Best Way to Insulate New Foyer on Slab

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Old 07-20-17, 09:39 AM
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Best Way to Insulate New Foyer on Slab

I'm having a garage and foyer addition built and need to determine the best way to insulate the foyer, which will be built one a slab with pressure treated 2x6's. There will be an 8" poured concrete wall around the perimeter that extends approximately 24" below grade (24" is the frost line). I've read that insulation can be applied underneath the slab, but am not sure what all this entails and if it is even that effective considering the soil beneath is encapsulated by a 24" deep 8" thick concrete wall. Should I use rigid foam and place it in the joist cavities directly on the slab? Or should I use 6" fiberglass batts to fill the cavities and also insulate against the band boards? Both? I should mention that one of the joist cavities will have a 6" duct running through it for a vent on the far wall as well as a plumbing line to feed a hose bib. I'm assuming I would want to insulate BELOW those things. Maybe I run them in the same cavity with rigid foam beneath then fiberglass in the other cavities. Should I put a vapor barrier directly over the slab, then joists and insulation on top?
 
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Old 07-20-17, 09:48 AM
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Here is a rough diagram of what I am working with:

Attachment 83256
 

Last edited by mossman; 07-20-17 at 11:02 AM.
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Old 07-20-17, 11:05 AM
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Feel like it's worth mentioning that there is a pop-out window in my home that I decided to insulate last year and ever since the floor has been ice cold in the winter. Prior to laying batts under the floor, there were only batts against the band board. The floor was exposed to the basement below, which kept the floor warm. Could I perhaps insulate only the band board in my new foyer and drill holes through the existing band board to allow warm air to circulate into/out of the joist cavities from the adjacent conditioned basement area?
 

Last edited by mossman; 07-20-17 at 11:41 AM.
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Old 07-20-17, 02:58 PM
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Should I do something like the following? Vertical foam board against inside of foundation wall and up to sill plate, then insulate with batts inside the joist cavities? The whole point is to insulate from the section of foundation wall that is exposed to the ambient air outside correct? The ground itself under the slab shouldn't get extremely cold should it?

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Old 07-21-17, 08:50 AM
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Talked to my builder, and this is what we're going with. Unless someone chimes in and advises otherwise:

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Old 01-08-18, 09:34 AM
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I ended up installing 2" XPS vertically against the inside of the foundation wall to below grade and also horizontally underneath the slab 24" around the perimeter. There are 5" sleepers spaced 24" on top of the slab. Even with the XPS insulation, the slab is still very cold. I initially planned on leaving the space between the sleepers/slab/floor void, but now I'm thinking I should insulate in between. However, I have a piece of 3/4" subfloor laid down temporarily and it does not feel very cold at all. Should I fill the cavities with XPS from slab to subfloor or leave the areas vacant and only insulate the band board (which is actually three 2x4s stacked)?
 
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Old 01-09-18, 09:38 AM
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I normally wouldn't hesitate to just insulate over the slab/between the sleepers, but I did something similar with a cantilever floor in my kitchen and ever since the floor has been ice cold. Prior to doing so, there was R-13 batts laying on the bottom of the joist cavity and an r-19 against the band board. I put an R-19 batt on top of the R-13 to eliminate the air space. My best guess is that prior to doing this, conditioned air from the basement below was able to circulate in the cavity, which kept the floor above warm, but now that I essentially blocked it off, it is freezing cold. I must have air leaks along the bottom and along the band board. Hopefully if I seal everything up it will be resolved.
 
 

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