# Insulation over concrete,Inside or outside?



## Seasoned Oak (Dec 11, 2014)

Is it better to insulate the inside of a concrete wall or the outside. Before you say the outside keep in mind the wall has no insulation at the bottom or the top. Insulating the inside you could remedy that. The wall im pondering is an inside wall ,the other side is an unheated stairwell.


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## blades (Dec 12, 2014)

outside - insulate to a level about a foot below frost line- inside seal block with water proof paint designed for that purpose, then read up on proper way to install insulation on the inside of the full wall height-  need to install a vapor barrier correctly.  also need to seal all of the sill area in basement.


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## peakbagger (Dec 12, 2014)

The standard recommendation is insulate outside. Unfortunately in my area, the carpenter ants had a say in writing that recommendation. When I built my foundation 25 years ago I put 1" of foam below grade down to the footings and left the upper 2' un insulated as I didn't want to scar up the foam during construction. About 2 years later I did two of the exposed walls and coated the foam with a masonry stucco product for UV protection. It worked well in winter, previous to insulating, the snow rarely stayed up against the wall for long and I had nice green grass in the spring. Within a year I started having carpenter ant problems finding nest in odd places like my second floor. It took me  a couple of years of fighting chemical war until I noticed one day a hole in the exterior of my exposed foam and some frass on the ground. I cut into the foam and found tunnels in the foam from the ground to the sill. I cut back the foam a couple of inches below the sills and dusted the exposed edge with diazanon and got them under control and haven't had issues since except in one odd spot. On the two walls I didn't foam on the exterior, I installed half sheets of foam down from the sill from the interior. There is not a thermal break in the wall  but the two foot overlap cuts down transmission. The snow on all four sides now stays for weeks up against the foundation so both systems appear to work about the same. About three years ago I had my sills sprayed and they tied into the top of foam sheets.

Do note that 25 years ago putting any insulation on foundation walls was still frowned by many builders. Their claim was the heat going into the ground was needed to keep the ground from freezing which would swell and push the wall in . I have actually seen this happen on older homes but generally the reason was the person who built the basement backfilled with the hard pan clay that is found about 2 feet deep in much of northern New England. It swells when frozen. My place is backfilled with gravel and has a full perimeter drain so its not a concern.


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## Circus (Dec 12, 2014)

Both methods are valid. What ever is easier. The thermal mass of a concrete wall, insulated on the outside, will tend to buffer the temperature swings if you heat with wood or solar. Both will cut heating costs.
I'm contemplating insulating the dirt under my garage with 2 or 3 ft deep 1" styrene around the perimeter using a ditchwitch. Any comments?


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## peakbagger (Dec 12, 2014)

Read DIck Hills article on Frost Heaves http://www.hotandcold.tv/professor hill.html. It will  explain a lot.


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## DickRussell (Dec 12, 2014)

Here's a good read on pros and cons of doing it either way:
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall

I did mine both inside and out. I think that if I had to do it over I'd just do it all on the inside. The "thermal mass effective R" effect applies more to situations where there is both a fair amount of exterior wall exposed to air and also a day/night swing above and below the interior temperature.

On insulating garage wall foundations in a heating climate, for sure insulating the foundation wall ("frost wall") is the thing to do, and inside, down to the footing, is the way to go, and I'd suggest 2" foam rather than 1". If the garage is uninsulated, and you just want to use ground heat to moderate the inside temperature, even keep it from freezing, then don't put foam under the slab, so that ground heat can come up.


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## begreen (Dec 13, 2014)

Unless building new, inside is what I would expect to be the most cost effective.


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## Seasoned Oak (Dec 13, 2014)

Im leaning toward the inside.


begreen said:


> Unless building new, inside is what I would expect to be the most cost effective.


My thoughts exactly, other side of the concrete wall is an unheated stairwell. Not earth. Too difficult to address the top and bottom of the wall from outside.
Also space is not heated continously so thermal mass benefit is negligible. In this case i think ill be doing the inside.


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## razerface (Dec 19, 2014)

My new house with full basement was done on the outside of the 9 inch walls. My friend, who built a similar house at the same time, did not insulate. His basement is always cold, and he does not try to heat it any more. Mine, after running my wood burner for a week or so,,,the furnace will not kick on for 24 hours after the fire has gone out in 20-30 degree weather, and the walls are warm. It does stay cool in summer.


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## begreen (Dec 24, 2014)

If building new I would definitely consider exterior insulation or building with an ICF foundation.


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