When we bought this 1858 house it was gutted on the inside which meant we have a great opportunity to super-insulate it and “seal the envelope” to keep out air infiltration.
The original walls were basic 2×4 construction, so we decided to build a second wall inside the building at 12″ from the outside wall. This construction technique allows us to stuff the space between the two walls with dense-pack cellulose to get to an R value of 40. As a reference point, Energy Star houses aim for R25-30 walls. Another way to achieve super-insulation is to use closed cell foam. It would take about 6″ of foam to achieve the same R40, but in today’s market this is much more expensive so we opted for thick walls.
With the double wall construction we can also ensure that the insulation is continuous within the wall, not broken up by studs as is typical in a 2×4 or 2×6 wall. Dense pack cellulose provides insulation as well as an air barrier. Fiberglass, by contrast, provides some insulation but allows quite a bit of air to pass through it.
Which brings me to the second part of the “warmth” of this house, which is the air barrier. There are tons of ways that air can leak in (or out) of a typical house. The windows and door jams are the most obvious… but even the lighting fixtures and outlets can be sources of air leaks.
In order to seal out all the leaks in this very old house (the original wall sheathing is still in place), we used smaller amounts of foam in every crack, along the original boards, around all the windows and doors, and we ensure that all the wall penetrations (like outlets, switches, and vents) are well-sealed with the foam or the dense-pack cellulose.
We also used the foam around the sills on each level where there is a very large wood beam. Wood is not a very good insulator, so we beefed up these areas with foam.
And then there is the basement. Our basement is mostly crawl space with a dirt floor with a stone foundation. The best way to ensure a continuous air seal and insulation throughout the entire house was to foam the basement — floor and walls right up to the first floor. We have added some plywood boards where we need to walk to maintain the heat exchanger for the solar collector to storage tank. All other equipment is located in the utility room on the first floor.
Next week the cellulose arrives and we will be able to heat this building with a candle!