Damp Sheathing with Unvented Conditioned Attic
home26
| Posted in Energy Efficiency and Durability on
Climate Zone 1a 3300 sq ft home
I am planning on installing open-cell spray foam on the roof rafters and seal the attic. I read the article about damp sheathing and was hoping to avoid it.
1) Do I need to add HVAC supply to the attic to avoid humidity? If so how many CFM?
2) By creating a tight envelope will I need an ERV or a whole-home dehumidifier to bring in fresh air and keep moisture low?
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Replies
Hi Home.
Hopefully some of our more mechanically-oriented members will jump in with some thoughts. As you read in the article, damp sheathing with open-cell foam is more of a concern in colder climates than your. In general though, you'll want to watch humidity levels in the attic. Treat it like the rest of the conditioned space.
A tight home needs balanced ventilation, so an ERV is a good idea. Also, in a tight and well-insulated home, the air conditioner may not run enough to dehumidify the space, in which case, you may need a dedicated dehumidifier.
A good HVAC designer should be able to spec all of this for you.
> the air conditioner may not run enough to dehumidify
Imagine the day that is very humid and 75F - the air conditioner won't dehumidify at all. So unless that never happens, install dehumidifiers(s) and an ERV.
On the other hand, whole-house is expensive and in most cases, a couple portables will provide good enough comfort.
Would vapor diffusion ports at the roof ridge work? Dr. Joe Lstiburek created the concept. Wondering if that would work here with open cell foam.