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Frost on underside of roof sheathing


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Hi there. After looking for a similar scenario already in the forums I came up with only a few results which touched on my issue to a degree but not as much as I needed. So I decided to start a new thread. Read only if you've got some time on your hands, I'm afraid I wrote a book.

First a little background; I'm a general contractor in southwest Virginia. We do some residential new construction but most of our work consists of remodels, additions, etc. All that to say that I've seen a good amount of nasty renovation situations.

I was called by my former landlord to check out some gutters that were leaking at the single story apartment I used to rent. It was built 50 years ago and has old asbestos siding without any housewrap. The apartment is attached to another apartment but at a 90 degree angle to the other one. So if you were to picture an L shape except that both portions of the L were the same size. The roof pitch is a 3/12 with asphalt shingles and felt. Some of the sheathing is CDX and some is the old 1x horizontal bracing. The attic space has neither soffit, ridge, or gable vents. The single bathroom is unvented.

When I got there the current tenant asked if the landlord had asked me to check the water and mold in the apartment, I said no but that I could check it out for her. All along the western facing side of the apartment they had moisture at the baseboard and in some places up 2' on the walls. On a portion of the south facing gable end there was moisture along the baseboard and it actually was moist all the way up to the ceiling. Mold was forming in all these locations. I'm pretty sure that some of the moisture on the western facing side of the house is due to the siding "failing" due to the tremendous amount of rain and stormy weather we have received in the last 1 1/2 months. The water coming from the ceiling was another matter that I wanted to explore in the attic.

The next morning I went into the attic which resulted in finding that frost had formed on the underside of the roof sheathing and on the roofing nails. There was mold on pretty much all the rafters, rot at the sheathing by the eaves (as a result of the gutters overflowing and wicking up). There was a lot of black and white mold on the southfacing gable end (the same end that the moisture was from floor to ceiling) and on the roof deck around where the chimney is.

I went back later to inspect both sides of the roof after things thawed out and this is what I found. What I found was that on the outside the chimney which is attached to the exterior of the apartment and runs through the soffit area of the roof was not flashed with aluminum, but merely had tar applied to the intersection where it projected from the roof. There were 1/4" gaps with nothing to keep the water from entering the attic. There were also several issues with the intersection of the roofs where the 2 apartments meet. On going back inside the attic, the frost had melted and the underside of the sheathing was wet, not so much that it was dripping, but enough that it was damp. The sheathing at the peak of the roof was saturated with moisture. Now, there is no ridge vent on this apartment, infact the roofer merely cut down 3-tab shingles to use for the ridge cap. Then of course there was the water getting in at the chimney.

It's my theory that water is getting in through the gaps around the chimney (obviously), the ridge cap, and also through the shingles in general. Here in SW Virginia we have a lot of freeze and thaw due to fluctuations in temps so as things thawed out I think the water could have been wicking in under the tabs, as well as the other areas mentioned.

The obvious starting point would be to seal up the roof in the areas identified (or better yet, tear the whole thing off, replace wood rot, ice and water shield and flash it, then install new shingles). The other thing is that since the building envelope is the exterior walls and the ceiling joists (due to the insulation in the ceiling), you have essentially exterior space in the attic which is unvented. So it seems like a good idea to add soffit vents and a ridge vent.

My main question at this point is why has this not surfaced sooner, especially without the ventilation? I suppose it's feasible that as the leaks in the roof grew more prominent that the problem has only surfaced at this point? The other question; is it possible that frost was forming without the introduction of water from exterior sources? My concern would be that we fix the outside leaks and still have frost forming and thawing into the house.

I moved into this apartment myself 3 years ago, and lived there 2 years. We moved out last May. During that time we saw no signs of moisture or mold in the apartment. The only difference is that the landlord added 6 in. of cellelose (sp?) insulation in the attic about 2 years ago (1 year into our time there). Before then there was R-11 batt insulation that had fallen apart. I'm not sure if this could have changed the dynamics that drastically, we didn't notice any difference.

Thanks for any input. Again, I'm sorry to have been so long winded.


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Wow, sounds like a mess.

My main question at this point is why has this not surfaced sooner, especially without the ventilation? I suppose it's feasible that as the leaks in the roof grew more prominent that the problem has only surfaced at this point? The other question; is it possible that frost was forming without the introduction of water from exterior sources? My concern would be that we fix the outside leaks and still have frost forming and thawing into the house.

The problem may not have surfaced before for a variety of reasons (roof leaks or leaks more, heating system not used as much, you were more careful to open a window,if there is even a window in the bathroom when showering, there weren't as many people living there, etc. etc. ) Or, it has just slowly gotten worse and finally surfaced. The more moisture in the home/ attic, the quicker it will show.

Sounds like you need to repair the roof (or replace), properly ventilate the attic, and locate any other water source problems in the home.

I think you wrote that the bathroom was not vented (is there even a window)- big issue. And yes, moisture will get into the attic from the living space, so ventilation is needed in the attic no matter what.

I see more mold / ventilation problems in rentals for some reason.

Is the plywood sheathing delaminated yet? If not, it shouldn't take too long with all of the moisture issues.

Someone smarter will be along soon to answer your questions better than me, but 'til then...........

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I think Brandon largely covered things. First, ventilate the attic properly and seal the chimney cracks. But to amplify on his comments -- if the crack in the joints around the improperly flashed chimney did indeed grow larger after your time there, then the amount of moisture in the already inadequately vented attic would definitely increase. That would obviously reduce the ability of the structure to "bleed off" moisture to other areas as it may have in the past. Therefore, the problems seen now might not have been there when you were living there.

The rest of the issues I can't help you with. The siding/guttering/rain issues would have to be looked at on-site to be properly assessed.

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It's the tenant.

He/she is doing something that's causing the problem.

It's not caused by leaking.

Adding insulation will makes things worse, but it's not the source of the problem.

Measure the humidity in the home. I bet it's somewhere between 60%-70% @ room temp (70f).

I would re-interview the tenant. They are doing something to raise the vapor pressure in the home on a regular basis.

Chris, Oregon

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The bathroom does have a window. We opened it after showers, but I'll find out if this tenent is or not.

I should have also explained that the only heat used in the apartments is the electric plug in radiater heaters. One other change is that they installed a dryer in the cellar/basement, but when I looked at it it seemed to be vented properly.

As far as I know they don't use a humidifier but I'll double check.

Thanks for all the input.

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You said they replaced the batt insulation with cellulose because of the batts had fallen apart.

Insulation does not wear out but a kraft paper vapor barrier can, which tells me there might have been a moisture issue before they replaced the insulation. Did they actually remove the batts or just cover it with the new stuff?

My guess is the insulation changed things causing moisture to build up combined with the new tenants habits.

Ventilate the attic to get rid of the moisture.

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