We're planning to remodel the basement, and we will add a wine cellar
to it. The basement has 8" to 12" thick block masonry walls. I want
the walls and ceilings of the wine cellar to be insulated as well as
possible, while the floor remains uninsulated (so we get the thermal
mass of the ground 8' below the house, without getting the thermal
fluctuations through the masonry walls, or from the rest of the house
or the rest of the basement).

My plan is to build the wine cellar as a room with double walls: Two
2x4 stud walls, with 2" of gap inbetween. The gap is filled with
sheet insulation (polyurethane or polyisocyanurate), R-7 per inch. On
the two outside walls (towards the masonry outside), the sheet
insulation goes directly against the masonry wall.

Moisture is not an issue. Our basement walls are perfectly dry
(thanks to a benign climate, good drainage, and the correct soils
outside). The inside of the wine cellar is finished with drywall
(needed for fire protection reasons). I need the stud walls to hold
shelving within the room. I can't make the walls any thicker, without
loosing too much floor area.

The default would be to insulate the stud walls with R-13 fiberglass
insulation. This gives a total insulation of R-40 for the interior
walls (two times R-13 stud wall, plus 2x R-7 for the sheet
insulation), and R-27 for the exterior wall. There are minor
corrections to the R-value (due to the studs, bolts to hold the walls
together, and reflective surfacing on the sheet insulation).

Would it be advantageous to insulate the stud walls with sheet
insulation also? I could take 2" and 1 1/2" sheets of such foam
insulation, carefully cut them into pieces that fit the wall cavities,
and drywall over them. At the edges (where a perfect fit is
impossible), I could use spray foam insulation (the stuff from spray
cans). In principle, this would greatly increase the R-value, as 3
1/2" of sheet insulation is R-24.5, compared to the R-13 available in
fiberglass. But I worry that in practice, the gain is much smaller,
given that the sheet insulation never fits perfectly, while the
fiberglass batts can be carefully stuffed into the walls.

Within limits, money is not the issue. The room is small enough (10'
by 6' finished size) that the cost of insulation is not all that
large.

Does it make sense to use sheet insulation? Is there anything even
better I can do?

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Ralph Becker-Szendy _firstname_@lr _dot_ los-gatos _dot_ ca.us