Insulation for slab on grade with elevated stem walls
it is very exciting for me to be posting to this board. I’ve been a reader for years, and am looking forward to hearing from the experts on this site regarding my matter.
I own a home that my wife and I are renovating in zone 5A, built in 2012. The first floor of the home was built to be a garage, but we will be converting it into living space. As such, I will be doing some renovations, and have a few questions regarding the best ways to insulate the slab and stem walls.
A few quick notes on the home…
The garage/future first floor living space walls are sheetrocked, finished (painted), and insulated with R21.
The garage/future first floor has HVAC currently with three registers, but will need additional ducting run (the furnace is spec’d for the increased load already).
The garage/future first floor has R30 in the ceilings (between it and the second floor living space
There is no below slab insulation
There is a vapor barrier below slab
There is no thermal break between the stem walls and the slab
There is no exterior insulation on the stem walls, roughly 6-18″ are exposed depending on the side as the home is on a moderate slope
The garage is 1280 square feet, and will become a living room, dining room, and kitchen
There is absolutely no moisture issue with the slab (it is on grade/slightly above grade/a foot or so above grade as the house is on a moderate hill)
Because I cannot insulate below the slab at this time, I will be looking to insulate above it to help make the room more comfortable and energy efficient.
Based on the many readings I’ve come across on this site, it seems as though the preferred method to insulate the floor would be install at least 1″ (2″ preferred) of foil faced EPS or Polyiso with seams taped, followed by a layer of 3/4″ plywood or osb subfloor, or 2 staggered layers of thinner subfloor. This would then be followed by a standard flooring install per the manufacturers reccomendation.
Due to the home having elevated stem walls (above slab on grade by 8″) I want to confirm that the other things I’ve gleaned from this site are the proper way to proceed.
First, install the foam barrier on the slab as describe above. The floor insulation should run wall to wall with no break. Then insulate the stem walls in the same manner on their vertical face. Caulk/tape the seams where the horizontal floor foam meetings the wall foam. From here, I can install the subflooring, and any surface covers for the vertical stem wall interior foam layers.
Is this method appropriate for the interior of the building?
Next question would be that I’ve seen in multiple places on this site that exterior slab insulation to four down is recommended. Can I do this in conjunction with the steps mentioned above? Meaning foam on both the interior and exterior of the stem wall. I know there are additional steps to protect the exterior foam, but am trying to keep this brief as the write ups on that are simple to follow.
Does the above sound, well, sound? If not, what can I improve on? I want to get moving on this, but want to do this in such a manner that is energy efficient and able to prevent any long term issues. My thought is that due to this space being conditioned, and with this insulation approach, moisture below the foam should not be a concern.
I eagerly await your responses, and, again, thank you for the volumes of advice you give to this community. Martin, Joe, and Dana – I’ve spent countless hours reading your work, and appreciate the pains you go to make it digestible to even non-builders.
I’ve attached two images (roughly the same from different distances) to help if at all possible.
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