Semi-Flat Roof Insulation
September 11, 2010 7:11 AM Subscribe
My new home is surprisingly hot and energy-intensive in the summer months. It's clad inEIFS (the false stucco that is supposed to be a good insulator) but the problem is the roof. How can I insulate my mostly flat roof which appears to be too thin to allow "blow-in" insulation?
I've been wondering about the effects of adding solar panels: if the roof itself had a superstructure of panels on it, would that reduce the amount of direct sunlight radiating heat into my bedroom below? Plus, tasty power to reduce my energy costs.
That said... I'm open to anything.
I've been wondering about the effects of adding solar panels: if the roof itself had a superstructure of panels on it, would that reduce the amount of direct sunlight radiating heat into my bedroom below? Plus, tasty power to reduce my energy costs.
That said... I'm open to anything.
Is it possible to paint it white? That will reflect a lot of the incoming energy back up to space.
The same amount of energy will hit your roof with solar panels on it , but ~20% of that energy will be turned in electricity you can use.
You could look into a living roof. Are you in an area that gets much rain?
posted by Quack at 7:25 AM on September 11, 2010
The same amount of energy will hit your roof with solar panels on it , but ~20% of that energy will be turned in electricity you can use.
You could look into a living roof. Are you in an area that gets much rain?
posted by Quack at 7:25 AM on September 11, 2010
Response by poster: The roof is painted some sort of reflective silver. I live in DC.
posted by anotherpanacea at 7:29 AM on September 11, 2010
posted by anotherpanacea at 7:29 AM on September 11, 2010
Response by poster: The same amount of energy will hit your roof with solar panels on it
Won't it hit the solar panels, instead? I mean, if the solar panels are on racks on the roof, then the solar panels will get hot, produce electricity, and then radiate the rest of that heat back into the air above my roof, rather than into the roof itself. Or at least that's how I was imagining it. Am I wrong?
posted by anotherpanacea at 7:35 AM on September 11, 2010
Won't it hit the solar panels, instead? I mean, if the solar panels are on racks on the roof, then the solar panels will get hot, produce electricity, and then radiate the rest of that heat back into the air above my roof, rather than into the roof itself. Or at least that's how I was imagining it. Am I wrong?
posted by anotherpanacea at 7:35 AM on September 11, 2010
I'm not sure what you mean by too-thin: a flat roof is going to have fairly deep rafters. If the roof really is too thin to blow in insulation, the solution would be to redo the insulation with polyurethane foam, which isn't cheap.
Some cheap things you can do are to install a layer of reflective bubble-wrap (available at any Home Depot) spanning the rafters. This you can easily do yourself, just staple it up and tape the seams. Also, you can install a vent for the "attic" space and try to exhaust some of the hot air.
posted by ennui.bz at 8:48 AM on September 11, 2010
Some cheap things you can do are to install a layer of reflective bubble-wrap (available at any Home Depot) spanning the rafters. This you can easily do yourself, just staple it up and tape the seams. Also, you can install a vent for the "attic" space and try to exhaust some of the hot air.
posted by ennui.bz at 8:48 AM on September 11, 2010
Do you have any real access to the underside of your roof? I know a couple of people who have put up radiant barrier foil in the past year and are very impressed with the results. We may tackle it ourselves once it cools off enough here to work in the attic.
posted by Lyn Never at 8:50 AM on September 11, 2010
posted by Lyn Never at 8:50 AM on September 11, 2010
Also, if you are going to lay down the cash for solar panels, actually insulating the roof up to R40 is a much better investment and better for the environment too. If you have AC you will pay off the investment much faster than solar power and massively reduce the amount of coal burnt to cool your home (and heat it if you have electric heat.)
