Wall assembly
Ccloud
| Posted in Mechanicals on
I am not sure this is the correct place to ask this question but here it is. I am building a new home in NW Wyoming. I will have a sealed and conditions crawlspace under half the house. I have installed a radon system. The crawlspace has a poly sheet under a rat slab. The hvac is located in the crawlspace. Would it be a good idea to have the returns for the bedrooms and baths to vent into this crawlspace. It will also be insulated on the walls internally with rigid foam.
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Replies
Here is what I hate about crawl spaces: they become part of the envelope and you put your air through the and you can never ever clean there. Unless there is some crazy high water table, I don't get the point.
Joe,
Why would you draw air from your crawlspace?
Charles,
You don't want to deliver ducted return air from you home to your crawl space.
Here is a link to an article that explains the required details for a sealed crawl space: Building an Unvented Crawl Space.
Building codes require (and building scientists recommend) that conditioned crawl spaces have a little bit of air flow to help keep the moisture levels under control. As my article notes:
"The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)
"Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)"
Martin, Just for my own personal education, why is using crawl space air as return air bad? If the area is conditioned and has radon mitigation, why is this air nor good?
Charles,
Return air should always be ducted to the furnace or air handler. It's never a good idea to use joist bays or crawl spaces as plenums, because (a) Without a duct, you really don't know the source of the air being sucked into the furnace; and (b) In the case of a crawl space, there are always radon and moisture concerns, even when the builder does his or her best to mitigate these issues.