Bathroom fan installation in inaccessible attic

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KevP
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Location: Saint Paul, MN USA

Bathroom fan installation in inaccessible attic

Post by KevP » Thu Jul 04, 2019 8:56 am

hey fellas,
I have a bit of a weird situation on my second floor. I have already asked about it but during my drunken night, I came up with something that kind of make sense but might be a PITA as well.

Long story short, none of my bathrooms in my 1960 house has bath fans. 2 out of 3 of them have access from above. The house is a 2 story house with the 2nd floor sharing conditioned space with unfinished attic. The 2 bathrooms are right below the unfinished section on the 1st floor and i have access to them from above. However a large area above the bedrooms and the 3rd bathroom on the 2nd floor does not have any access from above. I give it at most 3 ft of head room at best.
I think they used the rafter ties (if I recall it correctly) as drywall backers. So there is no way to create an access panel and walk over them. I think they are 1x4 or maybe at most 2x4s which are nail to the rafters and do not sit on any wall.
I live in MN and winters here are pretty brutal. The ceiling has minimal insulation (very old mineral wool, i give it at most 3 inches but the old stuff might be around r9 at most).
Installing the fan from below requires opening up a ceiling a bit but thats not bad. Good thing is that it would be a 2 ft straight run, then a 90 and 3 more ft to hit the gable wall.

My only issue is with the fan and duct sitting in an attic with no insulation over them. The r8 duct insulation is nowhere near enough with winters that can go down to -20F. Last winter we had -35 for a week.

So here is what I think. What if I create a box around the fan and duct work made out of XPS. Let's say 2 inch xps. Have the box mounted over the joists (kind of like an extension to the joist) by screws and brackets, and seal all sims with foam. I can then install the fan and duct and then fill up the box with insulation (batting or blow in). I can get a decent R value with this.
I have talked to an insulation contractor before and he said that i have 2 options to address the insulation issue in my attic:
1) take some drywall down and insulate the ceiling which requires tons of patching and fixing
2) wait until the roof needs to be replaced. The roofer can take some of the sheets down and they can spray foam from above. He preferred this approach since the path work is only putting back plywood on the rafters.

I like the idea of boxing the fan and duct since I dont want them to be covered in spray foam in case a repair is needed in the future.
SO as for the foam, can that be left exposed on the attic side. I know foam cant be left exposed in a conditioned space and must be covered with 1/2 drywall as a fire barrier. Does it also need to be covered on attic side as well?

Or my idea as a whole is dumb?

Thanks for reading the long post!

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Shannon
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Re: Bathroom fan installation in inaccessible attic

Post by Shannon » Fri Jul 05, 2019 7:33 am

You have a perfectly simple solution for your problem. In fact I would build the styrofoam box with 2 layers of 2" rigid foam This will give you about R 20 and be way better then the rest of the ceiling anyways .
One more thing ,if you can avoid the 90 degree turn I would.Making it more gradual will help keep the air flow less restricted and also reduce the noise from the fan.
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KevP
Posts: 79
Joined: Wed Jul 05, 2017 1:37 pm
Location: Saint Paul, MN USA

Re: Bathroom fan installation in inaccessible attic

Post by KevP » Sat Jul 06, 2019 8:20 am

Thanks Shannon. Yeah the R20 sounds great. I will still insulate the duct and add more the the box so i can get close to R 40 or higher. As for the bend, my joists run prallel to the side wall. So i have to make a turn. I can do 2 45s. I will use smooth round duct just for future cleaning purposes. Im also thinking of upping the size 4 to 6 to compensate for the close bend.
It just sucks that i have to do all of the work from below :(
But it seems like my idea is not that dumb :D
Ant preference on foiled face Polysio vs XPS?

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Shannon
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Re: Bathroom fan installation in inaccessible attic

Post by Shannon » Sat Jul 06, 2019 7:44 pm

I would just use XPS.
the 2 45 elbows would be better and the larger pipe will help
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VanTry2DIY
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Re: Bathroom fan installation in inaccessible attic

Post by VanTry2DIY » Fri Jul 19, 2019 7:10 am

A one-way louver in your duct (blocking airflow from outside to the bathroom) might also be a good idea, if you were not already planning on it.

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Shannon
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Re: Bathroom fan installation in inaccessible attic

Post by Shannon » Fri Jul 19, 2019 8:33 am

VanTry2DIY wrote:
Fri Jul 19, 2019 7:10 am
A one-way louver in your duct (blocking airflow from outside to the bathroom) might also be a good idea, if you were not already planning on it.
Any bath fan I have ever installed came with this .
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VanTry2DIY
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Re: Bathroom fan installation in inaccessible attic

Post by VanTry2DIY » Sat Jul 20, 2019 3:18 pm

Shannon wrote:
Fri Jul 19, 2019 8:33 am
VanTry2DIY wrote:
Fri Jul 19, 2019 7:10 am
A one-way louver in your duct (blocking airflow from outside to the bathroom) might also be a good idea, if you were not already planning on it.
Any bath fan I have ever installed came with this .
True--my thought was the cold drafts from outside could either be stopped close to the roof vent or right on the other side of the fans built in louver. I'm sure it's a negligible difference in efficiency.

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