(C) Daniel Friedman Basement Insulation Location Insulate Under Basement Floor Slab?

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Basement insulation guide for floors, walls, ceilings: this article discusses the question of insulating under a basement floor slab, and just where is the optimum placement of basement insulation.

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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. The accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Basement Floor Insulation Advice

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

Insulation Advice for Full Basements - Do We Need to Insulate Under the Basement Floor?


I know that it is important to insulate basement walls, but how important is it to insulate basement floors? What type of insulation is recommended under a 3- to 4-inch concrete basement floor slab? -- Harold Murray, Bowie MD


Heated floor slab insulation detail (C) Daniel FriedmanResearchers at the University of Minnesota's Underground Space Center suggest that it is generally not economical to insulate under a full basement unless one edge of the slab is close to the finish grade, as in a walk-out basement - or if the floor is to be heated (our photo at left).

This is particularly true in climates with significant cooling loads, where the conductive losses through the basement floor slab will be appreciated during summer months.

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.

What about basement wall insulation?

Mineral wool insulation in an atticMr. Bliss prefers insulation on the basement interior for the same reason that apply to crawl spaces, including avoiding frozen pipes. He suggests using a section of rigid foam insulation to separate off hard-to-insulate basement locations such as steel bulkhead basement walkout doors.

Our photograph (left) shows sytrofoam insulating foam board used on the inside of a basement foundation wall.

We prefer to use solid foam insulation in any below-grade location that is at risk of period high moisture, because our field and lab work have shown up frequent hidden toxic mold reservoirs in fiberglass insulation that has been used in those locations.


Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original artivcle

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


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