Reducing Trapped Heat by Bringing Attic Into Conditioned Space
GREGZ23
| Posted in General Questions on
I have an attic with blown in insulation and an HVAC unit in it. The roof line has a ridge vents, but the attic is really hot. Can i foam the roof line? Do and condition the space? Do i need to remove the blown in insulation?
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Replies
The usual way to deal with this is to bring the attic into the building envelope and convert it to conditioned space. This means you insulate the roof instead of the attic floor, and you air seal the underside of the roof instead of the attic floor. Once this is done, you can remove the blown in insulation, but you don't have to.
Exactly how you insulate the underside of the roof depends on the type of construction. Sometimes you can block off vent channels and use batts or dense pack cellulose, sometimes spray foam is the only option. It's hard to say what would work best without some more information about your attic construction. Do keep in mind that if you go with spray foam, closed cell is usually the safest option when used on the underside of a roof.
Bill
What do I do about the ridge vents and soffit vents?
Would it be better to add power attic fans?
If you're converting the attic to conditioned space, you'll either be sealing up the ridge and soffit vents (if you use spray foam on the underside of the sheathing), or "ducting" them by building vent channels from the soffits all the way to the ridge (which would let you use insulating materials other than spray foam).
Power attic fans depressurize the attic, which tend to exacerbate any air leaks from the home into the attic. Power vents are essentially band aides, not usually proper solutions to attic problems.
Bill
Bill one last question
Like a condition crawl space has a dehumidifier can you not leave the cellulose insulation in place and put a dehumidifier
I suppose you could, but that's not the best option. It's better to seal things well and not have the ongoing energy cost for the dehumidifier.
Bill
From Lstiburek: "Again, I repeat, it is ok to use open cell low density spray foam in conditioned attics….but the attics need to be conditioned. There has to be a moisture removal mechanism. I recommend air change."
If you have insulation in two places (attic floor and ceiling) and seal it up, then you have a large semi-conditioned space. Which in many conditions will be very high relative humidity (a condensation/sorption/mold problem).
I do have a condition crawl space with a dehumidifier
You want a fully conditioned attic. Say it's 70F inside, 0F outside and 50F in the ground. A semi-conditioned crawlspace might be 60F where a semi-conditioned attic may be below the dew point at 35F (at which a conventional dehumidifier won't work).
If you have a perfectly good conditioned crawlspace then the smart move would be to move the system the install 25% smaller equipment and pay lower bills.
HVAC in an unconditioned attic is a very very bad idea for the person paying the utility bills.
Conditioning the attic is only a slightly better but still bad idea as you are making the surface area of the conditioned space 30-50% larger and forcing yourself to use the most expensive insulation per R so you are likely to buy less Rs.
Walta
Walta,
I am sorry I lost you on the last part. What is the best to do for the attic. Yes I am paying the bills
Greg
Hi Greg,
Martin Holladay lays out key considerations and details for bringing the attic into the conditioned space in this article: Creating a Conditioned Attic.
The best thing to do for an attic is to get the HVAC out, install good vents 60% at the soffit and 40 % at the ridge, air seal the ceiling and cover the attic floor with R60 cheep fluffy insulation.
Builder love to put the HVAC in the attic because it is low cost to install and uses zero square feet for equipment and the home owner pay the fuel costs.
HVAC in a conditioned attic is only about 10% better than HVAC in an unconditioned attic. Think of your attic is a triangle the bottom is your ceiling the other 2 sides are your roof if you happen to have 12/12 roof all 3 sides have an equal area. If you put the insulation on the ceiling you have half as much area to lose heat and insulate If you insulate the ceiling you can use the lowest cost per R insulation as gravity will hold it in place and you have room 20 or 30 inches cheap recycled paper( very green)
If you insulate the roof the only insulation that works is spray foam very not green stuff and the most expensive insulation in dollars per R. Since you need to cover twice as many square feet at 7 times the cost making it cost 14 times as much per R no one has money or room for an R60 spray foam roof but lots of people have R60 ceiling. The hucksters selling spray foam will try to convince you that there R 12 is somehow better than R60 of fluffy stuff, this is a bald face lie.