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Community and Q&A

Cellulose, Housewrap, Foam Board, and Rainscreens?

Aaron Carlberg | Posted in General Questions on

I previously asked, what is the proper wall assembly for dense packed insulation? After doing some research online I have discovered that dense packed cellulose can absorb and release moisture vapor or condensation without degradation to itself or adjacent building materials. However, any moisture absorbed by the insulation needs to be able to escape. The companies that manufacture cellulose do not recommend using vapor retarders in general. Interestingly, they use the term vapor retarder and vapor barriers interchangeably. The current International Building Code (and its companion codes) defines a vapor retarder as being 1.0 perm or less when tested with the desiccant method using Procedure A of ASTM E 96. Based on the response I received on my last post and this information, it appears the use of house wrap would be appropriate as long as it has a 1.0 perm rating or greater. This information leaves me with a few follow up questions.

Can anyone recommend a good house wrap for my situation?

I am thinking about adding some eps, xps, or polyiso in addition to the house wrap. 1) I have read that eps, xps, and polysio all have different permeability ratings; will the addition of such a product inhibit the perm rating of the house wrap and the ability of the moisture to escape the cellulose insulation? 2) Any suggestions on which type of foam board I should use in conjunction with the house wrap? 3) Should the foam be installed on top of or under the house wrap? 4) Can anyone recommend a specific product?

In regards to creating a rain screen, can the house wrap or foam board be used as a drainage plane? Any preferences on using furring strips in conjunction with Cor-A-Vent or drainage mats like Driwall rainscreen.

In addition, I plan on siding the house with fiber cement. Can anyone recommend a superior way of fastening the siding to the wall assembly? What length nail or screw should I use to penetrate through the foam board if I do not use wood furring strips? Can I apply the siding directly over the drainage plane?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Aaron,
    Q. "It appears the use of house wrap would be appropriate as long as it has a 1.0 perm rating or greater."

    A. Why are you confusing the definition of a vapor retarder with your preferred permeance for housewrap? The two issues are not connected. No housewrap is a vapor retarder; they are all designed to be vapor permeable.

    Q. "Can anyone recommend a good housewrap for my situation?"

    A. Any housewrap can be used with cellulose insulation.

    Q. "I have read that EPS, XPS, and polyiso all have different permeability ratings; will the addition of such a product inhibit the perm rating of the house wrap and the ability of the moisture to escape the cellulose insulation?"

    A. Yes, the use of rigid foam sheathing prevents wall assemblies from drying to the exterior. However, such walls are design to dry to the interior, and they work well. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "Any suggestions on which type of foam board I should use in conjunction with the house wrap?"

    A. If you want to conform to green principles -- and I think you should -- you should avoid the use of XPS, because it is manufactured with blowing agents with a very high global warming potential. Choose EPS or polyiso.

    Q. "Should the foam be installed on top of or under the housewrap?"

    A. Here's your answer: Where Does the Housewrap Go?.

    Q. "Can anyone recommend a specific product?"

    A. Are you asking again about housewrap? Or are you asking about something else?

    Q. "Can the house wrap or foam board be used as a drainage plane?"

    A. Yes. For more information, see All About Water-Resistive Barriers.

    Q. "Any preferences on using furring strips in conjunction with Cor-A-Vent or drainage mats like Driwall rainscreen?"

    A. Either vertical furring strips or plastic drainage mats work fine; it usually boils down to builder preference, price, and the type of siding you are installing. Most builders who install fiber-cement siding use vertical furring strips.

    Q. "Can anyone recommend a superior way of fastening the siding to the wall assembly?"

    A. To fasten siding to the furring strips, follow the manufacturer's recommendations. To learn more about attaching furring strips, see Fastening Furring Strips to a Foam-Sheathed Wall.

    Q. "What length nail or screw should I use to penetrate through the foam board if I do not use wood furring strips?"

    A. Check with the siding manufacturer. Again, I recommend the use of furring strips under fiber-cement siding.

    Q. "Can I apply the siding directly over the drainage plane?"

    A. If you are asking whether you can install siding directly over housewrap without a rainscreen gap, the answer is yes, although I don't recommend it -- as long as you're not using foam. If you are installing foam sheathing, you need an air gap.

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