Polyiso directly over Tyvek stucco wrap vs 1/4″ shims between
I am a remodeler, project is my own whole house remodel in North NJ (Morristown – Zone 5). After reviewing your excellent information and blogs the envelope planned is:
Latex paint / 1/2″ drywall / 2 x 6 conventionally framed walls (existing) / EITHER R19 6.25″ Kraft faced fiberglas insul OR R15 3 1/2″ KF fiberlgas insul (leaving 2″ gap in front of the sheathing) OR 3 1/2″ R14 open cell foam insul (which leaves a 2″ gap behind the drywall) / Sheathing (existing) is 1×6 boards (old sections) or 1/2″ plywood (new sections) – varies / Tyvek stucco wrap / Atlas R9 1.5″ polyiso w/ taped joints / plus vertical and horizontal battens with insect etc. detailing to support cedar shingles. (like the current FIne Homebuilding garage/workshop project.) Overall wall assembly seems to come in around R 27 + / – and my DOW dew point analysis shows .001 to .002 oz/day moisture accum. at the sheathing layer in all three of these insulation scenarios.
1. I notice when I made up my sample section that when I place screws in the polyiso to attach to the sheathing or to screw the battens through it seriously flattens the stucco wrap below the screw locations – particularly for 6″ around each screw – it doesn’t seem to maintain the 1mm gap (but I have no real way to measure it) So…should I also place vertical 1/4″ +/- spacers/battens between the tyvek and the polyiso?
2. Any better suggestions on the wall-cavity insulation?
Thank you in advance for your help.
Carl Bruen
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Carl,
Good for you for building a sample section and noticing that the Tyvek StuccoWrap was getting squished. Frankly, I don't think it matters much, because the chance that you will have liquid water between the polyiso and the Tyvek is extremely small. Even if you ever get moisture there, there should be some available channels for drainage to the right and left of each vertical batten location.
Why would you consider installing 3.5-inch-thick fiberglass batts in 2x6 stud cavities? I don't recommend that insulation option.
I agree with Martin to leave the Tyvek behind the polyiso... and I'll make the suggestion to use dense packed cellulose insullation in the cavities, and air-tight drywall approach.
Thank you Martin and Armando.
Martin -
Thanks for the reassurance about the squished tyvek stucco wrap.
An insulation rep had suggested the 3.5 in Fiberglas as a way to allow some "drying to the interior" since as the insulation increases to R19 Fiberglas the dew point modeling shows increased moisture. ( I guess as a result of the ratio of interior to exterior insulation changing.)
Armando -
I will check into the dense pack cellulose.
Carlton,
In your climate zone, with 2x6 walls, you can fill the entire stud bay depth with fiberglass or cellulose insulation, without any condensation concerns, as long as you have at least R-7.5 of exterior foam. Since you are planning to install R-9 foam, you are fine. There is no need to skimp on the insulation between the studs.
For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.