Insulating detached garage attic in central Florida
I’m about to start construction of a 30′ x 30′ concrete block detached garage in central Florida. I’m using attic trusses and plan to use a white raised seam steel roof. I was not planning in insulate the block walls. What is the best way to insulate the roof? I want to keep it a cool as reasonably possible – I might add a window or portable AC, if I have to, but will not run it often. Depending on the final use of the attic, I might even try to condition that space. I’m concerned about moisture buildup inside – wet cars could bring a lot of water in. I was thinking of doing vented soffits, rafter vents (maybe covered with spray foam or foam board?) and a ridge vent – but that might block the exit of moisture brought in by the cars, unless I leave the area by the ridge vent VERY open. I’ve seen newer houses with completely sealed attics, but the amount of mosture that could come in, and the fact that the spce will generally not be conditioned scares me away from that. Ideas?
Thanks
Kelly
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Replies
Kelly,
In winter, especially on cool days, you can remove humid interior air from your garage by operating a few fans that exhaust interior air and introduce exterior air.
During the summer, though, that option won't work. The only way to dry out the indoor air in your climate during the summer is to operate an air conditioner.
When Martin says "the only way," I think he means the only standard way. If you want to get creative, there are other possibilities. For example:
1) Used forced ventilation, operated based on the outdoor humidity and dew point. This is difficult, because if it is cooler inside than out, the humidity of the outside air will go up when it comes inside and gets cooled. But eventually, this strategy could raise the inside temperature enough that that effect goes away, and you can get drying even in the summer. For this to work you need to be OK with the higher indoor temperature, and you might need either a custom automatic control system or an energy and moisture obsessed person monitoring conditions and controlling the ventilation.
2) Use a conventional dehumidifier. Again, this will raise the indoor temperature. But with a higher indoor temperature, accidental or deliberate ventilation will hurt less and help more towards drying the inside.
3) Use a solar desiccant dehumidifier.
Those might not actually be good solutions for you--1) and 3) are not things you can buy at the local home center, and 2) will make it hotter whereas you said you wanted it to stay as cool as possible. So I'm mostly listing those to mitigate the absolute way Martin stated it.
Now responding to your question, one thought might be to air seal between the attic and the main level, and to vent the main level to the outside separately from the attic. If the attic is air sealed and insulated, you could then condition it, or if it is just air sealed from the main level, you won't get moisture coming into it from there, and the solar heat you do get, even with the light roof, will keep it dry.
In that case, if the materials in the main level are moisture tolerant, your need to keep the humidity low there is reduced. You might want passive vents through the walls near the floor and near the ceiling that you can open and close as needed, and a fan.
In general, I think that moisture management of spaces that are not fully conditioned is something that is fairly often important, but to my knowledge, it hasn't gotten much attention from building science, although I'd be delighted to find out that I'm wrong about that and get pointed to some good references.
What about radiant barrier backed plywood/OSB for the roof deck to help keep the garage cool? Would this help much? It does not cost much more than unbacked roof decking... I have to admit, I'm a little concerned about mositure, but it typically has tiny holes in the radiant barrier for that.
Also, I have seen IR reflective exterior house paint from more reputable companies, like Sherwin Williams - I used to be concerned it was too gimmicky, but with mainstream companies now offering it, maybe I should reconsider.
Any thoughts or experience to share?
Thanks again
Kelly
Kelly,
In your climate, it certainly makes sense to include radiant-barrier roof sheathing for a new garage. While it won't do anything to help with humidity concerns (the concerns that you expressed in your original question), it will help keep your garage cool on sunny days.
For more information on radiant barriers, see Radiant Barriers: A Solution in Search of a Problem.
Radiant barrier paint is a scam; avoid these paints. For more information on this issue, see ‘Insulating’ Paint Merchants Dupe Gullible Homeowners.