Foam sheet insulation under existing ceiling

[dougstove]

Hi;
I have a problem.
The attic in my 1967 range house is well insulated (~R50), with a vapour barrier, fibreglass batts (original) and blow in cellulose (more recent) on top. The walls are also well insulated, about R25, and the windows are good. The air sealing is fairly good.
The house is warm, we heat it with ~2.5-3 cords/yr (1900 sq ft.).

But it was built with ~2 ft wide eaves, with ventilated soffits.
The horizontal truss members run out to the end of the eaves, and act like cold-fingers, transmitting bands of cold to the ceiling, which is a heavy old plasterboard with plaster, including metal mesh.

So in cold weather (-15 to -20C), in some rooms, every 16", I get a patch of condensation on the ceiling, even though the humidity in the house is low.
Adding more attic insulation will not help, because the insulation is at the sides and on top of the truss elements.

I had a corner closet where the problem was very bad, so I experimented by mounting 2" foam board insulation to the ceiling with screws and washers and sealing the seams with spray foam. The condensation problem was solved.

I am wondering if I dare repeat the procedure on entire rooms?
My contractor friend is nervous about creating a double vapour barrier.

As an aside, the existing ceilings were originally oil paint, and more recent painting and plaster touch ups have not adhered well, so a complete new layer could solve that problem as well.
cheers, Doug

 

[EatenByLimestone]

Every 5" or so punch a hole in the existing ceiling to let air move through. The new foam will keep the insulation from falling.

 

[DickRussell]

Every 5" or so punch a hole in the existing ceiling to let air move through. The new foam will keep the insulation from falling.

Huh? Why would you want air to move through the existing ceiling? Anyway, with a tight foam layer under the old ceiling, what air will be moving through the hole? Perhaps you are addressing his friend's question about "double vapor barrier." When the house was built in 1967, I would doubt that anything like a sheet of poly was hung under the ceiling joists before it was sheetrocked. Doug, is the poly indeed there? Otherwise the only vapor retarder layer the existing ceiling would have would be whatever layers of paint were applied afterward, and that will dry upward slowly as the seasonal conditions change. If there is indeed a sheet of poly, the only thing between it and the foam layer would be the painted sheetrock. Unfaced foam at 2" still has some permeability and can dry downward slowly if necessary. With R50 above an R10 foam board,I wouldn't worry about condensation above the foam at all.

 

[EatenByLimestone]

Yes, double vapor barrier was the concern.

 

[dougstove]

Hi - thanks.
There is sheet poly in the attic between the joists below the insulation batts, but I am sure it is not well sealed.
I think my plan will work.
cheers, Doug

 

[Swedishchef]

Condensation occurs when there's moist air coming in contact with a cool surface. Are you certain the R50 is evenly distributed in the attic?

Andrew

 

[Hogwildz]

With 2" board, you might be ok., and there will be no concern of condensation IF you air seal well. Cut it short around the perimeter and spray foam the gap for a tight seal/ And tape the seams with good tape. I am using 2" foil faced iso with a R13.5 value, and foil faced weather seal tape.

 

[woodgeek]

I'd look elsewhere. Get a decent humidity meter. If the humidity (RH) is too high, you will get condensation somewhere, but the fault is not with the structure, its the high RH. I suspect you are over 35%. Do you have an HRV? Is it working? If you don't have an HRV, you can get timer switched for bath fans, that cycle them a programmable number of minutes per hour, 'AirCycler' is one brand.

My concern is that if you have too much moisture, fixing condensation in one spot will just result in condensation somewhere else, and you will be at it for a while.

If the RH is ok (<30%) and this is only a prob in very cold weather.....some folks have had luck with crown molding along the corner backed with foam?

Other option is switch the attic to gable venting and closing the eave vents. If your attic is airsealed wrt the house, this should be aok.

 

[maple1]

Another option I see is running a ceiling fan on low when it's real cold out, pushing air up. The air wash should eliminate the condensation.

 

[dougstove]

"Condensation occurs when there's moist air coming in contact with a cool surface. Are you certain the R50 is evenly distributed in the attic?"

The R50 is not perfectly even, but the problem is exactly matched to the location of the roof truss horizontals, no where else.

Humidity: Is low, ~35% or so, at the lower limit for occupant comfort. We have 1 h timers on the bathroom fans, and a good quality range hood that runs whenever we are cooking.

Crown moulding along the corner backed with foam: Done. It solved the problem in the corners, but the truss members are propagating the problem inward (much less than before the crown moulding).

Attic venting: There is a full peak vent; the attic is cold and well ventilated. This is contributing to the issue, because the roof trusses stay cold. Attic venting in cold climates is currently under review; it is better to air seal between the conditioned space and the attic (which I have largely done).

