Crawlspace – going from passive venting to sealed/mechanically vented
We just bought a 47′ bungalow in climate zone 4 marine. The crawlspace is without a vapour barrier and has some disintegrating batts in the floor joists.
I have gotten some quotes to fix up this problem but am leaning toward a diy option.
I’d like to follow the guidance of this (https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space). Hoping that someone can ensure my plan is sound.
1. Attach r22 comfort batt mineral wool to the interior foundation walls.
– The access is very limited making rigid foam difficult.
– Considering the hilti product but this will cost as much as my insulation. Any ideas?
2. 1o mil vapour barrier on floor and wrpapped up walls and secured to the sill plate with acoustic sealant.
– any suggestions on products here? my local suppliers – in BC, Canada, don’t really have much to choose from.
3. Combo of closed cell spray foam and rigid foam in the joists bays. R-20/ equivalent.
4. 1 x mechanical vent to exterior – 1 cfm per 50 sq ft.
5. I x passive vent to conditioned living space.
I am a little concerned about venting mechanically to the outside, but our current heat source is a gas fireplace that heats the whole 1200sq ft, with electric baseboards as backup.
Any feedback would be great! Thanks.
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Replies
1- You shouldn't be using batts here. Use rigid foam against the walls. If you were to try to insulate your crawlspace walls with batts, you're likely to have mold problems. Try to cut the rigid foam into manageable pieces outside of the space, then assemble those pieces once you're in the space. It helps to have a helper you can call out dimensions to and have that helper pass you the cut piece from outside. If the "hilti product" you're referring to is the press-in plastic insulation anchors, they work very well but they're not cheap. You could also use tapcons drilled through the foam with a plastic washer (they make big plastic washers to help hold foam) on the head.
2- I would use urethane caulk and not acoustic sealant because acoustic sealant is very messy to work with and tends to get everywhere. Polyurethane caulk can act as glue to hold up your liner so it has additional benefits besides just air sealing. For a poly liner, I like Americover products, but I don't know if they'll ship to Canada. You really just need a relatively thick (10+ mil) polyethene sheet to use as a liner. White is usually best because it gives a bright, clean finished appearance.
3- What is on the exterior of your rim joist? If it's something that isn't vapor permeable, you may want to modify your interior side insulating plans. I used 2" EPS on my own home for this reason because EPS is a little vapor open and allows for a little inward drying. I "glued in" the pieces with canned foam.
4+5- You only need to vent to the INTERIOR once you've sealed the crawlspace. This can be accomplished in a number of ways, vents on existing HVAC systems usually being the easiest. Once the crawlspace has been sealed, it's considered to be INSIDE the building envelope and no longer needs to be connected to the outdoor air.
Bill
Thanks Bill.
1. Note taken. Will cut the sheets down and tape together in place.
2. Thanks for the tip. Never used acoustic stuff.
3. Think I'll do the same as you.
4-5 We do not have an HVAC system at this time. Will passive venting be good enough into the living space? mechanically vent from living area to the crawlspace and a passive vent else where?
Jason,
The BC Building Code has two methods of exchanging air in a crawlspace. In the absence of mechanical ventilation you can provide vents between the space and the house above, The size of which are determined by the square footage.
The downside of that is you are, as Bill said, making the crawlspace part of the conditioned area and exchanging air with the rooms above. That's only a good idea if you have a very clean, well-sealed crawlspace. The other method is to add an exhaust fan to the outside.
I know this is apostasy here on GBA, but the most common method of insulating new crawlspace walls here in BC is with encapsulated fiberglass batts. In most cases it works fine. If you go that route I'd sub it out.
Once the newly unvented crawlspace is brought into the conditioned area of the house, I don't see many benifit to insulating the floor cavities.