temperature. It would certainly increase roofing temperatures, which isn't ideal. I can say that if it was an effective solution you'd see it more often. Stick with what's known to work.

I'll reiterate and overlap what's been said in other answers. You need two things:

  • Better ventilation. I'd install continuous ridge vent and add or improve soffit ventilation.
  • Better ceiling insulation. I'd blow in more cellulose over whatever is there now. It's easy, inexpensive, and reliable.

How you achieve that depends on the nuances of your situation, which you haven't thoroughly conveyed to us.

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    Ridge and roof vents are typically installed as part of a roof installation or replacement. I'm not surprised it is hard to find a roofer willing to add them later. May 14, 2023 at 2:11
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    You don't need a company for that. It's a very easy job, assuming you don't have a challenging roof slope. You just cut a channel with a circular or reciprocating saw and nail the vent and new ridge cap on.
    – isherwood
    May 15, 2023 at 12:49
  • isherwood is right, it's very easy to find someone to do it, thank God. (I appreciate that a handyman might do it himself; I prefer to use seasoned experts even though it costs a few bucks.) {For example, we just had some ghastly popcorn ceilings removed. Of course, anyone "can" do that ("damp, scrape") but the folks we had do it do nothing else year in and year out, so they were incredibly good and fast at it.}
    – Fattie
    May 16, 2023 at 15:08
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    That outlines the process well, but I would not use a pneumatic nailer for ridge vent. It's likely to either mash the vent or punch through the shingles. I always hand nail those.
    – isherwood
    May 16, 2023 at 15:11
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    @isherwood the difference is utterly amazing - almost unbelievable. I can now easily get the second floor some TEN degrees cooler than previously. in the attic itself the difference is staggering, it went from sauna-hot (perhaps 40? more? degrees warmer than outdoors) to a simliar temp. as outdoors, a little hotter only. it's madness. it took me YEARS to figure this out !!
    – Fattie
    Jul 9, 2023 at 21:52
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+50

Note that there has to be airflow along most of the underside of the roof, to let the wood dry and discourage rot. There are channels you can buy to hold that space open; you would then install any insulation over them.

(Or you could install sheet insulation over the face of the rafters, letting them create that airflow space, but then you have to install wallboard or similar facing over that to meet fire code; I'm not sure whether spray foam has similar requirements.)

Note however that the alternative, if you don't intend to use the attic as living/storage space, us just to let the roof get hot and focus on isolating the attic from the rest of the house.My attic is several feet deep in blow-in insulation; cheap and easy.

Note too that before doing this, the single most cost-effective way to improve efficiency is usually to air-sealing the house. Heating and air conditioning both work much better when there is less loss of conditioned air to outside. The ideal house might be perfectly sealed except for heat-recovery ventilation units, which allow airflow with much reduced heat flow.

(Side comment: The south-facing side of my roof benefits from being shaded behind the solar panels...)

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I would put a layer of batt insulation on the attic floor. An R-30 batt over the blown in stuff that you have. That would keep the heat from penetrating to the living area.

Then check that your soffit vents are open and free of debris. If you don't have any, it would help greatly to add some. The more the better.

Finally if your fans are just that (fans) discard them and have thermostatic controlled attic vent fans installed right at the gable vents. Those combined with soffit vents will pull a lot of air and keep the attic much cooler.

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    I agree--better ventilation and an improvement to ceiling insulation are the solution. I would just blow in more cellulose, though. It's much easier to get good, uniform, unbroken coverage than with batts.
    – isherwood
    May 12, 2023 at 12:50
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If you are going to spray foam, find a spray foam roofing company and have them spray foam the OUTside of the roof and finish with a white or silver coating. Other than that option, the other answers cover the usual issues - ventilation from low to high in the attic, so it self-cools, and more insulation (between attic and house) are key. An insulated unventilated deck under a dark roof will ensure that your dark roof is heat damaged in short order, so you'll be able to replace it with a light colored roof, but you can skip that part by insulating on top of it and changing color at the same time.

Another approach to dealing with ventilation in a poorly designed/built attic is to build a new, well-vented roof on top of the roof you have, with continuous soffit and ridge venting and plenty of space for air-flow between the top of the original roof and the underside of the new roof.

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Your problem is solar gain vs the mass of your building.

The sun is powerful. A dark, 1 square foot panel that is facing dead square on with the sun, will absorb 90 watts or about 300 BTU/hr from the sun's rays. From the sun's perspective, how big is your roof? I would bet 1000 square feet or more, so 300,000 BTU/hr coming at you. As you probably know, this leads to a dark roof that you can fry an egg on.

