Moisture in Attic
There is moisture in my attic in the winter (rusty nails and wet plywood) on the front (north) side of the house where there is no ventilation. There also appears to be some staining that occurs on the siding at the back (south) side of the house where these is venting. Humidity seems to remain high in winter particularly on the 2nd floor. Indoor temp is usually between 66F-70F and indoor relative humidity ranges between 42-47 percent when outdoor temps range from high 20F to low 40F
I added a fan that runs continuously to move 100CFM out of the basement, but it didn’t seem to help much with my moisture problem last year. Where should I go from here? Add more attic ventilation? Below are the facts.
Facts
1) Purchase house with mold on the roof plywood in attic.
2) Removed moldly plywood when I got a new roof and fixed bath vent that was blowing into attic and vented it through the roof.
3) Got an energy audit from MassSave and they came in and air-sealed the house and blew more cellulose insulation into the attic and added 2” rigid to the front sloped rafters. See pic. for more details.
4) MassSave blower door test before air sealing was 2050/CFM 50 and went down to 1340/CFM 50. Based on my building volume this is about 3.2 ACH/50 (moderately tight).
5) House has full venting on the back side of the house ONLY, nothing on the front. Per MassSave it meets the IRC minimum of 1 square foot of net free ventilation area for every 150 square feet of attic floor space.
6) Mini Split Heat Pump heating system.
7) Climate zone 5 (Boston area).
8) No poly below basement slab and water table is somewhat high because water remains just below the basement slab for 4-6 months of the year.
9) The basement always remains dry. No standing water. Most of the house appears to have clay soil around the basement. I used Dry-Lok paint in a few areas on the slab and most of it has bubbled up.
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Replies
A vented attic should ideally be completely air sealed from the interior of the house, so interior conditions shouldn't be a factor. The problem is that north face that isn't vented. Even though the roof overall has sufficient venting there needs to be a way for moisture to get out of that section of roof.
I see two ways out. One is to clear a path from the soffit vent to the attic above the blown-in insulation, to allow ventilation. The other would be to make the whole north side unvented, which probably means replacing the insulation with spray foam.
This article explains the issues:
https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
I would guess that the staining of the siding is unrelated.
So would you spray foam just the lower section of the north side? Or would you spray foam the lower and top of the north side and go all the way to the ridge?
I addressed the high humidity levels in my encapsulated attic by adding an Ecor Pro desiccant dehumidifier. Still monitoring the results but it is keeping the RH lower than 60%.
I think your biggest issue here is that you don't have a continuous ventillation path at the first knee of your gambrel roof, where the "mostly vertical" part transitions to the "mostly horizontal" part. You need air to be able to move relatively unimpeded from the soffit intake vents up to the ridge exhaust vent for the vented actic assembly to work properly.
Since you've already done air sealing work, you probably need to focus on improving ventilation. I see two ways to do this here. See if you can open up air passages through that solid blocking you mentioned. You can sometimes do this with a hole saw if you can get enough access to the space to work. A series of holes can accomplish this without causing structural issues if you don't go overboard with your hole size or placement (look up pics of the ribs in an aircraft wing, you want to do something that looks like that). I'd do that as a first choice, since it will look better from the exterior, but it's more labor. The other option is to install some roof vents down low on the upper part of the gambrel roof (the "mostly horizontal" part) to provide an alternative air intake for the upper attic. Note that you probably have stagnant air in the upper part of the lower attic (the "mostly vertical" part) too, since that air doesn't have an upper exhaust vent, again due to that solid blocking.
Bill
It's not clear that the mostly vertical part is properly vented, the drawing just shows "HD fiberglass." If there's no vent channel behind that fiberglass that's a problem.
The most vertical part is definitely NOT vented properly. The fiberlgass is pushed all the way against the roof plywood. Attached is a pic. that shows a blown up version of that north corner. I think I'm going to start by trying to drill 1" holes through the plates into the attic in a few bays and remove the rigid/fiberlgass and add the vent channels to get the air-flow from bottom to to top.
So here is a blown up pic that shows that north corner. I think you are correct that the first place to start is drilling 1" holes through the top plates in each bay to connect the air-flow from the top to bottom. It will be a pain to-do but I think its necessary. Do you think 1 - 1" hole per bay would work?
How big are the soffit vents? The holes are going to need to have at least the same area.
Atop the top plate you're going to need some sort of baffle to keep the blown-in insulation from blocking the vents.
So the soffit vent is a vinyl vented soffit. attached is a picture. It has 9.19 sq. in. of "Net Free Area of Ventilation" per sq. ft. I'll probably just use these generic rafter baffle proper vents below from HD. I'll just have to make sure they extend from bottom to top and that they don't get "crushed" during the install process.
https://www.homedepot.com/p/Owens-Corning-Raft-R-Mate-22-1-2-in-x-4-ft-Attic-Insulation-Rafter-Baffle-Proper-Vents-10-Pieces-70RM/204848302