How well sealed should exterior foam board be?
External Rigid Foam Insulation Board Question: How well sealed should the foam board be?
I am building a house on the Gulf Coast with 7/16 OSB sheathing on the exterior walls (required by code here). I plan on using Tyvek “DrainWrap” on the OSB then Install R5 Dow Foam board over the Tyvek. The cladding will be brick. I have most of the details about sealing and taping the foam board figured out via various GBA, EEBA and BSC publications but am still unsure if I should caulk and seal the top and bottom of the foam board at the top plates and the sill plate. I understand the need to prevent air flow behind the foam boards to achieve max thermal benefit but is there a need to leave a “path” for a little ventilation behind the foam board for drying purposes?
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Replies
Leroy,
The Tyvek Drainwrap is optional. (Of course, every wall needs a WRB, but it doesn't have to be Drainwrap, and it doesn't have to be installed in that location.) If you want to install housewrap, you can install it under the rigid foam or over the rigid foam -- your choice. More information here: Where Does the Housewrap Go?
Including a wrinkled housewrap like Tyvek Drainwrap between the rigid foam and the OSB provides an increased margin of safety in terms of moisture management, at the (slight) expense of a (slight) degradation in thermal performance. If you want the Drainwrap, then you should probably skip the caulk -- because you want some drainage at the bottom and a small amount of airflow at the top.
Martin, thank you for the response. I understand and agree with leaving the top and bottom of the foam board unsealed. The house wrap as a WRB is required by the local inspectors but it must be behind the foam board due to driven rains concerns. Building science is not well understood or widely practiced along the Alabama Gulf Coast yet. I am hoping to introduce my local code officials to it via my house as I build it. Nevertheless, they are using the 2012 IRC and IECC but their approach to enforcing the IECC is to have the engineers and architects deal with it as everything built here has to be stamped by one or the other due to hurricane concerns. Of course very few of them have any idea what to do with energy codes. Which, hopefully, is a good opportunity for me. Thanks again for the input on the foam board.