Climate Minute #3 – Roof Insulation for the Mid-Century Modern Home

Deck Houses, Techbuilt Houses and other mid-century modern homes all share one key feature: roof insulation is usually installed above the roof deck and below the roofing material. Therefore, upgrading insulation to improve the energy-efficiency of the home is normally done only when the roofing is replaced. Because the roof is one of the greatest sources of heat loss and gain, good roof insulation is critical for moving a home toward zero carbon emissions. Residents of mid-century modern homes with insufficient roof insulation should plan to upgrade insulation the next time their roof needs replacement.

How much insulation is enough? U.S. Energy Star guidelines for insulation in Massachusetts currently call for R49-R60 in a roof. Allowing some insulating value for the roof deck, that’s roughly 8-10 inches of the best foam insulation (polyisocyanurate @ R5.6/inch). Click here to learn how to estimate how much insulation is in your roof.

What’s the long-term impact of replacing a roof without bringing the insulation up to the recommended level?

• The most notable difference would be missing out on the extra comfort of having a well-insulated roof.

• Greenhouse gas emissions, of course, would be greater.

• There is a growing sense that resale values on poorly insulated homes will be lowered in coming years.

• Heating and cooling costs would be noticeably higher.

For a cost comparison, assume:

• A 2-story home with a 1,250 sq. ft. roof

• The roof has an R-value of 20 (from 3” of the best rigid foam plus the roof deck)

The difference in heat loss over thirty years between an R20 roof on the above home and an R49 one would be about 150,000,000 BTUs. Providing that heat would cost an estimated $5,000-$6,000 if heating with oil or heat pumps at current rates (less for natural gas), though actual costs will almost certainly be far higher.

The calculations above do not consider likely heat loss from (hard-to-quantify) air leakage through the existing roof. Including that heat loss in the calculations would bump the above costs even higher.

One would need to add about 5” of the best foam insulation to bring the roof to R49. The insulation would cost roughly $5,000. We don’t have good numbers for labor costs.

If you currently have only 1-2 inches of roof insulation, then talk with your builder or roofer about potential moisture issues that might develop if a lot of insulation is added, and ways to avoid them.

Although MassSave rebates cover 75% of the cost of attic insulation, there are currently no MassSave rebates for insulation applied above a roof deck. That’s puzzling, and CFREE has begun exploring the rationale for that policy and what might be done to change it. We’d love to hear from anyone with insights about how best to update that policy.

Michael Moodie