Conditioned crawlspace sealing details (floor)
Gene_Davis
| Posted in Energy, Heating & Insulation on
The attached section is taken from Building Science’s site, and shows details for a conditioned crawlspace for a house built in a very cold climate. Please provide tips and techniques, material recommendations, etc., for the vapor barrier on the gravel floor fill, its taped joints, and the fastening of the turned-up edge to the foam insulation that is applied to the inside of walls. This looks less expensive than a rat slab, and we want to get it right.
Replies
The detail you posted looks good.
The last crawlspace we used hex head tapcons and strapping to hold the poly tarps we put on the ground in place. The tarps are more durable than polyethylene and survive subsequent crawlspace activities better. (If your project is new construction with reasonable clearances and a flat consistant ground cover as opposed to a nasty old house reno this may not be such a concern.)
The rigid perimeter wall insulation we also fastened with hex tapcons this time though we used washers. Fender washers in conjunction with large site made washers fashioned from round container lids. Not elegant but effective and demountable. Small holes around fasteners and gaps were with filled with spray foam.
Taping the floor is a good idea and we haven't done that yet. I figure that is a detail best left untill further expeditions into the crawlspace are unlikely (at least in the near term) and at this point any other tarp repairs can be made as well. We have already been back down there several times, most recently to replace a section of 4" cast iron waste line.
The poly tarps we used were white and brightened the whole area up. In addition they provided a surface one could slide on which was much easier than trying to wiggle along in some of the more constricted areas.
The Building Science advice is good. Here's a good site for the floor poly, sealing tape, and sealants (and building gaskets):
http://www.conservationtechnology.com/building_films.html
You also could install some foam panels above the vapor barrier on the floor if you want a little more insulation, but you should look into your fire code requirements to determine if you can leave it uncovered -- a rat slab could go on top if it's in the budget.
Billy
If you are using this detail, I assume it's new construction. For old work, the XPS would have to be installed on the exterior because you can't dig inside a crawler. However, I like exterior foundation insulation for two other reasons, 1. The thermal mass effect, and 2. The warmer and dryer a concrete foundation is, the more durable it is. The B.S. detail shows it on the inside because they haven't figured out a thrifty way to make foam as durable as a concrete wall. There are several ways, though, such as metal flashing, stucco, and roll membranes designed for the application. I personally believe that if the exterior backfill is sloped at 10+% and has 4" of gravel as the top layer, then you can run that fiber cement siding right down to grade, and it will last as long as the siding on the rest of the house. Would that look better than raw concrete? you betcha.
In either case, fastening the foam to the walls isn't too difficult because the backfill holds it tight. Some duct tape from the top of the stem wall down to the foam will hold it for a few hours while you backfill.
i recommend against foam in termite country - they use it to hid their tubes and gain unlimited undetected access o nhe structure - I've seen this many times
and i recommend against vapor barrier up he walls in older construction with limited exterior exposure of the foundation - it can lead to rising damp and rot of the framing (which is unlikely to be PT in those older houses)
Again, I've seen this man times.
I think the detail lacks the tape/sealant between seams on the poly in the crawl. Personally, I'd run the poly up the side and under the mud sill ... then that is one big seal I'd never have to worry about.
Other than that, I think I've seen this detail before. The radon stack will keep the area under the poly under a negative pressure ... I've seen this done w/ out the radon stack and the poly blew up like a balloon up against the floor joists due to the soil gas pressure!