2×6 shed dormer ceiling insulation
I have a finished attic space with 2×6 framing in the roof, including a shed dormer over a bathroom. I have around 1500 square feet of surface to insulate, and I’m probably going to end up with a hybrid approach, with some amount of CC foam plus blown in cells or fg. I’m in zone 4/5.
My question is related to the bathroom in the shed dormer. I want to install lighting over the shower, and need to install a bath fan. I nixed the idea of cans over the shower because of the penetration of the insulation space (I think I’ll use low profile globes instead), but I’m struggling with what to do about the fan. The smallest fans I can find will JUST fit into a 2×6 space, leaving 0 room for insulation. I’m assuming that will be a problem spot, especially with the bathroom humidity. I am now considering installing a wall-mount fan from Panasonic, but I’m wondering if that will be any better.
Do any of you know how well they are sealed from the outside when they are not running? Is it just like opening a hole in your house? Also, I think I’d need two of them installed, since they are only 80 cfm and my bathroom is 13×8.
Thanks for any advice.
chris
Replies
I would use
a through-wall fan in this case because the overall size of the hole will be smaller (i.e. fan only vs. fan + ductwork for a ceiling mount). You don't need more than 80 CFM, so just install one.
My experience is that the backdraft dampers in bath fans--even the good brands--are not that effective, and a lot of air leaks in (and out) through them. Sometimes you can improve them with some careful work. I have installed a second damper in the outlet duct on several occasions.
Fan/light etc. @ tub/shower
As an aside note, you might want to check IRC for clearence requirement for electrical devices in or near shower / tub areas.
Anyone have any experience that would indicate what the ramifications would be of going ahead and installing a ceiling mount fan in that space? I think it would leave room for basically no insulation. How big of a problem will that be if it's air sealed?
The biggest danger would be air leakage. In such a confined space, if the outlet of the fan isn't perfectly sealed you could have major moisture buildup.
I also considered furring the rafter down a couple of inches so I could at least have two inches of foam between the roof deck and fan, but I hate to lose the headroom. The shed is 6'8" at the low side.
I don't know how I could install a remote fan. The intake box and duct would chew up the insulation space the same way as a ceiling mount fan.
So, it sounds like a wall-mount fan is my best option? And live with the backdraft issues?
ccampbell
To limit the exterior flapper from allowing air to pass, make sure you get a spring loaded wall cap that has a bit of weatherstrip on the door. If you wish to "help" it seal out the air when not in use-adding a large fender washer or two to the door will weight it so wind will have to work to lift the door. To attach, silicone the washers (along w/foil tape to hold it while the silicone sets) to the inside of the flapper so it's not seen from the outside.
Keep adding weight till you have it right. The power of the wall fan will open it no problem.
another fan option
I just discovered a Panansonic fan called the WhisperValue that says it can be installed in 2x4 construction. This should allow me to install 1 and 3/4 or 2 inches of spray foam above the fan. I am thinking this might be enough to stop condensation.
Thoughts?