Correct bathroom exhaust assembly for cathedral close-cell insulated roof
tschieggm
| Posted in General Questions on
Hey folks. I’ve got a conditioned attic renovation under way with closed-cell spray foam directly under the roof decking. There needs to be a bathroom exhaust system that presumably goes through the roof.
I’m wondering if there are best practices to prevent condensation from making its way into the decking between the shingles and the close-cell foam along the exhaust? I understand this is an unpopular and risk prone way to insulate a space but it was the only way to meet local energy code for the cathedral attic without seriously reducing head space or ripping off the roof.
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Replies
tschieggm,
Cut the hole for the duct in the roof sheathing so that it leaves a 3/8" gap and fill with foam.
Another option is to frame up a bulkhead in the bathroom and run the exhaust out the wall.
Thanks for the suggestions. I don't think a bulkhead to a wall will be an option as the bathroom is entirely within the sloped attic space. Are there specific types and placements of dampers within the exhaust system that should be used to mitigate unconditioned air from backdrafting?
tschieggm,
Good question - and I'm afraid not one I know the answer to. Depending on where you are it might be a bad idea to have a damper on the roof termination, which could freeze up. As (unlike installations out a wall) the pipe will be sloping back towards the fan housing, you probably don't want any dampers in the vertical portion of the duct which could collect condensation either.
But I'm just musing out loud. Hopefully someone who does this regularly will chime in.
> as the bathroom is entirely within the sloped attic space.
Just to visualize this,
Are you saying this bathroom is effectively bordered by hips or valleys and has no gable-ish walls on its exterior or on the other end of interstitial space bordering the bathroom?
You can run the ducts down and through the floor between the joists to a wall. Going through the roof is just trouble.
You would recommend a fan at the bathroom ceiling with a duct that U-turns back downwards? I was contemplating this approach but worry about exhaust moisture creating issues at the soffit junction.
tschiegsgm,
If you go that route use a direction termination and locate it near the outside of the soffit. I have two. One for the dryer and one bathroom fan. On cold days when the exhaust is visible you can see they throw the air well clear of the house.
I have pretty much your setup in one of the bathrooms. The exhaust fan is mounted on the sloped ceiling with the duct running under the spray foam between the rafters. Before the outside wall, the duct pops out of the ceiling, drops about 1" and exits through a regular wall cap.
Because the fan is pointing slightly downwards, the damper on the unit won't work (I removed mine). You'll have to rely on your wall cap damper to seal, so try to get a decent one with a gasket.
Bonus, because all the ducting is sloping down, it will always drain any condensation.