Automatic vents for crawl space?
My builder proposes to install automatic vents in the crawl space of a small cabin in Maine. He is proposing a slab with 2 feet of cinder blocks, 2″ of foam under the slab and outside the cinder blocks. The vents say
The Foundation Auto Vent Replace manual vents. Fits standard 8 X 16 inch vent openings. Automatic replacement foundation vent for ventilating crawl spaces. Automatically fully opens at approx.70°F and closes at approx.40°F. Saves energy and lower risk of frozen pipes. Helps prevent foundation decay. 57 square inches of net free ventilating area. Quality you can trust.
While I understand the energy saving, for moisture control it seems backwards. In the summer it lets in warm humid air that will cool and deposit its moisture. On the winter the only air in the crawl space will have come from the warm house and it will cool and deposit any moisture. What am I missing?
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Replies
David,
If the slab and stem-walls are going to be insulated, what is the advantage of venting the crawlspace - either manually, or with these automatic vents?
How is the slab protected from frost heave?
The slab will be on a thick bed of gravel and crushed stone.
Instead of venting should try to keep the crawl space the same temperature as the inside of the house?
David,
There are basically two types of crawlspaces, vented and unvented.
Vented crawlspaces are outside the conditioned area of the house, so there is no point in insulating them. The boundary between outside and in is really the floor above.
If you insulate the crawlspace perimeter, it should be closed it off from the outside and made part of the conditioned area of the house. It needs both the humidity and temperature to be regulated - but that doesn't mean you have to keep it at the same temperature as the house above, just warm enough to prevent moisture problems, and keep the floor above comfortable.
Relying of porous aggregate to keep a foundation safe comes with some risk. It can work, but only if there is a path for water under the slab to drain. The two usual methods of avoiding frost heave are: going down below frost level, or using a frost protected shallow foundation (FPSF).
Thanks Malcolm
I design homes and renovations in Maine and I have two rules for crawlspaces: they are either fully exterior or they're fully interior. I don't think I've ever been in a conventionally vented crawlspace here where someone hadn't sealed the vent holes with foam, or where the fiberglass insulation wasn't hanging down.
I generally fully condition crawlspaces in the same way I do basements; they are just short basements after all.
Or I make a fully sealed floor system and put the project on piers with as much airflow as possible.
Anything in between is risky, in my opinion.
Michael,
You missed describing my favourate neither fully in or out: A house built on piers with a skirt.