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Ventilation problem

Thank you for all the awesome information here. I wish I had known more about this site as a resource when my inlaws built their new cabin. Here is my concern/question...

Collectively, we built a new 40'x30' cabin in the Adirondaks , NY. The crawl space foundation was poured and box set by the builder and we took it from there with a few exceptions. The floor sheathing is Advantec, the roof and side sheathing is Zip-system with taped seams. Pine ship lap siding on the outside, metal roof over perlins, and the roof system is not ventilated. The house was built with scissor trusses in the front third and attic trusses over the bedroom/bathrooms to creat a loft. Insulation was spray foam and done by a contractor and was at least 3". Over the insulation went tounge and groove pine directly. The house is heated with a propane fired forced hot air unit. The only ventilation the house has built in are the bathroom fans. I'm worried about condensation that gets very heavy on the windows and what that means for every other place the vapor is driving through the interior siding and finding a cold spot. Am I worried about nothing? Do we need an hrv or efv to fix the problem? Could we just use some stand alone dehumidifiers? Thanks for the help.

Asked by user-7015801
Posted Feb 9, 2018 8:32 AM ET
Edited Feb 9, 2018 10:34 AM ET


5 Answers

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How often is the cabin occupied?

Answered by Steve Knapp CZ 3A Georgia
Posted Feb 9, 2018 9:25 AM ET


We are up there almost every weekend in the winter. When we leave, the heat is kicked down from 65 to 50. In the summer, we'll be up there a lot.

Answered by user-7015801
Posted Feb 9, 2018 9:30 AM ET


Here are links to two relevant articles:

Designing a Good Ventilation System

Revisiting Ventilation

Every tight house needs a mechanical ventilation system. The simplest ventilation system, which happens to be applicable to your case, is an exhaust-only ventilation system. While you are occupying the cabin, leave one or both bathroom fans on for 24 hours a day. That should clear up the condensation issues.

Construction moisture may take a year or more to dissipate, so you may need more ventilation during the first year of use than you will later.

If you want a more sophisticated approach, you can put one or both fans on a programmable 24-hour timer. If you want to upgrade, you can always install an HRV.

By the way, 3 inches of closed-cell spray foam has an R-value of about R-18, while 3 inches of open-cell spray foam has an R-value of about R-11. Either product falls woefully short of minimum R-values required by code for roofs (R-49).

Answered by Martin Holladay
Posted Feb 9, 2018 9:39 AM ET


If you don't have a humidistat controlled HRV, then when it's unoccupied, I'd turn off the ventilation and set the humidistat on a dehumidifier. Almost certainly less expensive to run, more accurate and will work in all weather.

On the other hand, some negative pressure (from exhaust only ventilation) is good for your walls/roof in Winter.

Answered by Jon R
Posted Feb 9, 2018 11:31 AM ET


Thanks for the info. I'll have them stick with the bath fan running and see how that works out. Also, in going back and looking at the spray foam (closed cell) pictures during construction, there is more like 5-6" of foam in the roof and walls. The foam is doing its job at keeping the roof insulated. The snow only slides off the roof when the weather warms, not when the heat gets bumped up to 65. Also, the propane company mentioned that we are barely using any gas, so it isn't requiring a lot to keep the place warm, even when it is -5 outside and the house is set to 50 for the time we are away. Again, thanks for the help and I'll dive into the articles you suggested.

Answered by user-7015801
Posted Feb 9, 2018 12:12 PM ET

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