Condensation in eaves with non-vented soffit

[tlj87]

At the beginning of the month, we moved into what was previously a three season, weekend only home. The building was constructed in the early-1970's and has non-vented soffit and no ridge vent. The walls and entire roof are insulated with foil backed fiberglass insulation (all space is finished including the attic). While in a kneewall today, I pulled back some of the insulation to look at the eaves and found the west end of the building (in photos, the end to the left of the dormer) were damp about 2-3 feet up from the soffit. I cut out the damp insulation and have fans drying the areas out. There is no obvious mold, punky wood or other issues although there is some slight discoloration which leads me to believe there has been some moisture here before. It is winter (almost) and any drastic measures are not feasible until spring, so I seek any advice to get us through the winter or maybe even completely address the issue. I've attached a few photos for reference. Thanks in advance!

 

[MikePA]

Our house, built before 1900, had a similar problem. Although our attic was not finished, it didn't have any eave ventilation.

I could reach the eaves from the attic and I cut rectangular openings in every other space between the joists and screwed aluminum, screened vents in the soffits. I also removed all insulation (rock wool) from the attic, put down a plastic vapor barrier, and poured in loose insulation (vermiculite) since the spacing between the joists was not consistent and didn't match up with modern insulation rolls. The key was the plastic vapor barrier and the eave vents. Fortunately, there were small windows in the end attic walls that allowed for cross ventilation with the eave vents.

It sounds as if you've done all you can do for now, unless you want to start cutting soffit vents before the snow flies.

 

[/pine]

FWIW...I would make sure it is not a roof issue rather than 100% condensation...

 

[dave1949]

Sounds like a tough problem. I could believe it is all condensation but it never hurts to check the roof. That condition would not happen much during winter in a 3-season house that was left unheated and no humidity sources present like cooking, showers, and breathing .

The soffit vents need somewhere to vent to, either gable end vents or a ridge vent; those two aren't supposed to be combined. A vapor barrier is needed too. With soffit vents alone I wouldn't be surprised to see icicles hanging from the vents.

Is there a possible airflow path from the soffits to the ridge?

 

[tlj87]

How would I know if it's a roof issue? The roof was replaced in 2008.

 

[/pine]

How would I know if it's a roof issue? The roof was replaced in 2008.

That can be some tough troubleshooting...

First some info on non-vented/insulated spaces...the key is making it sealed...i.e., solid (bird) blocking between the rafters over top of ext. walls is a must (any cracks/gaps should be calked) if moist air can't get in...there should be no moisture to condense...

If the present moisture is only evident in a localized area...and it does not appear that the issue has been an ongoing thing (based on your description) I would tend to think that something has changed and usually this means the roof...i.e., a nail has popped up etc... however if there is not solid blocking between the rafters/trusses (that seal between the top of the wall up to the roof decking)...then damp air could be permeating the insulated space via the soffiting...

Sometimes a popped up nail will allow rain water to penetrate but instead of dripping directly below the nail...the water can run down a rafter etc. then spread horizontally if hits a cross brace member, "rat run etc., etc...

Also if the "eaves drip" is not sealed properly at the very edge of the roof...water can get sucked (capillary action) up below the roof membrane but this usually only effects the first several inches of the eave... if this was the case there would most likely be some sign on the fascia board/drip edge...the most likely places are at the joints of the them eave drip metal...

 

[tlj87]

Some good info here, Thanks! I have been looking at some diagrams online and am wondering, Would it make sense, once I get it dried out, to cut some rigid foam insulation to fit vertically between the soffit and the roof (essentially between the painted white plywood gussets in the first photo)? If yes, how would I tie that into the existing foil face fiberglass insulation that starts above that (since I already trimmed it back)? Would that keep the eaves isolated from interior moisture? Would this cause ice issues?

 

[dave1949]

Some good info here, Thanks! I have been looking at some diagrams online and am wondering, Would it make sense, once I get it dried out, to cut some rigid foam insulation to fit vertically between the soffit and the roof (essentially between the painted white plywood gussets in the first photo)? If yes, how would I tie that into the existing foil face fiberglass insulation that starts above that (since I already trimmed it back)? Would that keep the eaves isolated from interior moisture? Would this cause ice issues?

Condensation, or ice if the condensation freezes, happens when warm moist air--which is normal in any home--contacts a surface that is below the dewpoint of that warm moist air. It is easy to have surfaces below the dewpoint in winter, it doesn't take much moisture in the air to do that.

For example at an air temperature of 72F and a relative humidity of 50% (an ideal indoor value), the dewpoint is 52F. 52F is way above the winter air temperature of the eaves so condensation happens there.

What it comes down to is to not allow the moisture present in the interior house air to contact a cool surface. That is the purpose of the vapor barrier. The purpose of the insulation to slow down heat transfer, and to keep the vapor barrier warm enough that it does not become the condensation surface. The foil face on the fiberglass is an example of that concept.

Anything you do that follows those principles will work.

 

[MossRoad]

Temporarily, I'd put a fan or two to circulate the air and possibly a de-humidifier until spring. If it gets below freezing in that space, though, a de-humidifier may get frozen up and damaged. If its a heated space, run a hose from the dehumidifier down to a drain somewhere. That's easier than emptying a full tank twice a day.

 

[dave1949]

Looking at the first pic of the interior eaves, I assume you are behind a finished knee wall to the inside of that area?

That knee wall should separate the heated from the unheated area IMO. The interior knee wall would be one section of the conditioned space "envelope". What is outside of that should not be insulated and you want free air movement there too. The issue probably originates from the first floor ceiling area that is below that knee wall floor. That part of that ceiling is also an envelope barrier area.

If you put down a plastic vapor barrier on the floor behind the knee wall, then put un-faced fiberglass batts on top of that you will improve that part of the envelope. Add soffit vents to allow what makes it through the envelope to get some ventilation.

The vapor barrier for the knee wall floor really belongs on the warm side of that--the first thing on the first floor ceiling--but that would be a major project. You may see some condensation under the vapor barrier because of that but it could get you through the winter without major issues.