Attic Vapor Barrier Problem

[James White]

I am having a mold issue in my attic. Not all that bad. But I don't want any mold period. The problem is that I did not use a poly vapor barrier before installing the 2nd floor ceiling drywall. Then laid the fiberglass batt insulation from the attic side. So the vapor barrier that is the paper on the bats do not have a proper seal. The paper tabs do not lap over the face of the studs. However the worst areas are where there are interior partitions. Seems that the moist air is migrating inside the wall cavity and condensing once it hits the underside of the plywood floor of the attic. I would have never known, had I not removed a few pieces of the plywood. What would be the best way to remedy this problem?

By the way I used a poly vapor barrier on all of the exterior walls. Besides that the insulation was applied with the tabs lapping over the studs. Therefore there is in effect a double vapor barrier on the exterior walls.

So far I have thought of painting the second floor ceiling with a Sherwin Williams ceiling paint that is supposed to act as a vapor barrier. The other option would be tear out all of the plywood in the attic. Then remove all of the insulation. Then use one of the closed cell spray foam kits to spray a thin layer on the back side of the drywall as a vapor barrier. I wold have to do this a half of the attic at a time. Otherwise I would have to remove all of the content of the attic. That would include the plywood and insulation. No way am I doing that.

Is there a better way to do this? I was a carpenter for many years and never came across this situation. I guess leaky construction has its benefits after all.

Another point to consider is that I heat my house with a wood stove centrally located on the first floor. The boiler is always switched off. That is with the exception of the odd night here and there in the shoulder season. We will generally keep a pot of water on the stove in order to keep the RH in the low 30's. So the temp on the first floor is typically in the mid 70's with an RH of around 33. Then the second floor will be around 65 with an RH of about 45 or so. I am not totally sure as I do not pay much attention to the RH up there. We live and sleep on the first floor. But the temperature differential is enough to drive the RH higher on the second floor. I would think that this would not make any difference. Because the amount of moisture that reaches the attic will be the same regardless of the shift in temp and RH on the second floor. But I could be wrong. That is why I am providing as much information as possible.

I can't believe I made such a stupid over site regarding the tabs on the insulation. But I would never have imagined this would be the result. Again I have never seen this happen and people never put vapor barriers in there homes in the past.

James

[Bryan Slimp]

Why do you think the water vapor is from inside of the walls? I think there are a few pieces left in this puzzle.

Is the floor of the attic completely sheathed? Is the attic sealed to the outside world or does it have vents?

My first thought is that the attic is not properly vented.

[James White]

Why do you think the water vapor is from inside of the walls? I think there are a few pieces left in this puzzle.

The problem is the worst in the bays that have the wall plates in them. Especially those of the bathroom and not just the wet wall. There is no evidence of any leaks and it is not just near the bathroom anyway. There are wiring penetrations that are not sealed in the wall plates of the bathroom. I did not do the wiring. I thought that the penetrations where sealed for fire code. But they are not. I wouldn't be surprised if air can travel from the basement all the way to the attic.

Is the floor of the attic completely sheathed? Is the attic sealed to the outside world or does it have vents?

The center strip (about 2/3) of the attic is sheath. The two areas on the long sides of the house (colonial) where the roof rafters are the lowest are unsheathed. There is continuous vent in the soffits and ridge. There is no evidence of condensation anywhere other than under the attic floor sheathing.

My first thought is that the attic is not properly vented.[/QUOTE]

[Jim Matthews]

+1 on the ventilation problem.

It sounds to me like the vapor tries to escape, and condenses on the first unheated surface.
A scoring cut all the way along the bottom and a vent strip (http://www.airvent.com/professional/products/intake-theedge.shtml) in place would allow a path for the vapor to escape these walls.

It will just condense elsewhere, unless vented to the exterior.

[James White]

Hi Jim,

Continuous soffit and ridge vents are already in place.

James

[David Helm]

Where do your bathroom fans vent to. I can't tell you how many times if find bathroom fans venting in the attic (in my job as a home inspector), putting warm moist air into the attic and, hence, growing generalized mold.

[James White]

Hi David,

They have proper thru the wall outlets. Actually the first floor bath is through the wall the second floor is thru the soffit. But we do not use the second floor bathroom anway.

James

[James White]

For anyone that is interested. This podcast explains why I am having problems.

http://www.greenbuildingadvisor.com/blogs/dept/building-science/air-barrier-or-vapor-barrier-building-science-podcast

[Brian Effinger]

For anyone that is interested. This podcast explains why I am having problems.

http://www.greenbuildingadvisor.com/blogs/dept/building-science/air-barrier-or-vapor-barrier-building-science-podcast

Yes, definitely an air barrier problem. All of the ventilation in the attic is causing a chimney effect and sucking warm / moist air from the house into the attic where it is condensing on the cold surfaces. Sealing all penitrations (electrical boxes, access hatch, etc.) will go a long way to helping you. Also, remove the insulation over the second floor interior partitions and caulk the visible top plate to the drywall on either side.

One other thing you can do is reconfigure the attic ventilation. While ventilation is a good thing, it can cause problems, as you have seen. One theory out there now (and I think it may have come from Joe Lstiburek) is to have more ventilation at the overhang soffits and less at the ridge. That will create a pressurized attic space and should help to keep the chimney effect at bay.

[James White]

Hi Brian,

Thank you for your input. I think what you are recommending is what I will do. I was going to buy a spray foam kit or two to do a light coat over everything. But that was because, I thought it was a vapor retarder issue. Now that I know it is an air barrier problem. The solution is almost obvious. My ceiling light junction boxes are not sealed and a few wall plate penetrations. I think some spray foam cans and caulk will take care of the issues. I will only tack the sheathing back down. This way I can check on things this winter. I also think I will reduce the amount of water I put on the wood stove. Basically move the pot of water to a cooler area on the stove top.

James

[Shawn Siegrist]

It sounds like your attic is improperly vented. Are your soffit vents blocked? Is there a ridge vent or gable vents? Proper airflow from your soffit vents up to the ridge vent or gable vents will prevent moisture from building up and mold growing.

[James White]

Shawn,

If you read the thread those items have been addressed. Besides that. That fresh air has no way of getting to the underside of the floor sheathing of the attic.

James