I’m punching a hole into the attic of my old San Francisco Victorian to use it for storage space. The ceiling is insulated, but here in Fog City the daily temperature stays between 50 and 70 degrees nine months of the year so houses don’t need to be very tight. I was surprised, though, to find that the uninsulated attic space was probably in the 90s, a good 30 degrees above the air temperature of the upper floor of the house. Guess I’m not going to be storing any wine up there…
Question 1: Is this bad (the hot attic, not the wine idea)?
Question 2: Should I vent the attic space to reduce the temperature difference?
Thanks,
Tom
Replies
First thing I'd do would be to be sure there aren't leaks in the heat system. We had the same thing happen. Turned out one of the ducts had come loose and we were blowing heated air into the attic.
"A completed home is a listed home."
It is true that there is a secondary furnace in the attic space, but the primary heating is done from a downstairs furnace and the attic one almost never fires up. The source of the heat is probably leakage from below (three floors of warm air gathering at the top) with perhaps a small contribution from the furnace pilot light.
- Tom
Then it seems to me that you should insulate the attic floor to keep the heat in the living space. Maybe use the fan to recirculate the upstairs heat through the house. We used to live in Bernal Heights so I know it's almost always chilly and damp in city. And heat is too expensive to be venting outdoors!
Plus, if you start to heat the outside on a regular basis you will eventually raise the average temperature of the Western Hemisphere, bringing about the melting of the polar ice caps and the eventual destruction of life as we know it on this planet. Conversely, 50 degrees is a very nice temperature for a wine "cellar". ;-)"A completed home is a listed home."
Tom, here's my take on the subject, with a tale from the trench...er...from the attic.
Most attic heat comes not from the liviing space, but from the sun.
The sun heats the shingles, the shingles conduct heat to the sheathing, the warm sheathing then acts as a radiant panel and radiates heat into the attic space. Fiberglass insulation is transparent to radiant energy...radiant passes right through FG. Radiant is absorbed by cellulose, and rigid polyisocyanate panels wiith a foil face act as a radiant barrier.
Passive venting of your attic, while removing some of the warmed air, really isn't going to do all that much to lower the attic temperature. You need to block the radiant gain.
Now, that tale from the trench (attic):
When I finished my own attic, it was July. Attic space was wide open with 3/4" subfloor. Gable walls on either end, 5' knee walls on sides, the floor space is 18' wide between the knee walls at the gables and 46' long along the knees. Ridge board 11' high. Soffit and ridge vents with unobstructed air channel from one to the other. Asphalt shingles w 5/8ths ply sheathing on the roof. Inside the attic, the rafter bays were stuffed with unfaced FG. Temp at the ridge was 127degrees, with the windows on each gable wall open.
I tacked up sheets of rigid foil-faced polyisocyanate insulation. Gapped the sheets and foamed the gaps wiith canned foam.
The next day, which was a few degrees hotter than the previous, the max temp at the ridge was 77 degrees. The polyiso simply blocked aany further radiant gain from the roof.
The comfort level in the second floor bedrooms was greatly improved as well.
Back to the beginning...is the elevated attic temp bad? Probably not. It is adding a heat load to the house, and it could be putting a slight strain on any mechanicals located up in that space.
When dealing wiith attics though, it can be an "every action has an equal and opposite reaction" type of thing. Before messing with a loose attic, it's often wise to look at the rest of the envelope, too, starting in the basement.
Long-windedly,
Mongo
Edited 5/30/2003 1:38:48 PM ET by Mongo
Did you attach the polyiso panel directly to the sheathing?
My own attic is pretty toasty in the summer, and a condensing rain forest in the winter. Was built with gable vents which didn't work well. I put on a ridge vent, then drilled 2" holes in the soffit about 32" on center. I know I need more soffit vent, but there's an aesthetic problem: the soffit-long 1" wide vent looks too modern to be on a colonial.
At any rate, I've contemplated doing what you did with the polyiso, but am afraid of ruining the sheathing by trapping moisture between the decking and polyiso panel.
I never met a tool I didn't like!
Polyiso was nailed to the attic side of the rafters...the underside.
I then ripped 3/4" ply into furring strips. Screwed these to the face of the polyiso. Strips ran horizontal, perpendicular to the direction of the rafters, 16" oc. Furring strips attached with 4" screws run through the polyiso and into rafters. 5/8ths drywall then screwed to furring strips per usual.
The plane of polyiso, properly detailed and foamed, will make the attic ceiling as tight as a drum. No moisture will get from the living space, through the foam, into the rafter bays, and condense out against either the cold sheathing or the cold nails.
Because excessive house moisture will no longer escape from the previously leaky attic, pre-existing moisture sources, from the basement up, need to be addressed before tightening the attic.
Got it, thanks.
Ever hear of anyone using the polyiso as the vapor barrier/attic floor insulation (cut to fit between the joists, foam in to seal)?
My triple layer of FG batts are green with mildew in a number of places, indicating leaks. I tried sealing, but with no luck. I'm wondering if the polyiso could be cut and foamed in to provide the seal, but then I'd probably be trapping the moisture between the polyiso and the drywall, creating a potential nightmare.
Thanks
I never met a tool I didn't like!
My take is that you're asking if you can remove all insulation from the rafter bays...then cut polyiso and fit it between the rafter bays, holding it off the underside of the roof sheathing to allow for your soffit-to-ridge air channel...then foam the edges so the polyiso is sealed tight to the rafters?
Yes, it's fine to do it that way.
Technique? When I have a lot of polyiso to cut, I rip the sheets on the table saw. If it's new construction where the bays are uniform, you can rip the polyiso a tad wide, then cut a rabbett...about 1/4" wide by 3/4" deep...on the attic side of the edge of each ripped sheet. I'm talking 2" polyiso. Friction-fit the sheet betwen the rafter bay. It'll be a nice tiight fit. Then foam the rabbett. That'll give a super-tight seal.
Your moisture concern is still valid, but a couple of points: Again, with good foaming, no moisture should make it through the polyiso and into the soffit-to-ridge air channel. As part of that, you'll need drywall between the polyiso and the living space for fire protection. Properly installed and painted drywall can act as a barrier to limit water vapor from getting to the FG batts that you'll lkely install between the drywall on the face of the rafters and the and the polyiso tucked deep into the rafter bays. IF any water vappor gets iinto the rafter bays, the idea is to have the polyiso thick enough so that the bottom side of the polyiso remains warm enough to not become a condensing surface. You want the dew point to occur somewhere within the thickness of the polyiso, NOT in the FG batts that are between the polyiso and the drywall.
All that said...the sources of moisture...starting in the basement...need to be addressed before tightening up the attic. Otherwise you'll end up with nice, dry, rafter bays...but the attic space itself will be a moist and humid place.
There is a noticeable difference in the intensity of the sun when we visit my wife's hometown of Monterey, California and there are some similarities in the bay area.
Compared to southwest Virginia, where we live, the sun is more searing and heats up the interior of a car in a hurry but the air is cool/cold when the windows are rolled down, even on a sunny day.
This intensity is felt to a certain degree even when the daily heavy fog rolls in.
I can imagine what this would do to a roof and the resulting heat that is transmitted into the attic.
No suggestions, just my observations.