Gap or no gap?
SOTASOTA
| Posted in General Questions on
I have a 10″ cinder-block wall (1970’s) which I am insulating on the inside like this:
[Cinder Block] [2″ XPS Foamboard] [3/4″ Gap] [2×4 frame] [Fiberglass batt] [Drywall]
Question is should I have a 3/4″ gap? I read on this forum that the 2×4 framing should be mounted “tight” against the 2″ XPS Foamboard.
Thanks!
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Replies
Russell,
No gap.
Awesome...thanks!
Martin - With block walls above grade, and presumably susceptible to rain intrusion, shouldn't there be a gap between the block and the foam? This would facilitate drying from any bulk water that got past the block. I think furring strips attached to the block first would be cheap insurance.
Kevin,
Russell told us that he was installing rigid foam against the block wall, and was asking (if I understood correctly) whether he needed a gap between the rigid foam and the fiberglass batt.
The dampness of the CMU wall will depend on what type of siding or cladding he installs on the exterior of the wall.
Hi Guys,
The block wall has no siding or cladding. It has 3 x layers of Behr Marquee Latex paint, and a single layer of latex paint on the interior. So, yes there is a possibility of rain intrusion to some extent. Does this change anything? :-)
Give yourself at least a 1/4" gap between the CMU & foam as a capillary break & vent channel, and use 2" polyiso instead of XPS. Polyiso is easier to reliably air seal using foil tapes, and will have a modest performance advantage in most US climate zones, inch-for-inch. Polyiso is also a lot greener than XPS due to the differences in the blowing agents used. Pentane has only about 0.5% of the 100 year global warming potential of HFC134a, the predominant component of the XPS blowing agent soup. As the HFC blowing agent leaks out over the next handful of decades the performance of XPS drops assymtotically toward R4.2 per inch down from it's labeled R5/inch.
Drying of the vent gap can be promoted with vent holes to the exterior near the top, and weep holes near the bottom. Without a vent gap and foam tight to the CMU, if the cores aren't both empty and vented to the exterior the moisture content of the CMU can rise, leading to more rapid paint failure.
If I were in that situation, I'd put all the insulation outside of the block wall and cover it with whatever siding/cladding is appropriate for the neighborhood. That would keep the blocks warm and dry, and make their mass into a thermal asset. The wall would be a lot prettier, too.
Just not feasible in my situation. But thanks! ;-)
Russell - You'd probably get more good ideas if you gave more info. Dana's recommendations sound good to me. Where are you located? Are you gutting the house? Single story? Ceiling? The vent space suggested has to be part of the unheated space, and well separated from the heated space. The sealed polyiso needs to be sealed to the floor and the ceiling. It may be possible to build the stud wall on the floor with the foam sheathing and with plywood or wood spacers that help to hold the polyiso on and create the vent gap when the wall is stood up. I wish you the best of luck with this project. Keep asking questions and the more info. you provide the better.
Tried to find the balance between too much info and not enough! ;-)
All good. I have all the info I need now. Thanks to all!!