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Help me with insulation over recessed fan in a ceiling right under the roof

Help me with insulation over recessed fan in a ceiling right under the roof


  #1  
Old 06-21-22, 05:11 PM
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Question Help me with insulation over recessed fan in a ceiling right under the roof

Hi all:
Moved into a new place last fall, so the problem didn't become evident up until now. Upstairs bath is right under the unventilated asphalt shingles roof (2x6 joists), and ancient exhaust light/fan would get scorching hot when it barely got to low 80's. The rest of the ceiling has R-19 and seems to be doing a great job. Heat spot is due to the lack of the insulation.

Replaced with low profile IC rated fan/light combo mounted outside on the drywall and that gave me 4 " clearance inside the joists. First tried to get fancy with double radiant barrier layer at 2" gaps, and it doesn't do much once temperatures start approaching the 90s. Realistically the space only allows for effective use of R-13, but I feel that with summers here going often into mid-90's that's not the greatest solution.

What would you recommend? Any help is much appreciated. While I can certainly relocate the light, exhaust fan must stay.

Thank you.
 
  #2  
Old 06-22-22, 05:17 AM
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All you can do is insulate it the best you can. Foams typically offer higher R value per inch but getting other than spray foam in there may be difficult.
 
  #3  
Old 06-22-22, 10:05 AM
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Thanks for the response. So I'm looking at stacking two R-10 2" XPS rigid boards together, which theoretically should give me R-20. Should I use liquid nails to glue them together or something else? What's the best way to transition to surrounding R-19? Should I use can spray foam or is there a better way? Picture below for reference. New fixture internal frame has 12x12 footprint, so there's space of about 3" on each of the sides.

Photo of the ceiling situation https://imgur.com/a/oSLDspH
 
  #4  
Old 06-22-22, 11:50 AM
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For that small space I'd just use fiberglass. After all, it's what's in the rest of your ceiling. While foam has higher R per inch installing it so you don't have gaps with the existing fiberglass will be difficult.
 
  #5  
Old 06-22-22, 12:54 PM
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R-13 is the best fit in there, anything with higher rating will be compressed and useless. I put it in there for a day with a high of 85F. When got home from work the fixture was 100F. I've got to solve this :/
 
  #6  
Old 06-23-22, 05:16 AM
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I bet you will find the whole ceiling is hot. That area less so because there is no sheathing covering it but if the rest of the ceiling is only insulated to R13 then there is very minimal benefit to try and super insulated less than one square foot.
 
  #7  
Old 06-23-22, 08:47 AM
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Yep. The rest of the ceiling has R-19 and it's doing a good job. Heat spot is pounded by radiant heat since I can't seems to figure out how to use that 4" clearance to provide proper insulation. Surrounding R-19 just compounds the issue. Once temps here start climbing into the 90's the temperatures in that spot will elevate beyond what's safe for electric.
 
  #8  
Old 06-23-22, 11:36 AM
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beyond what's safe for electric.
60* C wiring is good to 140* F, 90* C to 194*F. Unless the fixture or housing is plastic that might melt or deform at 100* F +/- you should not be concerned. An LED driver could possibly be affected. Does the fixture/fan manufacturer give any temperature limits?
 
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  #9  
Old 06-24-22, 03:21 PM
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My concern exactly. With temps over 95F, 140F is a guarantee. Of course real world safe and white paper safe are not always the same, and this build was like this since early 1980's, but found the problem, so now got the solution.

Posted this question elsewhere too, and so far what's working is a combination of XPS board and fiberglass. Does better with XPS flush to the roof plywood, but of course couldn't trim roofing nails, so put some stain on them, marked the board, carved out groves which filled and cured with canned foam. The rest packed properly with old fashion R-13. No way could fit x2 layers of XPS in there - unworkable and too many gaps on sides.

Mid-80's all day today arrived home to room temperature fixture. Normally it would climb to 100F. Real test is tomorrow - 90's for two days in a row. Fingers crossed.

Thanks for all your help guys.
E: fan/light combo... it's all plastic and aluminum, light is just external LED pancake, so not a significant heat source, but motor and body would surely get damaged at the temps I was getting. Aero Pure ABFS-0511-L6-Series
 

Last edited by SkyMa; 06-24-22 at 03:28 PM. Reason: replacement fixture info
  #10  
Old 06-25-22, 07:49 AM
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It's a complete and utter fail. Just ordered 10mm Aerogel insulation.

If this best insulation material offered to mankind doesn't fix the issue, I'm giving up.
 
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Old 06-25-22, 09:15 AM
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Oooh, high tech. Make sure to post back with pictures and a how it is to work with the Aerogel. I've never used it.
 
  #12  
Old 06-25-22, 09:57 AM
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If it's good enough for land rover on Mars, I am hopping it's gonna help my disaster. Some of the heat is coming from exhaust duct, so hopping that insulating that and fixture itself will help. We will see.

 

Last edited by XSleeper; 06-29-22 at 06:56 AM. Reason: aerogel video
  #13  
Old 06-25-22, 01:00 PM
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Oh, important to mention if another poor soul in a similar situation decides hop on this miracle aerogel product solution, USE PPE (respitory mask, gloves, long sleeves, etc). There are many places you can get blankets for DIY, but many of them won't mention the protection measures you need, so do your homework.
 
  #14  
Old 06-29-22, 06:43 AM
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Did you consider running the fan continuously or on a high heat thermostat to provide air flow that might lower the temperature in and around the fixture? Not the most energy efficient since it will be pulling conditioned air from inside.
 
  #15  
Old 06-29-22, 03:41 PM
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It's a good idea, but insulation is needed not just to keep heat out in the summer, but to keep heat in during the winter.

I reckon 8 layers of aerogel blanket should do it - it's more than enough for my available clearance (each 10mm blanket has R-4 rating). I want to protect electric wiring too, just waiting for the stuff to arrive.
 
  #16  
Old 07-07-22, 03:23 PM
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Well, it works. 4.9F difference, which is due to the radiant heat from vent/duct (not insulated at the moment). This is a full day run from high of 89F today. Done for apples to apples comparison, and photos speak for themselves. As I layer on more aerogel blankets it should improve. But at upwards of $30 sq/ft it's not something I'll be using instead of fiberglass anytime soon.


91.8F 5cm (5 layers x 10mm) 1.97" Total: R-20



86.9F 2x6 neighboring joist cavity: R-19 sandwiched between roof plywood and ceiling drywall.
 
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