(C) Daniel Friedman Is it Necessary to Insulate Under a Slab in a Superinsulated House?

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Slab insulation placement guide:

Tthis article discusses the question of insulating under a basement floor slab, and just where is the optimum placement of basement insulation.

Do we insulate just the slab perimeter or the whole floor? Can we use uninsulated floor slabs to help with cooling? Critical insulation under radiant floors & passive solar floors. Advice for building insulation priorities.

This article explains that the location of building insulation is as important as its quantity. Sketch at page top is courtesy of Carson Dunlop Associates.

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Floor Slab Insulation Advice for Well-Insulated Homes

Heated floor slab insulation detail (C) Daniel FriedmanInsulation Advice for Full Basements - Do We Need to Insulate Under the Basement Floor?

The accompanying text is adapted and expanded from the original article with permission from Solar Age Magazine - editor Steven Bliss.

Reader Question:

Is it necessary to insulate under a slab in a superinsulated house? I assume an uninsulated slab would help cool the house in summer. -- William Lee, Cave Creerk AZ


Experts [in 1985] did not agree as to whether or how much to insulated below a slab. It is most important to insulated the perimeter of a slab down to the bottom of the footing.

Second most important [to insulating the slab perimeter down below the frost line] is to insulate under the first few feet around the edge of the slab.

In [moderate] climates where cooling as well as heating is an issue, it may be best to insulate only the perimeter and leave the underslab uninsulated, as you suggested, for cooling.

Full insulation is indeed recommended under the slab in some situations.

These include radiant heat delivery

or passive solar storage

Also, if wood flooring or carpeting isn't planned, underslab insulations will make the floor feel warmer to bare feet.

And if your concrete floor is a "floating slab" without below-frost-line footings, such as our thoughtless contractor installed in Northern Minnesota, insulation under the entire slab may be critical to avoid pumping most of your heat into the ground during the heating season.

If you are installing a radiant heat basement floor system, insulation under the floor slab and the location of the heating tubing are critical -

The material of choice for insulating a slab would be high density, closed-cell rigid foam board insulation, such as extruded polystyrene.

Thermally break the interior finish of the basement floor from the concrete (e.g. with wood or solid foam board) to increase comfort and to control condensation if that's a problem in your area during summer months.

If you are installing a radiant heat basement floor system, insulation under the floor slab and the location of the heating tubing are critical - see RADIANT HEAT FLOOR MISTAKES.

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

The question-and-answer article about Under-Basement Insulation , quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.


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