How Much Insulation is Needed?

Standard residential construction in much of the country is 2x4 framing with fiberglass insulation, achieving a paltry R-10 or so in the walls. If insulation is installed at all on the foundation walls, it's rarely more than an inch thick, and insulation is almost never put under slabs. In Vermont, we typically do a lot better. Act 250, enacted nearly four decades ago, required developers to improve energy performance and that led to a widespread switch to 2x6 framing in home building.

But 2x6 wall construction is still woefully inadequate in my book. A well-built 2x6 frame wall insulated with dense-pack cellulose or fiberglass will achieve only about R-17 or R-18 (accounting for the "thermal bridging" through the more conductive wood studs). If we want to have a chance of achieving the carbon-emission-reduction goals that climate scientists tell us will be needed--80% reduction by 2050, or even sooner--we will have to start insulating houses much better.

So what's a reasonable target?

Building science expert Joe Lstiburek, Ph.D., P.Eng., of Building Science Corporation in Westford, Massachusetts, argues that for any house north of the Mason-Dixon Line we should follow the "10-20-40-60 rule" for R-values: R-10 under foundation floor slabs; R-20 foundation walls; R-40 house walls, and R-60 ceilings or roofs.

Getting to these insulation levels is a challenge, but it's far from impossible. Here are a few ways in which the 10-20-40-60 insulation goals can be achieved:

R-10 under concrete slabs. This can be achieved with 2" of extruded polystyrene or XPS (for example, Dow Styrofoam), 2-1/2" of high-density expanded polystyrene (EPS), or 2" of spray polyurethane foam put in by a skilled insulation contractor. In cold climates like Vermont's I think sub-slab insulation levels should be boosted even further--to about R-20, with 4" of rigid foam.

R-20 foundation walls. This can be achieved with either interior or exterior foundation insulation or with insulated concrete forms (ICFs). With exterior insulation, most common is XPS, but I'm a big fan of rigid mineral wool, such as Roxul Drainboard, which provides R-4.2 per inch and comes in thicknesses up to 2-3/8" (so two layers of their thickest product will get you to the R-20 goal). If insulating on the interior, a reasonable approach is to add a 1" or 2" layer of rigid insulation against the foundation wall then add a 2x4 or 2x6 frame wall with cavity-fill cellulose or fiberglass insulation. With ICFs, many products are available with at least 2" of high-density EPS on both the interior and exterior faces, so that the R-20 goal can be achieved fairly easily.

R-40 above-grade house walls. Achieving R-40 in walls is a challenge. Here are several options that get you pretty close to that: a 2x6 frame wall with dense-pack cellulose plus three inches of foil-faced polyisocyanurate foam on the exterior; a double 2x4 wall separated by at least 3" with the resultant cavity filled with dense-pack cellulose or high-density fiberglass batts (at least 10" overall thickness); and a structural insulated panel (SIP) system with 9" or 10" panels.

R-60 ceiling or roof. Such a high insulation level in the ceiling (unheated attic) can be achieved with 16" to 18" of cellulose insulation or high-density fiberglass batts--you may need somewhat more to achieve the recommended R-value after settling. If the roof is being insulated (above a cathedral ceiling), getting to R-60 will typically require a combination of cavity-fill insulation in the rafters or trusses and rigid insulation on top of the roof sheathing.

Combining these insulation levels with a compact design, modest passive solar features, triple-glazed low-e windows, and high-efficiency lighting and appliances should get the energy consumption of new homes to less than a quarter that of standard new homes. The energy requirements for such homes should then be low enough that most, if not all, of the remaining energy needs could be satisfied (now or in the future) with photovoltaic (solar-electric) panels to achieve net-zero-energy or carbon-neutral performance.

Over the coming weeks, I'll examine various issues relating to extremely well-insulated houses as well as what can be done with existing houses--the concept of "deep-energy retrofits."