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How to fix a condensation problem possibly caused by closed-cell foam in the roof?

We have a mid-century modern home with a very low roof pitch. We pulled off the roof, blew 6 inches of closed cell insulation between the rafters and then put on new plywood and a new rubber roof. We have bathroom ceiling fans that vent through the roof, canned lights in the living room, and a light fixture that's set into the ceiling in the dining room.

Since we put on the new roof, these ceiling fans have been dripping with condensation on days with big temperature swings. We also see condensation in some of the canned lights and see a water stain coming out of the canned light fixture (which is in a direct line down the roof slope from the bathroom ceiling fans). We also have soffit vents that we did not seal up.

Most contractors and roofers we've spoken with are stumped because they haven't used this insulation in a low-pitched roof. That said, after a couple months of talking with people, we believe we need to close up the roof and vent out the side of the house. We also think we need to either check the canned lights to make sure they are surrounded by the insulation or to remove them, and we're not sure if we need to close the soffit vents. Do you have any thoughts about is happening and could weigh in on how to resolve the issues we're having with condensation and moisture? Thanks in advance!

Asked by Midcentury modern
Posted Jun 19, 2017 11:44 AM ET
Edited Jun 20, 2017 9:04 AM ET


10 Answers

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What was the exact insulation product that was retrofitted into the roof assembly? (Brand name, and product name.)

Where are you located?

Answered by Dana Dorsett
Posted Jun 19, 2017 12:18 PM ET


It seems probable that there are airway paths that allow interior air to reach your roof sheathing. When outdoor temperatures are low, moisture is condensing out of the interior air on cold surfaces (most likely, the roof sheathing).

Unless your ceiling has a well-detailed air barrier, these types of pathways will lead to problems.

Answered by Martin Holladay
Posted Jun 19, 2017 12:32 PM ET


Thanks, Dana, I'm trying to dig up the brand name and product name so be back shortly. We're in the Boston area.

Answered by Midcentury modern
Posted Jun 19, 2017 2:22 PM ET


Hi Martin, Thanks for your reply. The interior air may be going through the canned lights, then, as well
as the bathroom fans--and even through any cutouts in our ceilings it sounds like. Creating a well-detailed air barrier--maybe we need to close up all ceiling cutouts (canned lights, etc.) so we have ceiling-mounted versus in-ceiling fixtures? Or, can we pull out the canned lights and spot-insulated around them?

Answered by Midcentury modern
Posted Jun 19, 2017 2:29 PM ET


As you probably know, GBA has consistently advised readers that recessed can lights should never be installed in an insulated ceiling.

For more information on insulating low-slope roofs, see Insulating Low-Slope Residential Roofs.

Answered by Martin Holladay
Posted Jun 19, 2017 2:31 PM ET


Dana and Martin, do either of you have thoughts about whether we should close up the soffit vents?

Answered by Midcentury modern
Posted Jun 19, 2017 2:31 PM ET


The best fix is to remove the recessed can lights and to replace the recessed can lights with surface-mounted fixtures with airtight electrical boxes. When this work is performed, the void in the insulation should be repaired with canned spray foam, and all work should aim for airtightness.

With a shallow low-pitch roof, it's much better to route the exhaust duct from your bathroom fan to a wall, not the roof. I'm not sure if that's an option in your case. Wall-mounted bathroom exhaust fans are available.

Answered by Martin Holladay
Posted Jun 19, 2017 2:34 PM ET
Edited Jun 19, 2017 2:35 PM ET.


Hi, Martin, I actually didn't know about the GBA and canned lights--I'm a homeowner who had a contractor and roofer who came highly recommended and suggested this approach, but clearly didn't know what they should have. I've been trying to figure this out for a few months now and have been unable to find a contractor who has worked successfully with closed-cell insulation in a low-slope roof. This is incredibly helpful. Many, many thanks.

Answered by Midcentury modern
Posted Jun 19, 2017 2:39 PM ET


From the description of the foam installation it sounds as if it was being sprayed directly on the ceiling gypsum, but they probably kept some amount of clearance from the recessed cans for fire code reasons(?). If that's the case you probably have copious amounts of air movement through the light fixtures even when they're off, but it'll speed up an order of magnitude or more when the lights are on.

There's no excuse for not spraying the foam to seal the bathroom vents, but they may have maintained a clearance there too (?).

The humid conditions air condenses on the colder surfaces in the cavity between the foam and the roof deck, then puddles & flows downhill to the nearest available leak point, which doesn't take a lot of slope.

The better (and probably cheaper) solution would have been to put rigid foam above the roof deck, and seal soffit vents. But until this gets under control you need whatever drying air that the soffit vents can supply.

Answered by Dana Dorsett
Posted Jun 19, 2017 2:49 PM ET


Thank you, Dana. We'll keep the soffit vents open for now, then. I'm hoping we can get this under control sooner than later now that I know what steps to take.

Answered by Midcentury modern
Posted Jun 19, 2017 2:51 PM ET

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