Replies

1. Expert Member
   Dana Dorsett | Dec 09, 2013 03:56pm | #1

   Yes you can use rigid foam on the interior side of the plaster, but you need more than a tin ceiling to meet code. While the sheet metal ceiling is an ignition barrier, it doesn't meet the timed thermal barrier for use in a conditioned occupied space. Half-inch OSB or half-inch gypsum between the tin ceiling and foam would meet code though.

   Mansard roofs are a common place to find insulation gaps, and it can be very difficult to insulate them while preserving code-required venting, but rigid foam on the interior works.

2. 
   mcgster | Dec 09, 2013 04:10pm | #2

   Dana,

   Thanks for the response; I’m glad I checked!

   Is it best to leave a gap between the rigid foam and use spray foam around the sides or but the pieces up against each other and use tape?

   I think half inch OSB may be the best way to go as it would provide a nailing base for the tin. I would just have to ensure I had it adequately secured to the ceiling joists.

3. 
   user-980774 | Dec 10, 2013 11:57am | #3

   If you have a true R50 ceiling over the bedrooms, adding a couple inches of foam will not be cost effective or noticeable. My guess is the problem is the mansard roof / interior walls.

4. GBA Editor
   Martin Holladay | Dec 10, 2013 12:14pm | #4

   Mark,
   Richard is right. If you have R-50 in your attic, you're all set. Put the foam on the walls instead of the ceiling.

5. 
   mcgster | Dec 10, 2013 01:31pm | #5

   I'm not able to do the interior walls at this time, it would require a much more extensive renovation that i'm just not able to do. My thought is the heat is moving up and out and the insulation is not acting as it should due to the design of the ceiling.

6. Expert Member
   Dana Dorsett | Dec 10, 2013 02:41pm | #6

   (Don't ask me why, but...) I was assuming by "ceiling" you were referring to treating the sloped-mansard wall interior plaster, not the flat-level ceiling with the R50 above it.

   The insulation in the attic is working fine, but you have a large thermal bypass with a vented mansard edge/wall that is uninsulated. When the floodgates are open it doesn't matter how high you make the dike, the flow is the same. Nothing you do at the flat-ceiling side is going to affect the heat flow through the mansard or walls, which are probably getting on to 90% of the heat loss, compared to 2% for the R50 ceiling.

   Buy an infra-red thermometer and compare the wall & ceiling temperatures on a cold night, or just feel them with a bare wrist (much more temperature sensitive than palms.) It won't be a mystery where the heat is flowing out.

7. 
   mcgster | Jan 09, 2014 12:08pm | #7

   Dana,

   Thanks again for the answer, i will take my IR thermometer tonight and check the mansard wall and the ceiling to compare.

   I had another thought since i have to do some work to prepare for the tin ceiling anyway, and i have bags of insulation left over from the project. If i were to frame up the ceiling (flat) with 2x4's and use my left over bags of batt insulation between them this would essentially be free. This would provide more insulation as well and also give me some room to run new wires for lighting. If i were to do this would i use vapor barrier to help prevent air leakage?

   I understand the exterior walls would be ideal, and this summer i will try and do a plan to get them insulated!

   Thanks again.

8. GBA Editor
   Martin Holladay | Jan 09, 2014 12:27pm | #8

   Mark,
   Q. "Would I use vapor barrier to help prevent air leakage?"

   A. You don't need a polyethylene vapor barrier. Interior vapor barriers can cause problems, especially if your house is ever air conditioned.

   You do need an air barrier above your tin ceiling. The best air barrier would be drywall.

9. 
   wjrobinson | Jan 09, 2014 01:05pm | #9

   pictures would be worthy to aid all in helping