1 inch of polyurethane foam has about an R10 insulation rating. So if you have four inches under your roof, you can get it up to R40.
posted by ennui.bz at 8:52 AM on September 11, 2010
1 inch of polyurethane foam has about an R10 insulation rating. So if you have four inches under your roof, you can get it up to R40.
posted by ennui.bz at 8:52 AM on September 11, 2010
What about getting a home energy audit? There are companies in your area that do that and can tell you where your energy problems are (rather than assuming, though you may be right on the money) and how best to fix them.
posted by cecic at 9:02 AM on September 11, 2010
posted by cecic at 9:02 AM on September 11, 2010
Best answer: You need insulation, but you also likely need air movement between the insulation on the upper side of the ceiling and the lower side of the roof. Too much heat build up there will make your house energy intensive and also impact the life of the roof itself. Roof rafters are probably going to be 2x8, so you may be able to get fiberglass batts in there with some creative maneuvering, but you also probably need to get a fan up there to keep air circulating up there and prevent heat/moisture build up. They are typically whole house fans/etc, or maybe you just need some ventilation in the eaves. Talk to some roofers.
The insulation is really important though, you may have to go foam if you don't have a lot of clear space. If you're having this much heat transfer in the summer you're going to have a ton of heat loss in the winter which will be pretty expensive long term.
posted by iamabot at 9:19 AM on September 11, 2010
The insulation is really important though, you may have to go foam if you don't have a lot of clear space. If you're having this much heat transfer in the summer you're going to have a ton of heat loss in the winter which will be pretty expensive long term.
posted by iamabot at 9:19 AM on September 11, 2010
Response by poster: Two loose fill contractors who use the "drlll and blow" method have said they'd only be able to supply an R-value in the 20s and perhaps even the 10s, which is seems perssimistic but is roughly consistent with EPA estimates.
I just clambered out to redo the measurements, and the roof is 8 inches deep. Four inches of that is the rafters for the top layer, and four inches are the rafters to which the interior ceiling is affixed. To access the roof for foil or foam would require us to open up the interior ceiling of our bedrooms completely, in which case we'd lose the use of the top floor of our house during renovations. We're loath to do that in the short term because it's a newly renovated house. But perhaps that is the best longterm solution?
posted by anotherpanacea at 9:27 AM on September 11, 2010
I just clambered out to redo the measurements, and the roof is 8 inches deep. Four inches of that is the rafters for the top layer, and four inches are the rafters to which the interior ceiling is affixed. To access the roof for foil or foam would require us to open up the interior ceiling of our bedrooms completely, in which case we'd lose the use of the top floor of our house during renovations. We're loath to do that in the short term because it's a newly renovated house. But perhaps that is the best longterm solution?
posted by anotherpanacea at 9:27 AM on September 11, 2010
Four inches of that is the rafters for the top layer, and four inches are the rafters to which the interior ceiling is affixed.
Could you clarify this?
posted by jon1270 at 10:01 AM on September 11, 2010
Could you clarify this?
posted by jon1270 at 10:01 AM on September 11, 2010
Response by poster: Sorry, I may be using the wrong terminology. On a low-slope roof, I guess they're called joists?
Four inches of that is the rafters for the top layer,
The joists to which the roof is affixed stick out of the back of the house to hold the roof overhang back there, and the visible part of it is 4" from top to bottom.
four inches are the rafters to which the interior ceiling is affixed.
There's 4 inches between the drywall layer you see inside the house and the start of those joists.
I'm measuring from a windowsill, and subtracting the inside measurement from the outside measurement. There's 8.25" left. Since I can see the the roof joist outside, I can see that they start 4" above where the inside ceiling is. The inside ceiling is also attached to something, so I'm guessing there's another layer of ceiling joists that parallels the roof joists.
However, I haven't yet punched a hole in the ceiling to see what's up there, so perhaps it's a single, 8" high joist, and for some reason only the top half of the joist sticks out to support the overhang?
posted by anotherpanacea at 10:36 AM on September 11, 2010
Four inches of that is the rafters for the top layer,
The joists to which the roof is affixed stick out of the back of the house to hold the roof overhang back there, and the visible part of it is 4" from top to bottom.
four inches are the rafters to which the interior ceiling is affixed.
