Enclosed, vented unconditioned crawl space — buildability question
I am putting an addition on an existing house and the foundation has to be a vented crawlspace. I found this article by Joe Lstiburek helpful: https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces . He recommends a layer of foil-covered polyiso below the joists, foil side down, with seams taped. I would also like to add plywood below that to keep rodents out. I figure if I do a inch of continuous foam and a half inch of plywood it will be the same thickness as the mudsill and will fit snugly between the foundation and the bottom of the joists for a nice tight seal. At the rim joists I would also put foam insulation board, and then rockwool in the cavities.
The question is how to build this. The ground under the crawl space varies in height, but it’s not much and as low as 18″ in spots. I have to meet an existing floor so I can’t raise it.
Option one would be to do the insulation after the addition has been dried in. This would mean working in the dark lying down. I worry that it would be difficult to do a careful job.
Option two would be to do the insulation as the floor is framed, and somehow try to keep it all dry until the roof is on. This would be easier to get right, but if the insulation gets wet it could be disastrous.
Option three is to pick different materials. Use EPS for the continuous insulation, and add the fluffy insulation after the roof is on. Either put batts in through lifted pieces of subfloor, or blow in fluffy. With this I worry about being able to do a thorough job with the insulation into blind crevices.
The addition is roughly 20×20, the joists are 12″ TJI’s.
Thoughts?
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DC, I'm dealing with a similar situation right now. I know you said that it has to be vented, but is there any chance you could do a raised slab instead, and backfill the interior of the foundation walls? That does away with all of the problems a crawl space presents. You would build the walls and roof on the gravel fill, then once you're dried in you can add insulation and either sleepers or framing on edge and a subfloor. The framing could be undersized and rest on the foam. Hopefully that's clear; I've designed this for several projects now but so far the builders have requested other approaches. But I don't see why it wouldn't work, unless you have specific requirements.
I can't disturb the earth in the crawlspace due to tree preservation. But it's an interesting thought to see if I could put the floor joists in after the walls and roof. Like old-school balloon framing.
DC,
Pre-build the floor upside down in sections, the size of which will depend on how much labour or machinery you have to flip them. Insulate and sheath what will be the underside remembering to block at bearing p0ints. Sequencing is a bit difficult as you need to insulate the cavities from above before laying the subfloor, making that insulation vulnerable to moisture during the rest of the construction.
The alternative - laying on your back in an 18" deep space while securing and sealing two layers of sheet goods, borders on impossible.
I don't think I can do them upside down, they're 20' long. But the idea is if I put the bottoms on before the floor sheathing and the walls I don't have to do as much crawling around, no edge is ever more than 4' from a spot where I can stand up. I can inspect from above and make sure every fastener hit a joist, and seal the seams from above. I just have to make sure no water gets in.
DC,
I somehow missed the part about the crawlspace being enclosed. If so, I think you could probably do a good job of insulating the cavities and adding foam to the underside , as long as you move your rodent protection to the crawlspace walls, and forget about sheathing the underside in plywood.
Can you scrape down the area underneath to give you 2 ft clearance? It would make a huge difference.
I can't dig at all. This is in Washington, DC, which has a tree protection law, you can't disturb the roots of a mature tree. The addition is going to be on piers but the crawlspace overlaps the root zone of a mature tree.
DC,
So piers with a skirt enclosing it. I'd go back to my first suggestion of flipping the floor system. With a skirt you definitely need some protection on the underside against rodents and insects.
I've had to work with some tight spots like this in the past. What I did was use pieces of pipe (EMT conduit actually, which is lighter weight) suspended by light ropes to hold the panel up under the floor structure while adhesive set. You would put a bead of adhesive on the underside of the joists, working from the top, then slid the panel underneath with the pipes under the panel. You then pull the ropes to bring the pipes up, securing the ropes to keep the panel pressed against the underside of the structure until the adhesive has cured.
This works, but it's very slow going. I would try to avoid doing something like this unless you have no other option. If possible, I'd try Malcolm's suggestion, possibly leaving a lip on one edge so that the floor sections could interlock a bit (for better air sealing).
The other option would be to put some heavy poly over the joists to tent things to keep rain off until you're further along with the project. You'd need to make sure the poly is somewhat taught to keep water from pooling though -- I've seen poly used like this fail before when lots of water pools up on top. It's amazing how much weight can put put on things from water pooling on poly sheets and tarps.
Bill
Have you looked at a SIP floor system and have it installed?
This might be a good place for some ZipR. For the cost of the material, it is not adding all that much but can save a lot of labour. I would install it the parallel (the wrong way) to the joists as the joists go down, tape the seams easily accessible and tape the rest once the floor is finished and the place dried in. You can also do this upside down as suggest above, 20x4 section is not that heavy but not a one person job.
One note on your stackup: the foil face down is to inhibit radiant heat transfer. If you put plywood over it, you lose that benefit. You could use foil faced plywood instead. or you could use a layer of hardware cloth instead of plywood, so that the radiation still sees the foil on the polysio. Or reverse it and have the plywood right under the joists, followed by the polyiso under that. The rodents could chew the polyiso but wouldn't get further than that.
As far as sequencing, if you want to keep it dry, you could lay the subfloor down without fasteners, and put the tarp on top of that, to lessen the pooling opportunity.
Or panelize the whole structure so it all goes up in a few dry days.