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Critique my wall assembly! (Tstsuds, wood fiber insulation, CDX, Tyvek)

danielhowe | Posted in Green Building Techniques on

Dear GBA Brain Trust:

I am 1-3 years out from a new build, most or all self-built. While I find a lot of building science talk interesting, I get worried that some of it disguised marketing, puts homebuilding out of reach for all but higher-end pros. Having grown up in two houses built by my parents (the second one superinsulated and ahead of its time for 1990) I’d like to do the same. Priorities are simplicity of assembly, insulation, repairability, low-chemical, cost. ASHRAE climate zone 5A (coastal Rhode Island). 

Kicking around ideas for a wall assembly:
– 2×8 Tstuds, 24″ o.c.
– blown-in wood fiber insulation from TimberHP (they have been saying their price point will be similar to cellulose)
– 1/2″ CDX plywood sheathing
– standard (Tyvek?) house wrap
– nonwoven fabric-type rainscreen
– either Hardi or cedar shingle exterior; cementboard or PVC trim

As of now I’m thoroughly confused over the state of the “cold sheathing problem” debate. Should I be prioritizing an insulated sheathing produce (i.e. Zip-R)? (I’m not a big fan of continuous exterior insulation. I am suspicious of a lot of layers: certainly for an amateur like me, there are a lot of opportunities to get it wrong.)

I’m also open to thoughts on Tstuds and on the new wood fiber products coming out of TimberHP. 

Can answer more questions if needed. Open to all serious suggestions (as well as non-serious suggestions). 

– dh

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  1. walta100 | | #1

    To my eye your plan lacks an affective air barrier. When I see house wrap install on real construction sites it never looks air and water tight to me on the day it gets installed, 6 weeks later when the siding crew comes to cover it is always ripped and flapping in the wind.


    1. jollygreenshortguy | | #2

      Yes. One approach to an air barrier in this assembly would be to do a thorough taping job of the CDX sheathing before the tyvek goes up. Make sure the sheathing is dry. Use pressure and a primer if necessary.

  2. andyfrog | | #3

    You might consider a lower permeance housewrap, something in the 10-15 perm range. Tyvek Commercial would be the budget option, but there are many others.

    The air sealing notes from others are important.

    Before you plow ahead, it would definitely be a good idea to understand the value of continuous exterior insulation.

    This is a good lecture on that subject:

    Christine Williamson: - Stop worrying about the dew point

    In short, exterior insulation prevents condensation by keeping your sheathing warm. There are definitely additional constructability challenges with exterior insulation, but it is by far the technically best way to improve wall assembly durability versus moisture.

    If you want to stick with your existing assembly, read up on various double stud wall options. That's the type of assembly you'll find guides for that is relatively speaking, the most similar to the one you're proposing.

    For double stud and thick cavity insulated walls, a really important factor is the bulk water details.

  3. boxfactory | | #4

    I have been following the TimberHP news articles for a few years now, very glad that they seem to be up and running.

    Has anyone used their blown in product themselves? How did the ordering and delivery process go? How did the installation go?


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