"Condensation in one spot will just result in condensation somewhere else, and you will be at it for a while": Yes, that is an issue, but if I keep all surfaces warm enough to be above the dew point, I will be happy. I have been picking away at cold spots for years.

HRV: I do not have a true air recovery unit (they are required by code in modern houses here now). But I do have an amateur system that brings outside air in through my cold room, and then to my stove room to provide makeup air.

To be fair, the humidity load in my house is high (5-6 occupants, long showers, 4 dogs, alot of home cooking).

 

[woodgeek]

Sounds like you have given this some thought!

Ok. If you use http://www.dpcalc.org/ and say that T = 68°F and RH = 35%, the dew point is 39°F, or 30°F lower than the air temp.

If you use an IR thermo, is it really getting that cold on a sustained basis? I would think with wood being R-1/inch, and an air film being R-0.5, then a few inches of wood (your cold finger) coupled to an air film at the ceiling plane, the ceiling might reach equilibrium at a temp 10-20% of DT below the room air temp, where DT is the indoor-outdoor temp difference. If it was 20%....condensation is only expected if DT = 5 * 30°F or 150°F or above, corresponding to -82°F or colder, which I doubt you hit frequently.

So, IMO it seems more likely that your RH is higher than you think (very common with hygrometers), or the problems, like in the closet, are in rooms/area where the air temp is significantly lower than where you are measuring RH. With an IR, if you measure the temp of the wet spots, it is the the wet-bulb temp, and allows you to compute the RH given your air temp in the room.

Something is not adding up....

That said, Maple1's idea of a fan, by transferring the heat to the ceiling plane (reducing the R-0.5 to a lower value) should help the problem a lot.

OK, another option is that your cold fingers are more conductive than R-2.5. Was your house built with steel joist hangers or some sort of similar metal bracing? If you were humid enough to condense at R-2.5, your windows would all be running water, unless you had very nice windows and no curtains.

Hmmmm.

 

[dougstove]

Thanks for the advice.
Thinking back, we had a weird December, with 24 h fluctuations between rain at ~15C and -15C or lower.
I think during the rapid temperature drops the RH in the house rose, and outran my dehumidifiers and ventilation, so the cold points dropped below the dew point.

So my RH is probably bottoming out at ~35%, but peaking a good deal higher immediately after warm wet days.

But, these wild temperature fluctuations seem to be the new normal here; we are getting oscillations between arctic and southern air.
I think the trusses are all stick-built, and have not seen any steel joist hangers, but there is a significant metal mesh in the plasterboard ceilings.

 

[woodgeek]

My airsealed house (ACH50=5) doesn't swing humidity that fast, unless I am running active systems to change it. More like a few % a day tops. My humidity tops out in the Fall when its too cool to run the AC, but the dewpoint is still high on warm rainy days. I find it takes a month of decent wintry weather to get the joint dried out to 35%, when I break out the humidifier (in my climate, around xmas). I've got an ecobee logging tstat so I have a web-based plot of RH versus time.

Its possible you need to crank up your ventilation system in the early winter, as soon as the outdoor dewpoint falls off, to get the place dried out before the real arctic blast arrives. This makes sense if you have a cooler, more maritime weather pattern in the summer without a lot of AC use, your house could be 'charged' with water.

Is your amateur ventilation system 'forced' so you have a sense of airflow, or is it passive? Winter condensation is usually taken as a sign of inadequate ventilation.

Do you have a **reliable** hygrometer? I have found that my 'weather station ones are ±5% at least, and go more out of whack after a couple years.

 

[maple1]

I could initially understand a possible issue and what was going on. But after some more thought on what happens in our place I'm puzzled. Our trusses also have some overhang but not quite that much. Maybe 18"? We have a Venmar but it never cuts in this time of year - but that also means the bathrooms see high humidity periods after showers. I have never seen condensation like is being described.

Is there strapping between your rafters & drywall? Kind of at a loss to understand this phenomenon.

 

[dougstove]

The heavy plasterboard is mounted direct to the underside of the rafters, no strapping.
I agree about excess humidity.
My make up air is passive, pulled by the stove combustion air, the dryer vent, the stove hood and two bathroom fans (when they are running). But there is a significant inward flow at the intake vents.

Woodgeek is also right about stored water, the wood floors etc. store alot, and with our fluctuating Nov/Dec there was no drying period between warm/wet and sudden deep cold. There is no air conditioner, we only have about about 6-12 h/y where we would want cooling.

I read online about similar problems in a batch of houses in North Dakota, with a similar roof truss design.