Wall Insulation Retrofit Best Practices
My house is next to multiple houses that burned to the ground in the recent Marshall Fire in Colorado. My house is a spec home built in 1992. Near Denver. Climate zone 5B. Cold in the winter (a few days below 0F) and hot in the summer ( a few days above 100F).
Due to the fires next to my house, the wall insulation is contaminated with soot and ash and needs to be replaced. My local building department is telling me that if I pull out the smoky fiberglass batts, I have to upgrade to current building code of R21.
I am not 100% sure where they are getting the R21 number, since it seems the the current code for my zone is R20 or R13 + 5. The building department is very adamant that R21 is their requirement.
My house currently has 2X4 walls, therma-ply (carboard) sheathing, and fiberboard lap siding. No WRB unless you count the thermaply. R13 fiberglass batt in the walls. I have a 6mil poly vapor barrier behind the drywall.
While I am at it, I’d like to upgrade the siding to Hardie, replace the cardboard sheathing with OSB, and add a real WRB. Therefore it seems best to fix the insulation from the exterior by removing the siding and sheathing. That way I can achieve my other goals and not disturb the drywall and tile on the interior.
What is the best “bang for the buck” to meet R21 in a 2X4 wall?
1. Closed cell spray foam, sprayed from the exterior? I realize that this isn’t the optimal solution from an energy point of view. It does nothing to address thermal bridging of the studs. I think that it is the cheapest and easiest option. Are there any caveats to spraying foam from the exterior side? It seems more common to take down the drywall and spray from the interior. Do I need to remove the poly vapor barrier, or just have the contractor spray foam onto the poly?
2. R15 fiberglass in the wall cavity with ZIP R6 on the exterior. I like this system the best, but the price quote I got for this was outrageous, about $10/sq ft installed. What should ZIP R6 cost in the Denver area per square foot? This would also require a lot of extra work to work around windows, doors, hose bibbs, etc to accommodate the extra thickness which adds to the cost.
3. R15 fiberglass in the walls, OSB, then R6 foamboard on the exterior of the OSB sheathing. I’ve read enough to be worried about moisture with this one. Is that as much of a concern in a place like the Denver area where we have really low humidity? Is there any advantage to this over the Zip system other than (possibly) reduced cost?
4. Other options?
Since this is a completely unplanned remodel, I am not looking for the absolute best solution. I really just want to meet the local code while getting the best value out of my money. Your help would be greatly appreciated.
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