Best Attic for Hot, Humid Climates ????
Such is the claim of a job-site diary currently active here. I have some reservations, points on which I am not clear.
It appears that the method udes in the blog is to completely seal and insulate the exterior faces of the attic, and to condition the attic as one would condition the house. They’ve simply added the attic to the house.
In doing this, their reasoning is that it makes little sense to pipe cold air through a hot attic. Have they really addressed this concern? I’ve had a few experiences with thissort of climate, and I’m not sure they know what they’re doing.
I agree that it makes little sense to pipe cool air through a hot attic. It’s not as simple as that, though. The conditioned air has it’s heat loss affected by not only the temperature difference, but by the surface area of the ductwork and the length of time the cool air is exposed. Since the blower does not -ordinarily- run all the time at full speed, and the ductwork is usually designed to be as short and small as it can be and still be effective, the losses are minimized.
This contrasts to the air in the conditioned air in the sealed attic, which is ALWAYS in contact with the entire hot roof, relying upon the insulation to prevent heat transfer.
I’ve tried ‘super-insulated’ roofs, and the resulany heat that comes through right back outside.ts have not been very good. One need simply enter an unused walk-in cooler, one that’s been sitting in the sun, to realize that even 4-6″ of foam isn’t all it’s cracked up to be. That roof collects heat, and the heat does transfer. At a slower rate, perhaps …. but all that does is delay the ‘hottest’ time inside from 3PM to maybe 7PM.
What has made a huge difference is, simply put, shade and ventilation. That’s what the ‘usual’ roof can do, if properly designed. The roof deck has the effect of putting the house beneath in the shade- and air flowing under the roof deck transfers any heat that gets through right back outside.
IMO, most attics ‘fail’ by not having enough airflow, directly under the entire roof deck. That air flowing right under the roof deck is what removes the heat. No air flow, and heat is free to radiate / convect right to the ceiling beneath.
Ironically, the bloggers might accidentally provide this with the addition of the solar panels; the panels will put the roof in shade, and air might be able to flow under them.
I’d appreciate your thoughts …..
Replies
I think, for a given amount of insulation, you're better off having some sort of a ventillated roof in a hot climate. This allows a substantial fraction of the sun's heat to be dissipated before it reaches the "envelope".
Otherwise, the main issue would be whether you can more effectively seal and insulate an attic ceiling vs a "cathedral". In most cases I suspect that the attic ceiling is a lot easier to seal/insulate, if such is planned in advance (retrofitting may be another matter), and it has the advantage of being a smaller overall area.
The one monkeywrench is that in warmer climates it's more common to have the HVAC equipment in the attic space, both because basements are rarer and because AC is the primary operating mode and it makes sense to have it come from overhead. And it's virtually impossible to tightly seal an HVAC system (and keep it sealed over the long term), so at least partially conditioning the attic may make sense.
I'm in the process of redoing the roof on my 1960 SoCal ranch. Not too hot or humid, but I'm building as if it is.
In one area, I'm furring up over my 2x6 rafters with 2x10s. Between the rafters I'm installing foam chutes for ventaltion - and as a secondary drainage plane in case of leaks. Under the chutes and bove the old deck I'm blowing cells, so I will have R30 when I'm done. I will also be using reflective shingles and possibly radient barrier OSB decking on top of the rafters. Lots of thermal breaks.
The furring 2x10 rafters become real rafters over the rest of the house - a vented catherdral ceiling.
I think the real advantage of the insulation at the roof deck plane is that you don't have to worry about penetrations into the unconditioned space anymore. Everything is sealed.
We did a very large attic on a Sanibel Island house, Very hot and very humid, about 3 years ago. Sprayed in 6 inches of closed cell poly and completely sealed the attic. The house and attic stay cool throughout the day, even in the summer. The highest temp I've seen in the attic was 85 but it was dry, not humid at all. if I had the money I'd have my attic sprayed and sealed tomorrow.