There's 4 inches between the drywall layer you see inside the house and the start of those joists.
I'm measuring from a windowsill, and subtracting the inside measurement from the outside measurement. There's 8.25" left. Since I can see the the roof joist outside, I can see that they start 4" above where the inside ceiling is. The inside ceiling is also attached to something, so I'm guessing there's another layer of ceiling joists that parallels the roof joists.
However, I haven't yet punched a hole in the ceiling to see what's up there, so perhaps it's a single, 8" high joist, and for some reason only the top half of the joist sticks out to support the overhang?
posted by anotherpanacea at 10:36 AM on September 11, 2010
Best answer: Another option might be to insulate on top of your current roof surface. That could mean sheets of foam insulation with a membrane system on top of that, or maybe a built-up spray foam roof. You'll need to be careful about weight and snow loads, though, with any kind of addition to your roof.
posted by Forktine at 10:36 AM on September 11, 2010 [1 favorite]
posted by Forktine at 10:36 AM on September 11, 2010 [1 favorite]
Best answer: A fairly cheap and non intrusive solution would be to add trusses to your existing roof. Either replicating your existing flat roof (just making it deeper) or converting to a sloped roof. This would allow essentially any depth of (cheap, cellulose) insulation to be installed and it would increase the ventilation of your roof. It'll probably be cheaper than spray in foam and it'll definitely have a higher potential R-Value. (FYI Foam tops out around R7 per inch once it's aged and the structural members (IE: wood) reduce the R-value of the complete roof.
posted by Mitheral at 10:37 AM on September 11, 2010 [1 favorite]
posted by Mitheral at 10:37 AM on September 11, 2010 [1 favorite]
Best answer: Given that this summer was a serious temperature outlier, you may want to hold off on making expensive changes until you've had a more typical summer. I live in a recent-construction high-rise in VA, and my electricity usage was extremely high this summer compared to last due to it rarely getting cool enough at night to open the windows.
Also, when I've lived in a multi-story house, I've found that summer is a great time to sleep downstairs.
posted by backupjesus at 10:50 AM on September 11, 2010
Also, when I've lived in a multi-story house, I've found that summer is a great time to sleep downstairs.
posted by backupjesus at 10:50 AM on September 11, 2010
How high are your upstairs ceilings? If you don't mind losing a few inches you could add some layers of rigid styrofoam and then strap/redrywall over top of that. Wouldn't be too messy as you wouldn't necessarily have to remove your existing ceiling materials.
posted by davey_darling at 11:06 AM on September 11, 2010
posted by davey_darling at 11:06 AM on September 11, 2010
In heating climes you can't just lay foam sheets over the ceiling gyproc. That'll give you two vapour barriers (the existing barrier and a new one created by the foam). Vapour will condense within the foam in the winter damaging the gyproc and possibly giving mold a place to grow.
This technique can be done but the existing vapour barrier has to be removed (IE: take down the gyproc and plastic sheeting). And it is difficult to properly manage the intersection of the foam and the partition walls.
posted by Mitheral at 12:15 PM on September 11, 2010
This technique can be done but the existing vapour barrier has to be removed (IE: take down the gyproc and plastic sheeting). And it is difficult to properly manage the intersection of the foam and the partition walls.
posted by Mitheral at 12:15 PM on September 11, 2010
I presume the roof is long-length galvanised iron sheet?
If so, pulling it off may be both a feasible and economic way of accessing your roof space, allowing ready access to insulate. Depending on a few things, you can stage the work too (ie do a part, then come back and do some more).
IF you are a DIYer, working with a roof like that is no big deal. Mind you, working a two-story roof introduces a couple of considerations that don't really apply to a single story.
posted by GeeEmm at 5:55 PM on September 11, 2010
If so, pulling it off may be both a feasible and economic way of accessing your roof space, allowing ready access to insulate. Depending on a few things, you can stage the work too (ie do a part, then come back and do some more).
