How to attach mineral wool to crawl space walls?
Hi,
I’m building an addition, currently insulating the crawl space walls, but I’m unsure how to affix mineral wool insulation to complete the job.
The concrete floor was poured over 3″ of XPS, and I installed (prior to the pour) poly over the XPS and up the walls and lapped over the top of the foundation.
I installed 3″ XPS in the joist bays, and 3″ XPS on the concrete wall, adhered to the poly with adhesive caulk. I sprayed foam around the XPS to seal the assembly.
I want to add a layer of 3.5″ Roxul mineral wool, both for the added R factor and as a fire barrier.
The crawl space is about 26″ from floor to under-side of 2×10 joists. The space has very limited accessibility; currently, I’ve left a 3×3 section of first floor sheathing unattached so I can enter the crawl space between joists. I anticipate some sort of trap door for future access to utilities (wires and pipes only; there are no appliances / furnace / etc. in the crawl space), but have no plans to store anything down there.
I plan to install Roxul in the joist bays in the usual way, with friction fit.
I plan to insulate against the foam that’s attached to the concrete below the joist bays with a single course of Roxul sitting long-side on the floor and a second course atop the first course, cut length-wise to fit beneath the joists.
I’ve thought about attaching with some kind of washer + long screw through the Roxul into the foam. Is there a better way?
I’m assuming the Roxul will suffice for a fire barrier to meet code; let me know if that’s an invalid assumption.
Thanks, Jeff
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Replies
Jeffrey,
I guess I would suggest that you use 1x4 furring strips attached through the insulation to the concrete below with long TapCon screws. Not easy or cheap -- but I can't think of an easier way.
With insulation under the slab and on the crawlspace walls, what is the point of the insulation between the joists?
A single 2x4 hung from the joists 3.5" out from the wall foam, and a second 2x4 glued/nailed to the slab would be sufficient for tucking in 26" tall rock wool batts. That would leave only 19" of unsupported exposure on the batts- no need for through-screwing 1x furring to the concrete wall with expensive (and thermally bridging) screws.
If it seems like the batts would sag over time, tack a vertical 1x4s to the 2x4s at every seam (with the flat side of the 1x4 facing the crawl space.)
Martin,
I agree, not easy or cheap. As with Dana's idea, I'm unsure about leaving exposed wood where there's supposed to be a fire block (if I understand the code correctly). Though, of course, the joists immediately above and across the entire area are wood.
Dana,
I wasn't clear -- I was referring only to the space adjacent to the rim.
The 2x4 retainer idea seems good - my concern with anything wood-based has been that it introduces a flammable substance to what I'd be viewing as a fire block (which seems required by code). Am I being too concerned with that aspect?
Thanks, Jeff