Best Practice for Deep Crawlspace
mhomedream
| Posted in General Questions on
We are building in Zone 5, and working within the confines of a track builder. We did upgrade to a 4 ft crawl and I would love some insight into best building practices for drainage, air flow, etc. We will need radon mitigation and do not want the crawl to be part of the building envelope. We are looking into the April Aire for radon and a 15ml Stego. Our excavator is about the break ground and seemed to think he needed to lay a foot of gravel over the crawl base. I can’t find anything that backs this up. Please advise…does he need to do anything at this point other than dig the foundation?
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Replies
mhomedeam,
Your excavator doesn't need to do anything special before the foundation is poured. The layers of fill necessary for radon mitigation and as a capillary break go inside the stem-walls.
By "not want(ing) the crawl to be part of the building envelope" do you mean you do not want it to exchange air with the rest of the house but still be conditioned? Or are you suggesting a vented crawlspace? I would strongly suggest the former.
I don't want the air to be exchanged with the rest of the house. I would love for it to be conditioned but don't want to use spray foam. Also limited by what the contractors are willing to do or what we could realistically do ourselves. The crawl will have vents as it's drawn.
ihomedream,
Vented crawlspaces are problematic in most climate zones. They often have moisture issues, and risk freezing services.
You don't need to use spray foam. Insulate the walls, ground, and rim-joists with foam board. The space will need to be conditioned. That can be done in a variety of ways without connecting it to the house above. Some of the more common ones are to:
- Run an ERV/HRV duct and return.
- Install a fan to exhaust air, and have a supply grill to the floor above.
- Use a spot heater and dehumidifier.
How the foundation walls deal with water partially depends on site conditions.
https://www.greenbuildingadvisor.com/article/dampproofing-and-waterproofing-foundation-walls
- Below the rat-slab you want a layer of clean free-draining material as both a capillary break and to connect with your radon mitigation system.
Thank you. Would pea gravel be an appropriate layer? And how would you drain the dehumidifier without a vent?
mhomedream,
Yes pea gravel will work for both.
Many codes require crawlspaces to have a floor drain - and it's a good idea even if they don't where you are - so if you need a dehumidifier it can drain to that.
Interesting. I have NEVER seen a crawl space with a drain but agree it's a very good idea. Thank you. Since this is a pretty severe mountain climate would there be any special considerations with the drain?
mhomedream,
Some codes don't require drains in unfrequented parts of a building to preclude flooding going unnoticed if the system back-flows.
The safest practice is to run this drain separately to daylight to preclude problems if the perimeter (french) drains clog, and include a mechanical back-flow preventer.
Floor drains in crawlspaces have saved my bacon a couple of times - although to be fair there were water heaters down there.
I recommend this article:
https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces
This article is from 2010. I've been in some pretty bad encapsulated crawlspaces and we do not want spray foam anywhere in our home. I thought maybe the science had advanced since this article was written.
There are a bunch of details you really should have worked out before you break ground. I'd say the things that give people trouble are insulating the crawl space/basement and the roof, and you need to figure out a plan for the walls before you frame them.
It doesn't get easier later on.