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New home — attic moisture

Our 2 y/o old home has had moisture issues in the attic since we moved in. It's a Cape-style home with closed-cell foam insulation and touted as a "tight" efficient home.

I noticed moisture on the roof sheathing, small spots of green and white mold, and rusty nails in the attic and musty stagnant smell in the upstairs bedrooms in the summer. I hounded the builder and original insulation company the first year. There were a few issues that I thought may have contributed but they were corrected; i.e., disconnected dryer vent in first-floor joist chase, poorly sealed bathroom exhaust in kneewall space, and a 3rd floor bathroom exhaust that traveled too far across attic.

Since then that bathroom exhaust has been vented directly through the roof. 3rd floor has kneewall entire length of house from and back. Attic ventilation is soffits with ridge vent.

Anyway, its been cold and snowy here, finally we had a slight thaw, we came home to water dripping from north gable ended window casing in 3rd floor bedroom. I inspected the attic and found frost on mainly north end gable sheathing and some rafters. The frost melted and leaked down into bedroom finding its way out the window casing.

The humidity in the house is low, dehumidifier barely collects any water, although we have condensation and frost on windows in the winter. I also noticed moisture in between foam and batt insulation on gable ends in kneewall space.

What gives? Is it an air seal issue from kneewall space to attic?

Asked by Brian Peck
Posted Wed, 01/08/2014 - 21:18
Edited Thu, 01/09/2014 - 06:26


7 Answers

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It's hard to be sure what is going on without a site visit, but all of the symptoms you describe (especially the fact that you have condensation and frost on the interior of your windows) are consistent with high indoor humidity.

A tight home like yours needs a mechanical ventilation system, but you haven't told us whether your home has one. For more information on this topic, see Designing a Good Ventilation System.

There is no need to operate a dehumidifier during the winter. If you want to lower you indoor humidity levels during the winter, the best way to do that is to increase the ventilation rate (assuming, of course, that your house has a ventilation system).

I don't know why your dehumidifier "barely collects any water." It's possible that your dehumidifier is broken.

Whenever a house has high indoor humidity, it's important to (a) identify the source of the moisture, and (b) increase the ventilation rate during the winter.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/09/2014 - 06:32

Helpful? 0

Thanks, I have measured my indoor humidity with a digital gauge and it reads between 30-45%, the dehumidifier is brand new (I thought the old one was broken because it wasn't collecting water). I don't have ventilation system but I have been trying to educate myself about them. Are there cases where HRVs installed to ventilate living areas have helped reduce attic moisture.

Answered by Brian Peck
Posted Thu, 01/09/2014 - 08:19

Helpful? 0

The source of your attic moisture is probably indoor air that is leaking through your ceiling. Blower-door-directed air sealing could be used to seal the air leaks in your ceiling.

Yes, an HRV will lower your indoor humidity level. Since your indoor air is almost certainly the source of the moisture in your attic, it will also help solve your attic problem. But you should also investigate those air leaks.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/09/2014 - 08:36

Helpful? 0

A reading of 45% RH @ 70F is pretty extreme for wintertime air, and high enough to cause all sorts of issues over time, whereas 30-35% is fine.

As Martin said, air leakage into the attic from the conditioned space is by far the most likely cause of the moldy attic. If the ducting for the bath exhuast vent (even the new one) is leaky, it's contributing big-time. Hopefully the seams & joints are all sealed with duct-mastic, and ceiling penetration for that (and all other ceiling penetrations) rigorously sealed with foam/caulk?

Homes often end up with plumbing or flue chases running unsealed from the basement to the attic too.

Kneewalls are also often notoriously difficult to seal properly, and often have thermal bypasses between the floor joists running between the attic spaces, often with air leaks even through the flooring. If it doesn't have air-dams in each joist bay at the kneewall, it's a necessary first-step to install them, and given how time-consuming that can be, do it prior to the blower-door testing so you can reasonably fix the remaining leakage during the test.

Answered by Dana Dorsett
Posted Thu, 01/09/2014 - 13:16

Helpful? 0

I'm wondering where your "closed-cell foam insulation" is located? it sounds like you have a lot of air leakage. Did the builder or the Energy Star inspector do a blower door test? What was the result?

Answered by Bob Irving
Posted Thu, 01/09/2014 - 16:43

Helpful? 0

Thanks for all the advice. So after doing more digging I found a large plumbing chase that has no air barrier just batt insulation with blown in cellulose on top. I also found that there is no air barrier in the rafters bays where it meets the knee wall. What technique can be used in that situation. I thought of cutting blocks of rigid foam and fitting it in each bay and sealing with foam. Question is will the expanding foam crush the plastic air channel vent that runs from the soffit. how thick of rigid foam is suitable for an air barrier? Thanks again

Answered by Brian Peck
Posted Fri, 01/10/2014 - 07:02

Helpful? 0

Here is a link to an article you might want to read: Two Ways to Insulate Attic Kneewalls.

You're on the right track. Rectangles of rigid foam can be used as blocking between your rafters. Ordinary canned spray foam can be used to seal the perimeter of each piece of rigid foam; unless your ventilation baffles are unusually flimsy -- and I hope they aren't -- you don't have to worry about the expanding foam compressing the ventilation baffles.

You might also want to read this article: Air Sealing an Attic.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/10/2014 - 08:48
Edited Fri, 01/10/2014 - 08:53.

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