Published on June 13th, 2016 | by Henri


Insulating basement of restored farmhouse

Q.  We recently restored an 1860s farmhouse, part of the work was building an addition with a full 8-foot basement. I use the lower section as a workshop and insulated the outer walls on the outside before backfilling. There is about a 2-foot thickness of #2 bluestone on the outside of the walls and perforated drain pipe leading to a gravity drain.

Our access on the lower level allowed us to put a stone veneer on the outer wall closest to the basement door. My problem is that wall is uninsulated above grade and my “workshop” is cold. What would you suggest on the inside? We placed polyisocyanurate with the reflective barrier to the inside on the outer wall and I am thinking of placing the same material inside. We would use furring strips, then polyisocyanurate-reflective covering to the inside and then cover with half-inch plywood.  Your thoughts would be appreciated.

A.  If I am interpreting your description correctly, my understanding is that:

  • The shop is underground
  • Polyiso insulation was installed on the foundation walls before backfilling
  • One of the new basement walls is above grade because the land slopes, and that allowed a grade level entrance door
  • And that this wall has polyiso on the exterior, reflective film facing toward the inside, and a stone veneer over it.

In that case, this wall IS insulated. It would have been better to use extruded polystyrene (XPS) on the backfilled outside foundation walls, as it does not absorb measurable moisture. The reflective foil serves no purpose on the outside of the foundation as, to be effective, it needs to be kept clean, and to have an air space between it and a facing material.

To increase the insulation on the workshop walls, I assume that you are planning to fasten the furring strips to the inside of the foundation wall and install the polyiso insulation over them. If so, place the reflective film facing the foundation wall for it to be effective.

An alternative is to install the polyiso directly to the masonry walls – the reflective surface facing inside — and the furring strips over the insulation, shot into the foundation with power-studs of the correct size. This will give you the air space the reflective foil needs to add to the rigid insulation’s basic R-factor. Be sure that you clean the foil well before fastening the finish material to the furring strips.

If my assumptions are incorrect, please give me more detail information.

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