Published on October 24th, 2013 | by Henri


Insulating a basement from the inside

Q.    I built a new house several years ago and did not insulate on the outside during construction. I am now planning to stud the inside basement walls and install fiberglass insulation.

I plan to place the studs on 16″ centers 1.5 inches away from the concrete. The insulation will end about a foot from the basement floor and the 1.5″ gap sealed off. What do you think?

A.    If you install 2-inch by 4-inch studs 1-1/2 inches from the concrete walls, you will have a 5-inch space into which to put R-19 unfaced fiberglass batts. This is fine, but you must cut strips of fiberglass insulation and place them in the 1-1/2-inch spaces behind the studs,  as you progress with the insulation of the stud spaces.

This will give you a complete job without anomalies. Be sure to staple a 6-mil plastic vapor retarder on the face of the studs before applying the wall finish. But why not use 24-inch on center framing to save money?

As to where to stop the insulation from the concrete floor, and assuming that your basement is dry, there are several important considerations. You must be sure that there is a working drain outside the basement foundation walls; that the foundation is backfilled with coarse, well-draining material; and that the grade slopes away from the house. If these three conditions are not met, you should not insulate lower than three feet below grade in order to allow some heat to warm the soil, so it won’t freeze and potentially crack the walls.

You should also consider insulating the foundation with 1-inch or 1-1/2-inches of extruded polystyrene adhered to the walls before studding and using R-15 fiberglass between the studs. This will give you a better job and be simpler to do.

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