The Problem With Framing It Out
The traditional method stood a stud wall in front of the foundation, packed fibreglass insulation between the studs and stapled polyethylene over the face. That insulation fails in a way anyone can predict.
Masonry stays damp. Absorbent insulation touching it takes that damp on. The cavity has no route to dry, and the sheet on the warm face guarantees it. What grows in there is what people discover when the finished basement comes apart five years on.
Closed cell spray foam insulation goes directly onto the masonry. It bonds to the face with nothing between, insulates it, and manages vapour with the same layer of insulation. There is nothing absorbent against the wall and no void for damp air to occupy.
Half The Value Is At The Top Of The Wall
Where the foundation ends, the floor structure begins, sitting on a thin band of framing that faces the weather. In most houses the only insulation on that band is a scrap of fibreglass pushed against it.
It is a small area, but it accounts for a disproportionate share of the leakage.
Two to three inches of closed cell spray foam insulation around that perimeter is quick work, and it is the insulation that stops the floor above feeling cold.
We take that on as a standalone insulation job as well, for people who want the strongest return before committing to a whole basement.
Older Foundations
A good number of houses on the island stand on stone or rubble, and plenty have had block or poured work added later. Neither troubles spray foam insulation, but both deserve a look first.
Spray foam insulation bonds happily to an irregular face. What it will not do is hold back running water. A wall that is actively leaking gets repaired before we spray, and we say so plainly rather than sealing the evidence in. The same approach applies under a crawl space, where the foundation wall is just as irregular.
Allow For The Board
Spray foam insulation in a basement you actually use has to be covered. A thermal barrier is required over it, and half inch gypsum board is what normally satisfies that.
Put it in the budget. A basement figure that looks unusually low is very often one with the boarding left out, and that surfaces later.