Is it okay to insulate against the underside of roof?
- Thread starter PabloCruise
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agomez
Frijoles
just fxxxing staple batt insulation to the inside of the rafters....
All this aside--do not insulate your roof! you will be replacing it(roof) in less than 5 years--no where for the heat to redistribute--the roof will be toast-esp if you have asphalt shingles-although tile or metal is not immune. Best to insulate the HOUSE against thermal transfer-and add exhaust fans to keep the attic ventilated-even if you have ridge vents.
Listen to 90FJ. Do Not insulate up against your roof. If you have asphalt shingles, they will melt/crack/deteriorate. If you have a metal roof, the heat build up will damage the exterior galvalume sealer.
You want the roof to have air circulation to keep it cool. That's why roofs are designed with ridge vents, etc., and no insulation against them.
The best insulation is air---dead air that is-that's why insulation is porous-traps air. (dead air). If you use this against the roof-the heat can't escape from the roof, and lo, you will be replacing your roof earlier than you thought.
If you insulate the roof, the heat can't get into the attic air space to be exhausted by whatever means you have(ridge vent/attic fan/soffet/etc. The result is the roof heats up to a point it can't handle, and fails.
Gable fans are a good option(if you have that type of roof)-set the temp at 110 Deg F
You want the roof to have air circulation to keep it cool. That's why roofs are designed with ridge vents, etc., and no insulation against them.
The best insulation is air---dead air that is-that's why insulation is porous-traps air. (dead air). If you use this against the roof-the heat can't escape from the roof, and lo, you will be replacing your roof earlier than you thought.
If you insulate the roof, the heat can't get into the attic air space to be exhausted by whatever means you have(ridge vent/attic fan/soffet/etc. The result is the roof heats up to a point it can't handle, and fails.
Gable fans are a good option(if you have that type of roof)-set the temp at 110 Deg F
That is far from true for my house. The key is the high density foam. I can be right under the roof on a very hot day and not be hot. As long as your shingles are not paper base which I think is mostly the cheap shingles nowadays, the roof under the shingles won't be any hotter than a non foamed roof.
You stop air movement, you stop humidity movement through the roof.
You stop air movement, you stop humidity movement through the roof.
I think my point is missed here. I'm not talking about how hot your attic gets-that's another question. I'm describing what will happen to your roof if you insulate directly under it. Keeping your attic cool at the expense of your roof is not a choice I would make.
gary
gary
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Heat can be an issue with dark roofing materials and hot climates but I'm more concerned about moisture, either from diffusion through the roof assembly or more commonly a roof leak.
2000, will you admit that roofs somtimes leak? can you tell me the fundamental differences between how a standard roof would deal with a bulk water intrusion versus how a roof directly insulated with spray foam would react?
When I get some time I'll run a WUFI model and show you what exactly the problem is.
2000, will you admit that roofs somtimes leak? can you tell me the fundamental differences between how a standard roof would deal with a bulk water intrusion versus how a roof directly insulated with spray foam would react?
When I get some time I'll run a WUFI model and show you what exactly the problem is.
Moisture does not permeate the high density foam. Because and because you don't get airflow, you don't get moisture build up on the underside.
Roof do leak. And the decking underneath can get wet. But it is less likely because the foam is structural. The roof system is more solid. Less flexing. So a longer lasting roof. And if your roofing material does fail and water leaks, it could rot the underlying wood. That is the big concern it seems. But so what? It would mean your shingles would fall off in a larger area than it already has. Now way easier to find. And the house underneath has not sustained any water damage. You replace the shingles, roof felt or water dam and small piece of plywood.
Now I have a paneled steel roof. It is on purlins so any water runs down the waterproof under roof.
And it is reflective so it doesn't get hot. Anyone concerned about their roof is not going to have a roof surface that doesn't reflect heat. Even in very cold climates. Best to have a mid to light colored roof and even deeper colors can have a reflective element.
So the big leak concern isn't the issue some fear. If you haven't noticed a huge pile of shingles missing, you haven't bother to replace thje shingles and they are 30 years old, you may complain that you have a little rot but that rot is way less damage that the historical raining ceilings that happen now.
