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Atlanta, GA
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773.398.5288

Advanced residential construction and home improvement consulting and owner's advocacy in Atlanta, using the latest building performance diagnostic and modeling techniques and tools. Airtightness, insulation, HVAC, ventilation, moisture, and air quality and EMF consulting for homeowners and building professionals alike.

Dear Corbett: How do I Protect Against Both Air and Water?

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Home performance articles and stories from the field with internationally respected building forensics guru Corbett Lunsford at the Building Performance Workshop. Hear new episodes of the Building Performance Podcast, see new videos from the Home Performance YouTube channel, and learn all about how diagnostic testing (more than an 'Energy Audit') can make home improvement and new home construction a proven process!

Dear Corbett: How do I Protect Against Both Air and Water?

Corbett Lunsford

Corbett - I have what I hope is a great 'old house' question for you regarding my home. I am getting my attic sealed and insulated, but I'm leaving the basement now to help troubleshoot if I am still pulling in air this winter.

I just discovered that my sill plate has plenty of gaps where one can even see daylight through the width of the entire foundation.  In unfinished part of the basement, the sill plate is easy enough to access and fill with cans of spray foam. 

However, in the finished areas, the drywall and wall blocking goes above the sill plate, and it is not accessible. It would be possible to use foam on the exterior, between the foundation wall and the original wood siding, which is underneath the aluminum siding.  This would also provide pest control, which would explain why I found a few ants this year.  

The question is, will that cause a moisture problem? Am I sealing in moisture that would normally drip out from behind the aluminum siding through the old wood siding?  Would it effectively direct the moisture or water into the house along the sill plate?  Is it simple to do, or would I be taking off the lower three slats of siding to properly seal?

Looking for details such as this online is very difficult, so I'm wondering if you already know of a great solution! ~Curt P.

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Thanks for your question, Curt! Though air sealing is of critical importance, when we're talking about air sealing from the exterior, make sure you don't create any moisture or durability issues for the house.

Spray foam used for air sealing is like a sponge, and it can wick water from under the siding into the house- I'd be leery of trying to fix it from the exterior for this reason.  If at all possible, seal the sill plate from inside, but if it's not possible, then just make sure you're able to monitor those areas that we're not sure about.

There's no standard solution I'm aware of for DIY repair of this issue, which is why I'd recommend having a professional take a look at it.  Just make sure they guarantee you a measurable test result of some type after home improvements have been done- that's the secret!
~Corbett