Cladding for basement rigid insulation
I have been planning to clad the above grade portion of basement exterior (rigid) foam insulation with 1/2″ PT plywood under painted steel.
Fastening the steel over the plywood is not an issue.
I have not figured out a good way to fasten the plywood to the foam.
One method I have seen people use around here with slab on grade is to put the foam and plywood into the forms with carriage bolts extending through the plywood and rigid into the concrete…
This seems to work fine for slab on grade but doesn’t seem to be applicable to a full height concrete wall, since the plywood won’t extend the full height of the forms.
Thermal bridging through the carriage bolts seems like it undermines the effectiveness of the thermal break.
The 3/8″ carriage bolts I have seen people use seem unnecessarily large for this application.
I would like to consider cement board as an alternative to the PT plywood.
Any thoughts?
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Replies
Lucas,
Why not use a foam-compatible adhesive supplemented by a few Tapcon screws?
Martin, I'm not sure Tapcon screws are long enough for this application...
Their website says they come in 4" but I only see 3-3/4" listed.
I have yet to find a concrete fastener that is long enough to penetrate the plywood and 4" of rigid foam.
Is it possible that an adhesive might provide enough holding power on its own?
Did you see John Abrams' article in the most recent (Jan. 2011) JLC? http://www.jlconline.com/cgi-bin/jlconline.storefront/4d518b3101cf424e27170a32100a05e5/Product/View/1101hig He has a nice detail they use for interiors, but I don't see why it wouldn't work on the exterior. They rip dovetailed PT 2X's and screw them, thin face out, to the forms. The concrete keys into it and holds it in place when stripped. Then they have nailing every 24" OC.
No, I don't have a membership.
What about something like this?
Do you want the foam on the outside? There's a system, whose name escapes me, to suspend the foam in the middle of wall, which makes a lot of sense to me. I met the company owners at the NESEA and JLC conferences last year. I know the product has been mentioned here - perhaps someone will chime in.
Thermomass.
Lucas, the description is that the PT 2x is embedded into the wall. It is beveled so that it stays in the wall. I wonder if the bevelling (dovetailing) would be necessary on an exterior wall. Once back-filled, the soil should eliminate the possibility of pulling out (or could be Topcon'd).
Martin, appologies. I see now there are Tapcon screws up to 6".
I sort of like the idea of the 2x2 nailers now though.
The ardox nails oughta hold up the nailers - no?
I've heard that "scuffing" the foam on the side that faces the concrete will let the concrete "grab" it and "hold" it securely enough until back-filled - no?
If the cladding doesn't extend more than a foot or so above grade, the mechanical fasteners can be considered temporary until backfill. You'd probably have conventional screws fastening the top edge of the PT plywood to the sill plate.
Hardiboard (1/4" fiber cement panel) is more economical than the combination of sheet metal and plywood.