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Moisture in wall assembly...

From doing a job recently (wiring) involving cutting into the sheetrock, I saw that there was moisture against the interior side of my plywood sheathing. This is a 36 year old post and beam house, owner built, using what was typical for the time, specifically, interior painted sheetrock, then 3.5 inch fiberglass either foil or kraft faced, plywood sheathing, tyvek wrap, then siding which in my case is cedar shingle. Perhaps the only non-convention is the use of rough cut lumber, so the 3.5" of fiberglass is in a 4" wall, but not sure if this would impact the situation.

This followed some very cold weather (we saw 15 below), but I have no reason to think this was a rare incident, as some mold was visible. However, it did not appear that the plywood was very far toward rotting. Not a great situation, - a bummer in fact, but in the grand scheme of things, not life and death!. What are my options?

Asked by Howard Gentler
Posted Mon, 01/06/2014 - 09:18
Edited Mon, 01/06/2014 - 09:20

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Howard,
Only you can decided whether you want to address this problem. If you leave things as they are, it's possible that your only problem for the next 30 years will be high energy bills.

If you want to fix it, you can. You can start with cheap & easy, and move up to expensive & difficult.

Your house is typical, but it has a big problem. The walls were built with no attention to airtightness, and they are poorly insulated.

The mold and moisture are signs that you have air leakage through the walls. Interior moisture is carried on the exfiltrating air, and the moisture in the air is accumulating on your sheathing.

The cheap & relatively easy fix is to perform blower-door-assisted air sealing work. (If you search the GBA site for "blower-door-assisted air sealing," you should get lots of articles to read.) Since you know that you have lots of air flowing through your walls, you might pay attention to the crack between the bottom of the drywall installed on your exterior walls and your subfloor or finish flooring, as well as leaks at electrical outlets and around windows.

The expensive & difficult fix is to open up your walls and install new insulation, or, alternatively, to add several layers of exterior rigid foam and new siding. This work should be performed by someone who understands air leakage issues.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/06/2014 - 09:38

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