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 (12)

Rich M's picture


I recommend you do some research on attic ventilation and attic fans at green building advisor and energy vanguard web sites. You should also research vapor barriers, where and when to use them. Improper use of vapor barriers will cause moisture build-up.

bwolcott's picture

Don't forget about adding enough ventilation. Most homes don't have enough roof soffits. You can also add a temperature controlled roof fan that kicks on when it gets to hot ( A temp you set - Norther tool supply sells a solar powered module ) cold/warm zones halve to breath or all the insulation in the world won't stop mold and mildew from growing, having vapor barriers on the warm side of the wall is important to prevent moisture build up.

Fred Golden's picture

Rich M makes a good point. Say that insulating with R-6 insulation over 1,500 square feet is $300, and installing R-19 more insulation adds another 3 times the cost or $900 more, then the change to R-19 will save 1,850 Btu's per hour, while changing to R-38 would only save the same as changing R-13 to R-19, yet cost a little over three times as much in material. At the same time, installing the R-38 insulation can reduce energy consumption and size of the unit by 1/6 ton, saving about $600 in material costs of the HVAC system.

Rich M's picture

I recommend you go to the Green Building Advisor website. You will find a lot of great information there. They have a great series on the Pretty Good House.

What is the cost incurred to reduce that 724 BTUH heat loss? How long will it take to pay back that investment in energy saved, its measured in years. How long do you plan to stay at this home? It may be cheaper to invest in PV panels.

Passive Solar using large heat sinks and large glazed areas has been shown to be problematic and is not as cost effective as tight, well insulated construction.

Foil/ bubble duct wrap is not insulation. They have little R-value.
They reflect Radiant heat back to the source. Radiant barriers need an airspace to work. If the wrap touches the ducts the heat will conduct thru it. If the radiant barrier gets dirty it will not be effective.

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.

zap101's picture

Hi Rich could you explain why it is not a good idea to reduce the 724 BTUh by adding R-22 to R-38 to obtain R-60. Might this additional 724 mitigate the heat gain through the attic insulation in the summer enough to keep the dwelling pleasant without HVAC?

zap101's picture

Hi again Rich, thank you for your response. My concern is cost for the life of the dwelling and my own comfort. Thank you for your reference i have enjoyed many of their articles too.
Happy New Year

Fred Golden's picture

Basically installing another $1,000 worth of insulation adds to the home construction costs.

So if 50% of your heat loss is from R-7 insulation, and installing another $1,000 will increase that to the current Oregon code minimum of R-21 walls, then that might significantly decrease your heating and cooling costs, and allow you to install a smaller heating / cooling system. Yet a change from R-21 to R-60 might only reduce the heat loss through the walls by another 5%. Most of your heat loss at when you have R-21 walls will be from air loss in the home, not heat loss through the walls and ceiling.

Actually increasing floor insulation might increase the need for summer cooling. While it will decrease the amount of winter heating needs, it is a trade off. I am considering a ventilated floor passive heat sink system. It was well described in this book,
Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian. Basically it is 6X12 blocks laid on their sides to create north to south air channels, and with the addition of 4" concrete floor above it, creates a heavy 400,000 Btu heat sink (for my size home) that the air channels can passively move heat into and out of.

It should absorb enough heat each day to avoid turning on the A/C in the summer, and provide much of my needed heat load in the winter. It includes software to calculate the heat loss through the walls, ceilings, and air loss. Then lists each heat loss. Heat loss into the ground is really insignificant, due to the slow absorption rate of the ground under the home. He offers a calculation of heat through a basement wall into the ground. With 15 MPH wind, 0F outside and 70F inside, heat loss through 100 square feet of above grade R-20 wall is significantly more than R-6 insulated below grade wall exposed to 45F soil temperature.

Also consider active heating in place of an additional $5,000 in insulation. Say your calculated heat loss at 0F outside air temperature is 25,000 Btu's per day. Making up that heat with a evacuated tube solar collector and in the floor tubing might replace all that lost heat. Yet if you spent your 'budget' on insulation, it will not 'produce' heat, just slow it's leaving.

Today, installing $5,000 more in PV solar system and running a electric heater is less expensive than spending a additional $5,000 in insulation and trying to keep in that last 724 Btu's of heat. Spending $5,000 more on a grid tied solar system can change it from a 6 KW to 8 KW or perhaps 9 KW system!

Good luck with your projects!

zap101's picture

Hi Mr Goldman, thank you for your response to my question. However i am interested, not for my oregon home, but one in Nebraska i am superinsulating a grand old victorian home there. Double stud walls R40+ and R-60+ in attic. My costs for insulation are material only- my hobby labor is priceless. I have experience with the block floor system your considering. Keep in mind cleaning the block cavities and preventing mold and fungus is an issue , depends on where the home is and how subsurface moisture is mitigated etc.
Perhaps foam under a slab on grade floor might be a wise alternative & incorporating a solar water heater + pex in the floor makes a superior outcome. Saw video on you tube recently about a heat sink water storage system that is inexpensive.
Best of futures an happy new year . Tim

Rich M's picture

Please see my new comment.

zap101's picture

Happy New Year to one an all.
Ok i am almost all thumbs but i figure adding more insulation, in part, if using fiberglass is that it is never uniformly installed and it compresses some when installed. Same with cellulose . An extra $1,000 + labor over the life of the dwelling or mortgage is almost mute.

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