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How does wall insulation work, and will cellulose insulation mold?

I am in the design phase of a small house approx 800 s/f, and have been planning on using 2x8" walls with staggered 2x4" studs. For insulation I am planning on wet blown in cellulose. I will have osb on the exterior, with Tyvek or some other vapor/moisture barrier and drywall/sheetrock on the interior, with no rigid foam. These walls should be pretty close to R-30,. In the attic, loose blown in to around R 40 to 50. I live in N. Colorado in zone 5B, and although we barely know what humidity is, winter temps can be -20F and summer temps +100F. This is not a passive house, but it will be fairly air tight in the winter, windows cracked weather permitting.
I started to worry about the walls after attending a talk about Net Zero houses put on by a local energy group, NCRES, and the speaker was from a SIPs manufacturing company. He said anything other than foam will rot. He was clearly biased toward SIPs, but it made me wonder, about dew points and how a wall actually works. We are making walls less and less breathable and cellulose is an organic material...
It is probably best to talk about the houses ventilation system in another thread, but we are thinking a ductless mini split, to supplemental heat and some trickle vents, with no HRV or ERV. We want inexpensive, mostly natural, and low tech solutions whenever possible.

Asked by Curt Lyons
Posted Thu, 05/08/2014 - 15:18
Edited Mon, 05/12/2014 - 06:28

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2 Answers

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The cellulose itself contains borate which prevents mold, but the sheathing can potentially have some risk of getting moldy, especially if it is OSB: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-risky-cold-os...

Answered by Nick Welch
Posted Thu, 05/08/2014 - 16:40

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First, Tyvek is a weather resistant barrier that is SOMETIMES also detailed as an air barrier, but is specifically desinged to NOT be a vapor barrier- it is highly permeable to water vapor, despite being relatively waterproof to liquid water. It's on the order of 10x more vapor permeable than standard latex paint(!).

In a zone 5B climate as long as you have a ventilated air gap between the siding and the OSB (aka "rainscreen" or "rainscreen gap" ) you will be fine with only latex paint as the interior vapor retarder. See:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_p...

An air gap between the siding & OSB qualifies as "Vented cladding over wood structural panels. "

I'm not clear on what your framing configuration is, and how thick the walls/insulation layer is, but if it's more than 5.5" (probably is) damp sprayed cellulose will take a long time to dry. If this is a foot-thick double studwall, if damp-sprayed in May it would be best to wait until October to paint the drywall. It would be better to use dry blown goods at 3.2lbs per cubic foot minimum, at which point it wouldn't matter how soon it got closed up & painted.

Cellulose can buffer quite a lot of wintertime moisture drive without damage or losing function, and thicker cellulose walls may not even need the rainscreen, as long as there is sufficient roof overhang to limit direct wetting of the siding. A builder in VT (zone 6A ) named Rober Riversong uses about 1' of overhang per story, and builds R50-ish cellulose insulated walls without rainscreens or vapor retarders with good results, despite being outside of IRC prescriptions. See: http://www.gree nbuildingadvisor.com/homes/thick-cocoon-cellulose-protects-superinsulated-house But note, his houses are build with pine or hemlock plank sheathing, not OSB, which allows the wall about 2x the drying rate that you'd get with OSB. Then again, his is a colder & wetter climate than yours (on average, not peak, and it's the averages that matter.) But with a rainscreen, properly lapped flashing, and good roof overhangs you'll be in good shape.

Answered by Dana Dorsett
Posted Thu, 05/08/2014 - 16:46

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