Roof do leak. And the decking underneath can get wet. But it is less likely because the foam is structural. The roof system is more solid. Less flexing. So a longer lasting roof. And if your roofing material does fail and water leaks, it could rot the underlying wood. That is the big concern it seems. But so what? It would mean your shingles would fall off in a larger area than it already has. Now way easier to find. And the house underneath has not sustained any water damage. You replace the shingles, roof felt or water dam and small piece of plywood.
Now I have a paneled steel roof. It is on purlins so any water runs down the waterproof under roof.
And it is reflective so it doesn't get hot. Anyone concerned about their roof is not going to have a roof surface that doesn't reflect heat. Even in very cold climates. Best to have a mid to light colored roof and even deeper colors can have a reflective element.
So the big leak concern isn't the issue some fear. If you haven't noticed a huge pile of shingles missing, you haven't bother to replace thje shingles and they are 30 years old, you may complain that you have a little rot but that rot is way less damage that the historical raining ceilings that happen now.
Ok, obviously not going to dissuade you, I'm assuming you're not in the building industry. I personally don't really care what happens to your roof, and it will probably be fine but the law of averages mandates that some roofs will unnecessarily fail because some dude with a foam rig is out schlepping his wares on unsuspecting homeowners.
asa lama lakum.
asa lama lakum.
And I guess I won't impress on you the new way of doing things. I am not a roofer and I guess you aren't either. Not that being a roofer means you understand new technology. I have found pro roofers that put asphalt shingles on decks designed for wood and vice versa. Even when my roofer installed my panels, I had him change the method of purlin installation. He later found out that my way was the mfg new method that they adopted after I emailed them.
I did not do mine without a lot of research. Found a lot of experts who knew as well as many who don't.
I do believe that building tech that works is a locality thing. The construction industry fails to see that a home designed for LA works as well in Anchorage or Chicago.
My house had full ridge venting, 2' deep full width soffit venting. My 15 year shingles lasted 30 years and were still serviceable. So I had a good system, I just wanted great.
I did not do mine without a lot of research. Found a lot of experts who knew as well as many who don't.
I do believe that building tech that works is a locality thing. The construction industry fails to see that a home designed for LA works as well in Anchorage or Chicago.
My house had full ridge venting, 2' deep full width soffit venting. My 15 year shingles lasted 30 years and were still serviceable. So I had a good system, I just wanted great.
PabloCruise
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- #53
I appreciate the posts here.
I am going to take some pictures of my garage setup in order to more fully help me articulate my questions.
I am going to take some pictures of my garage setup in order to more fully help me articulate my questions.
Sorry, but this rule does not change. The drawing is a representation of the Second Law Of Thermodynamics-heat transfer. It is a theory that has been proven--that's why it is called a Law.--Same Law that governs how your air conditioning system works, your furnace, your clothes dryer, your refrigerator, and yes, your carb heat shield.--Would you pack insulation all around your exhaust manifold??--Of course not. You would be replacing the warped/cracked part inside of a year. Make believe your manifold is your roof-if the heat can't get out, it will increase to the level of the heat input(sun)
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PabloCruise
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- #55
So I think I found a nice solution here...
A plastic Rafter Vent that will preserve an air space on the interior side of the vaulted ceiling, and then batt insulation over the top of those.
Soffit vents under the eaves and a gap up in the interior for airflow, I think this should work.
A plastic Rafter Vent that will preserve an air space on the interior side of the vaulted ceiling, and then batt insulation over the top of those.
Soffit vents under the eaves and a gap up in the interior for airflow, I think this should work.
PabloCruise
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- #57
I'll be putting an exhaust fan in the eave of the south facing wall - perpendicular to the roof slope...
ventlation is so critical, it cause builders and tax payers billions in losses with the improper installation of building envelops in Vancouver BC Canada. So, yes, need to ventation. Home builders even have created a 1 inche space half way up the wall all around the house for ventalation.
