ASK THE EXPERTS: Cooling & Insulation

Attic insulation
In the summer, attic insulation helps reduce heat gain through the ceiling to living spaces below. In the winter, it helps slow heat loss from the interior spaces.

I just read Claire Anderson’s article “Design with PV in Mind” in HP154. We live in Massachusetts in a 60-year-old home, although the second story of the house was added 12 years ago. During this past heat wave, when temperatures were in the high 90s, our bedroom temperature reached 88°F. Will adding insulation to the attic to bring it up to modern standards keep upstairs rooms cooler during the summer?

Denise Sheppard • via email

Added insulation in the attic will make the upstairs rooms more comfortable in the summer. It will also reduce your cooling and heating loads, saving you money on your utility bills.

Just to give a rough idea of your savings, consider this: Let’s assume your attic is 1,500 square feet and it is currently insulated with 6 inches of loose-fill fiberglass for R-13. On a hot summer day when the attic temperature is around 120°F, the heat gain into the living space through the ceiling is 5,800 Btu per hour. Nearly a half ton of air conditioning would be needed to take care of that heat gain.

Adding 16 inches of cellulose over the existing insulation will give you R-60, dropping the heat gain from 5,800 to about 1,250 Btu per hour—a reduction of nearly 0.4 tons of air conditioning, which will make it easier for your upstairs cooling system to keep the temperatures lower. (The winter savings for this insulation upgrade in Boston, if you are heating with electricity at $0.12 cents per kWh, would be about $430.)

Before adding new insulation, seal all of the air infiltration pathways between the living area and the attic—including around wiring, plumbing penetrations, ceiling light fixtures, vent fans, and the attic hatch. In many homes, these leaks are the single largest source of air infiltration, and plugging them will reduce your heating and cooling bills and reduce the chance of moisture problems in the attic. This sealing is a lot easier to do before the new insulation goes in. This is also a good time to seal and insulate any heating or cooling ducts in the attic.


Comments (2)

Fred Golden's picture

It is a great idea to seal the air ductwork, then bury it in the blown in insulation. Think about it - the very coldest air in your home in the summer is inside the A/C cooling duct, and it is only insulated with perhaps 1" of insulation at R-3? And if 10% of that air leaks out every hour, then you are basically cooling the outside of your house with the A/C unit. And heating a lot of cool attic space with the furnace!

Consider wrapping the air ducts with a product like "Reflectix" and taping all the joints to prevent air duct loss. It will increase the R-insulation by about R-5, while also dropping the air leaks significantly! Then bury it with about 4-6" blown in insulation, and you will have well insulated air ducts, and well insulated ceilings!

You should also consider the siding. If you where to install 2" rigid foam, covered with new siding, it will prevent 'thermal bridging' where the framework of the house conducts heat to the outside in the winter, to the inside in the summer. The siding can be taped to make it a air and moisture barrier too, so the curtains will no longer blow around on windy days!

If your current siding insulation consists of R-1 3/4" wood siding, then adding another R-12 or so will keep the heat load way down in the summer, and lessen the heat loss in the winter!

Rich M's picture

Should be aware of the diminishing returns of insulation and the importance of Air Sealing. The heat gain/loss reduction deminishes greatly after the 1st R-19 which stops about 90% of the Heat L/G Vs. R-1.

1500sf at R-13 and 50 Deg Delta T = 5,800 BTUh
1500sf at R-19 and 50 Deg Delta T = 3,947 BTUh + R-6 = - 1,853 BTUh
1500sf at R-38 and 50 Deg Delta T = 1,974 BTUh + R-19 = - 1,973 BTUh
1500sf at R-60 and 50 Deg Delta T = 1,250 BTUh + R-22 = - 724 BTUh

Adding that extra R-21 to existing R-38 to reach R-60 only reduced Heat Gain by 724 BTUh.

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