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Exterior wall assembly in NW Iowa

We are building a new house with geothermal heat and a lite form basement and lite deck first floor floor. The first floor wall assembly is board and batten cedar siding on Tyvek drainwrap on 7/16 OSB on 2x6 girts at 30" oc with 2x4 vertical framed walls. We have R40 SIPs panels on the roof under a metal roof. We are planning on 2" of closed cell insulation on the walls. I have been reading about including batt insulation as well and was wondering if anyone had a recommendation either way?

We are also including a fresh air exchange in the HVAC system.

Asked by New House
Posted Aug 13, 2014 5:19 PM ET


5 Answers

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Where is the 2" ccSPF going to be applied? If applied to the OSB it would be advisable to use a "smart" vapor retarder between the gypsum and studs, since the ccSPF would limit drying toward the exterior, and would be a woefully insufficient foam-R/total-R ratio for dew point control at the foam/fiber interface.

The 2x4 wall can be reasonably insulated with R15 rock wool/fiberglass batts if you get obsessive about the cutting/fitting/tucking. It would be a bit harder to batt-insulate 30" girts, since it's not a standard batt-width. (Damp sprayed or dense-packed cellulose could work.)

Answered by Dana Dorsett
Posted Aug 13, 2014 5:49 PM ET


The ccSPF is going between and over the girts on the OSB. There will be a cavity between the studs for electrical, plumbing, see attached photo

Girts 2.jpg
Answered by New House
Posted Aug 14, 2014 10:43 AM ET
Edited Aug 14, 2014 10:43 AM ET.


What is holding up the girts?

How much space is there between the OSB and (eventual) wallboard?

My mental picture had had the girts attached to the 2x4s, and rotated with 5.5" between the 2x4s & OSB. This is clearly this is not the case.

Batts are a lousy choice when the surface bounding them has 1.5" deep steps, since it can never properly fill the space, leaving thin spots & long lateral voids that become thermal bypass channels. Empty voids for electrical & plumbing between insulating layers also become thermal bypass channels, and may even fail fire-code standards. Sprayed fiber (preferably dense packed) would be the only way to completely fill the cavity without thin spots or voids.

In US climate zone 6 (most of NW IA is in zone 6), when putting foam on the exterior layer of fiber insulated cavities the foam-R/total-R ratio has to be at least 35% unless you are using an interior-side vapor retarder. With 2" of ccSPF you're looking at about R12, which means you're limited to a maximum of about R22 of fiber to the interior side of the foam. If you end up with 5" between the surface of the foam and the gypsum you can dense pack 1.8lb fiberglass and still have pretty good margin, but if its more than 5.5" it is potentially problematic, and cellulose would be a better choice (slightly lower R, and capable of buffering wintertime moisture accumulations. If it's more like 6.5-7" you'll need to use an interior side vapor retarder- either "vapor barrier latex" or MemBrain, Intello-Plus,

Answered by Dana Dorsett
Posted Aug 14, 2014 2:16 PM ET


The girts are attached to the "bents", it's a post and beam framed barn home. There is 7-1/2" between the inside face of the OSB and the gyp (2" girt, 2" gap, 2 x 4 stud). The plan is to fill the 2" space between the girts and also the 2" space behind the studs to provide continuous insulation. I want to make sure we have sufficient R value, but don't want to create a moisture problem within the wall.

Answered by New House
Posted Aug 16, 2014 10:16 AM ET


A 7.5" total depth you'll be fine with 2" of ccSPF and 5.5" of blown cellulose:

With 5.5" of R3.7/inch fiber you'd be looking at about R20 for the fiber layer and R12 for the 2" of ccSPF.. The total center cavity R would then be about R32, making the foam about 38% of the total which is sufficient dew point control for a zone 6 climate to be able to use just standard latex on the gypsum for vapor retardency.

But do you really have fully 7.5"? The girts appear to be miilled 2x6 which are nominally 1.5", not 2". Is the gap also 1.5", or is it a full dimension 2"?

With board & batten siding on the exterior of OSB sheathing it will be important to have a rainscreen gap between the siding & OSB, since rain penetration is going to be somewhat higher than other siding types. A mesh-type system such as Obdyke Rainslicker is fine- even the 1/4" stuff is 1000x better than board & batten tight to the Tyvek.

Answered by Dana Dorsett
Posted Aug 18, 2014 4:45 PM ET

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