Obviously some of that is re-radiated, or cooled by ambient air. But much of it soaks into your home's structure, heating it up significantly - also heating the attic well into triple digit temperatures and thus, heating your ceiling and here's your complaint.

Now if you're wondering about reflectivity of lighter paint colors, I asked my paint supplier about the reflectivity of their 3 lightest paints. 82.6%, 80.7% and 80.4%. All of these are called white. Off-whites head into the 60s real fast, and almost any "normal, aesthetic" roofs is in the 10s or single digits. So very little solar gain is being reflected away. Once I painted the roof of a small shed white, the people who sit in the shed all day had no way to know I had done that. They commented "it is MUCH cooler in here!" Anyway, all that to say, if you for a white roof, no half measures. A little not white is a lot not white. Ask your coating supplier.

But that significant building mass is definitely a player. The morning sun is blasting your roof with the exact same 300 BTU/hr per sf as the afternoon sun. It only feels different because the building has thermal "inertia" - it cooled off overnight and are only starting to be warmed. If you're the frog in a cook pot, the first couple of minutes probably feel real nice. That's why we get this goofy "duck curve" in electricity generation - the solar panels work exactly when the sun is heating your roof, but the building's mass adds many hours of delay before you feel the need for A/C. I got smoked out of my house about 4 pm today, even though sunlight is from 6 AM to 8 PM. The building's mass carries the heat well past sundown - that's why at 9:30 after dark, your A/C is still working hard. We can't use solar for that, the sun has set. (One answer is to overcrank the A/C when cheap clean power is available.)

In conventional construction like yours, the mass of the building is your worst enemy.

Of course you can add layers of insulation between you and the building's mass, but that's a holding action - the building's mass is still your enemy.

The best efficient designs actually "flip the script" on that, by putting the building's mass inside the insulation envelope. Now instead of storing solar gain, the building's mass is helping resist temperature change. That's a shout-out for Ecnerwal's suggestion of foaming the outside of the roof.

That also puts the foam outside the living space envelope, which I like for a couple of reasons. First, mixing errors can make professionally sprayed foam very toxic, causing odor and illness that just won't stop, ever. People have had to have their entire roofs torn off and replaced to save their homes. Second, all DIY foam and some commercially applied foam is quite flammable... but worse, when exposed to flame, it emits extremely toxic smoke that renders people unable to escape. Outside the living space is better. I would use fiberglass or rockwool on interior rafters, which are non-flammable and non-toxic.

In the west we are very skittish about soffit venting, because it scoops up embers during wildfires, defeating all other fire defense. But if that's not an issue, moving as much air as possible through that attic will help. It will break the conductive "greenhouse effect" happening up there, and make the top of your ceiling no hotter than ambient air outside.

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  • I had a shed like yours - grey zinc galvanising so quite silvery, not really "dark coloured" But slapping some layers of cheap white house on the outside paint easily brought the temperature down by 5-6 degrees C. Definitey the best midsummer evening's work I ever did.
    – Criggie
    May 15, 2023 at 3:27
  • while an absolutely fascinating idea, it wouldn't be practical to put insulation outside the roof here. ON another note, a critical piece of information I learned here, is that, if you are going to have a "light color roof" to reject heat, it has to be blazing white to be effective, whitish does not cut it! Amazing info!!!
    – Fattie
    May 16, 2023 at 15:31
  • @Fattie Yeah exactly, with the whites. No half measures... If Google complains that you blinded their satellite, you're white enough :) A better paint vendor can quote you the reflectivity/albedo. I get where "staple fiberglass batt to the outside of the roof" sounds insane :) but we went with a company that spray-foamed our entire roof (rusting tin roof), and I don't recall if it was naturally white or they spray painted it afterwards. It was not perfectly smooth, however. May 16, 2023 at 21:08
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I did something similar in my loft. I didn't spray it, that's not as common in the UK, but instead stapled insulating superfoil (an alternating stack of foil and insulating material) to the rafters. It reduced the heat radiating from tiled roof into the loft significantly, and it certainly felt cooler up in the loft.

I also insulated the floor of the loft which seemed to make the top floor of the house (the floor immediately below the loft) warmer in Winter and cooler in Summer.

I left a big gap in the insulation at the eaves to allow fresh air to enter and exit the loft.

If you're trying to cool the top floor of you house. I'll mention the obvious fact that heat rises, so if you can direct that out of your house it won't accumulate on the top floor. We found that opening the loft hatch a bit (it's at the top of the stairs) gave the hot air somewhere to go. Closing the bedroom doors off of that landing and stopping heat entering the rooms where possible on the sunny side (blinds and curtains) also helped. We'd then open the blinds curtains and windows once the sun had move round to get rid of the bit of heat that had accumulated. Whether that would work for you depends on the inside / outside temps because any air that's lost has to be replaced with outside air.

