Need clarity on vapor diffusion ridge openings of conditioned attic
alexdorf
| Posted in GBA Pro Help on
Hi all,
We’re looking at doing an unvented / conditioned attic in Southern California and struggling with the detail for a vapor permeable membrane at some of the ridges at different parts of a roof, with multiple hips.
We’re planning on spray forming and enclosing the attic spaces. Our greatest concern is that of moisture affecting any high point in the attics.
IF we have ample spray foam, would this not be an issue? We would be looking to go above and beyond the dew points. Thanks!!
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Replies
If it’s fully conditioned, with proper air conditioning supply and returns in the attic this shouldn’t be much of an issue. Install some humidity sensors up there to keep track and if needed you can put a plug-in dehumidifier in the attic space if needed. Or if you already have a whole-house dehumidifier planned just make sure to include return/supply ports to this attic space.
Edited to add: you are talking about closed-cell spray foam, correct? Lstiburek’s paper points out that open-cell is a bad idea in this application because it allows interior humidity to migrate up through the open-cell and condense on the underside of the roof decking leading to rot. Exterior rigid would also be a good solution instead of interior spray foam.
Somehow, I get the feeling the space will never really be conditioned.
Yes, some people do win this game of Russen roulette with an unvented unconditioned attic.
No one can predict the winners or losers in advance but living in a desert could be an advantage.
The safe bet is to build a code compliant vented attic.
The smart thing to do would be to not live in the attic and keep the HVAC system out of the attic.
The low cost thing to do is cover the attic floor with a thick layer of cheap fluffy insulation.
The green think to do is avoid spray foam insulation.
Walta
alexdorf,
If you use closed-cell, as Matthew said, you will be fine. If you use open-cell you need to keep the RH in the attic low to stay safe.
You can specify a permeable roof underlayment, but whether you will get any useful drying to the exterior depends on how your roofing is installed, and what type it is.