IF you are a DIYer, working with a roof like that is no big deal. Mind you, working a two-story roof introduces a couple of considerations that don't really apply to a single story.
posted by GeeEmm at 5:55 PM on September 11, 2010
I once had a house with a relatively flat tar/gravel roof. When it started leaking I had it redone with the spray foam that Forktine mentions. It is like the insulation in food coolers and is almost white, so it reflects sunlight too. It was great. My house was much cooler in summer (and warmer in winter).
It also makes for a long lasting roof, which was my original intention.
posted by eye of newt at 6:33 PM on September 11, 2010
It also makes for a long lasting roof, which was my original intention.
posted by eye of newt at 6:33 PM on September 11, 2010
Best answer: I grew up in a flat roofed Eichler which had literally no gap between the ceiling and the roof. The insulative properties bordered on non-existant and every winter we'd play whackamole with the inevitable series of leaks.
About four years ago my parents went with a polythurane foam on top of the existing roof and the difference has been dramatic on all fronts.
Here's a page describing what they had done.
posted by Tell Me No Lies at 11:00 AM on September 12, 2010
About four years ago my parents went with a polythurane foam on top of the existing roof and the difference has been dramatic on all fronts.
Here's a page describing what they had done.
posted by Tell Me No Lies at 11:00 AM on September 12, 2010
Response by poster: Thanks everybody. With the tax credit this year for energy-efficient renovations, there's a major incentive to deal with this sooner than later. I especially like Mitheral's idea adding trusses to build more roof on top of the current roof. Bonus: a taller house!
However, I find backupjesus's comments a persuasive caution: perhaps it's a bad strategy to immediately jump to conclusions about the longterm energy costs if this summer was a "temperature outlier."
Decisions, decisions: I guess this is what we signed up for when we bought a home in the first place!
Thanks again!
posted by anotherpanacea at 7:35 AM on September 14, 2010
However, I find backupjesus's comments a persuasive caution: perhaps it's a bad strategy to immediately jump to conclusions about the longterm energy costs if this summer was a "temperature outlier."
Decisions, decisions: I guess this is what we signed up for when we bought a home in the first place!
Thanks again!
posted by anotherpanacea at 7:35 AM on September 14, 2010
One last bit: GreenbuildingAdvisor.com is amazing. Just don't read every interesting article or you may miss your chance at the tax credit.
Also, check out the last few editions of Fine Homebuilding at your library, or get a subscription to their website (they're also affiliated with GBA). Their most recent edition has an article that might be of interest.
Also, I've always been sort of obsessed with the idea of a "cold roof" (perhaps this is not the term I'm looking for... er.. false roof?) - sort of a combination of the solar panel airspace and the trusses.
Basically - just a roof on top of the current roof, with a separation of a few inches for air to circulate. Here's a good pdf.
The most minimalistic designs don't give you added r-value in the cold, but I've always thought it was a killer idea for giving the finger to summer solar gain.
posted by BleachBypass at 4:14 PM on September 15, 2010
Also, check out the last few editions of Fine Homebuilding at your library, or get a subscription to their website (they're also affiliated with GBA). Their most recent edition has an article that might be of interest.
Also, I've always been sort of obsessed with the idea of a "cold roof" (perhaps this is not the term I'm looking for... er.. false roof?) - sort of a combination of the solar panel airspace and the trusses.
Basically - just a roof on top of the current roof, with a separation of a few inches for air to circulate. Here's a good pdf.
The most minimalistic designs don't give you added r-value in the cold, but I've always thought it was a killer idea for giving the finger to summer solar gain.
posted by BleachBypass at 4:14 PM on September 15, 2010
This thread is closed to new comments.
But none of these kinds of options are going to be cheap, and not all are going to be possible given the realities of how your house was built.
posted by Forktine at 7:21 AM on September 11, 2010