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Put some solar panels on the roof? Maybe it can absorb some heat, convert it to electricity to run your ac?

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I have closed cell foam on the underside of my roof deck. The attic is finished space. Before the insulation was sprayed in, the attic would get very hot in sunny weather. Now it's actually a little cooler than the downstairs: most of the heat comes in through the big southward-facing picture window. So yes, adding insulation to the bottom of the roof deck would make the attic less hot.

Unvented roofs with the closed cell foam sprayed in between the rafters are very common around new england. It's not a death-sentence for your house if you have this done. Here's an article in Fine Homebuilding that covers it: Vented and Unvented Roofs.

That said, I would advise against it. It's not insta-death for your roof to have this done, like I said, but it's finicky. It has to be done right. The insulation can crack and peel away from the rafters forming voids. This may not apply to you if you're in the south but here that means warm humid air gets in in the winter and forms condensation. That will rot the roof and could be so bad it ruins your ceilings below.

I regret doing my insulation this way. Won't go into the reasons it happened. I would love to have an actual unfinished attic. It's just a much simpler, more robust system. Not to mention if you ever get a roof leak you won't be blind to it. Pile on the insulation on top of the attic floor, add some baffles by the soffits to keep that insulation from blocking that air flow. Make sure the soffit vents are sufficiently free and either add a ridge vent or just force it out the gable vents with a big fan.

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  • Great advice. A lot of people are saying, just add MUCH MORE blow in insulation (I have only like 6 four inches. One issue though, the other part of the upstairs AC is indeed in the attic (it's one of the crap split systems; the downstairs AC is a Real AC). I imagine they'd have to make sort of a barricade around it or such, IDK.
    – Fattie
    May 15, 2023 at 18:01
  • @Fattie Yes, they should be able to build a barrier around it to keep the insulation away from it. There's a part of a mini-split in the attic? Is it a ceiling cartridge? May 15, 2023 at 18:35
  • right - it's the god-damned split type of AC, not a real AC. :/ :/ :/
    – Fattie
    May 15, 2023 at 21:47
  • @Fattie Mini-splits can be effective, they're real AC. I'm asking because I'm wondering if your upstairs AC is dumping heat into your attic. It should not have been built that way but crazier things have happened... May 16, 2023 at 14:26
  • ah that makes great sense! (1) the ac for the upstairs is indeed a split type. outside the house on a concrete pad there's part1. up in the attic is part2. as I understand it, and can see, cold water (I assume) pipes run from part1 to part2. thats's the facts I know (2) as I understand it part2 sucks air in (from a big hole with a filter in the ceiling of the upstairs hall/landing and then pumps out hopefully colder air to the 6 or so ceiling vents which blow out (slightly!) cool air. (As it happens, Part2 was replaced a couple yrs ago as it broke)...
    – Fattie
    May 16, 2023 at 14:48
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I had a similar problem several years back. I added several additional soffit vents and then added an exhaust fan at the gable near as possible to the peak. My electric bill went down a noticeable amount and the home was cooler. I had it wired to a thermostat set at 80F to turn it on so it was automatic.

The fan was set up as an exhaust fan, drawing in from the soffit vents and out as close to the roof peak as possible. Going in the attic after this was operating there was a large noticeable temperature gradient at about 24" above the insulation (on the ceiling of the room below). The area close to the roof was much warmer. I turned the fan off and it got much hotter in a relative short time.

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  • Gil, just TBTC your powered vent near the gable was blowing OUT, correct ??
    – Fattie
    May 15, 2023 at 18:01
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Result:

summarizing the advice here, I had a couple guys come and slice the roof open:

enter image description here

Dead easy.

I'l report in a few weeks on whether it drops the temperature!!


The temperature drop is ALMOST UNBELIEVABLE.

I can't think of anything, at all, in any field, I have ever personally seen that has been so effective. It's mind-boggling.

On a given extremely hot day of a specific temperature, when it would be impossible to get our second story below 85 degrees, I can now get it to 73-74 easily.

Whereas during summer months (indeed until the DAY BEFORE the miracle ridge vent installation) the second story was almost never below about 80, I can now keep it at 71 or 72 as desired all the time.

The difference is almost unbelievable, flabbergasting.

The men in the pic installed a ridge vent on every possible part of the roof, all wings long and short of the structure. It may depend on your region etc but the difference is nothing short of astounding.

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