SubSeal-40 as air barrier?
I have a client (Zone 4a) that is thinking about using SubSeal-40, made by MFM Building Products Corp, as a weather/air barrier on exterior plywood sheathing. The siding would be cedar shingles with a rainscreen. Interior 2×6 cavity insulation would be R-21 fiberglass batts, but may change to a flash and batt system.
I have never heard of the product and was wondering if it would work as an air barrier. I usually suggest Henry Blueskin.
I am also concerned that since it is rubberized asphalt, it may not allow the sheathing to dry to the outside if needed.
Link to technical data sheet below –
https://buymbs.com/Images/MFM%20Images/SubSeal%20Specification%20Sheet.pdf
Any thoughts would be appreciated.
Thanks in advance.
Gordon Grant
HERS Rater
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Replies
Gordon, I have not heard of that product but looked at the data sheet. They do not provide test results for an air test, but the vapor permeance is 0.01 perm (US) and it passes the ASTM water resistance test so it can be assumed that it's airtight. That is a very low perm rating; the product appears to be similar to ice and water shield. It would be ok with exterior insulation but would lead to moisture accumulation if you don't have enough exterior insulation to control the dewpoint.