so I was wondering about the foam insulation. And how some people spray the underside of the roof deck. While there are some advantages to this wont you be heating a lot more area by heating your whole attic? Instead of heating just the house like normal with insulation on the attic floor. Now all of your heat will transfer through the 5/8 drywall and go up to fill your entire attic. Any thoughts on this. Seems real counterproductive to me like bending over a dollar to pick up a dime?
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Spraying the underside of a roof deck is a way to insulate when you've got a cathedral ceiling or difficulty venting properly. I know what you're saying, but I dont think its something people do just to say I sprayed foam. Theres scenarios where its a problem solver.
Real trucks dont have sparkplugs
Except for one instant of time it does not cost anything to heat extra space.
What cost is the amount of transfer out of that space into the exterior.
Now with the roof you have a little more area than the ceiling, due to the sloped roof. And you have the gable ends.
But the big difference is that you roof is seal. You will only have a couple of penatrations and they are enclosed by pipes and then sealed by foam. Not much air will leak out of them.
Compare that with the house to attic transistion. You have literally dozens, if not hundreds of holes where warm conditioned air will excapse in to the attic and then venting will take it out of the house.
That includes electrical wires through the top plate, electrical boxes for ceiling fixture, can lights, plumbing vents, and flue vents. And last, but not least usually an access panel or stairs of some type.
And this is without HVAC equipment in the attic.
At that and you not only have the leaks from all of the duct work, but also the leaks at the vent openings into the house. But also the ducts and equipment are in unconditioned space. There is just so much that you can do with sealing and insulating them.
Very very good points!
I second what Bill said. Volume is immaterial. Surface area is what counts. Add in the air sealing qualities, and the fact that any attic based HVAC stuff is now in the building envelope, and you've got a useful combination.
Andy
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In my area all HVAC stuff is in the basement but I see what you guys are saying about small penetrations in the ceiling, and cathedral space of course.
If you have an attic ( not a cathedral ceiling), my vote is to keep all lights & cans and other penetrations out of the ceiling and have the foam guy spray one inch of foam on top of the sheetrock ( from above). Then add appropriate fiberglass or cellulose above that to reach r 42 or whatever. You can then do a conventionally vented roof system.
just my penny and a half.......
you said too keep all lights and cans out of the ceiling ..... then what are you suggesting for lights? I must be misunderstanding you
wall sconces and switched lamps are often enough, even along stairways. If you must put one in the ceiling, you can build a little sheetrock box around it , seal it as best you can and if you're using foam above then have it sprayed.
As others have said it can solve some problems like venting or insulating (well) tight spaces and bringing HVAC stuff into the building envelope.
But it adds the problem of not being able to tell if your roof is leaking AS EASILY as you would with a conventional attic.
I see it as an exchange of one set of problems for another.
I think heating the added volume/surface area is a secondary concern.