Q&A / 

Basement Insulation

DEAR TIM: I have a question about insulating the basement ceiling in a house that is only 2-years old. The first floor is made from 2x10s and the cost of fiberglass batts to put in the joists is about $1,000.00. How much benefit would I receive if I only added batts say five feet in from the poured concrete walls all the way around the floor, leaving the middle of the floor space open? Is this worth it? We do not use the basement much, but I am hoping to keep the first floor a little warmer. As for heat supply in the basement, there are only two vents for heat. Bill Bosken, Toledo, Ohio

DEAR BILL: My answer is going to bring tears to the eyes of the insulation salesmen and sales women around the world. You should take that $1,000.00 and buy a superb value stock that might jump up in price 30-50% in the next 6 months. Spending it on insulation between a tempered basement area and a heated first-floor is perhaps one of the poorest investments of capital you can make.

Insulation is designed to slow the transfer of heat. Although I do not have a PhD in thermodynamics, I can tell you the transfer of heat is a non-linear function. This simply means that heat loss is great when the differences between the two temperatures is significant. If the temperature of two objects is fairly close, the warmer object slowly cools to the lower temperature. You can prove this very easily.

Take two 16 penny nails and put them in your oven. Set the temperature to 400F and let the nails get up to that temperature. Take the nails out of the oven and place one in an empty metal cake pan and place it in your freezer. Place the other hot nail in a metal cake pan that you set on your kitchen counter.


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After one minute, open the freezer door and quickly touch the nail. I am confident you will be able to touch it without getting burned. Wet your finger tip and then touch the other nail that is sitting on your counter. I'll bet you get a quick hiss of steam as that nail is still quite hot.

This simple experiment should tell you that the heat transfer between your heated first floor which is probably 70-72F and your basement which might be 60F is extremely slow. The amount of energy you would save in fuel costs would be measured in pennies and possibly nickels each heating season, not in dollars. It could take you decades before you break even on the initial investment of $1,000.00 for the fiberglass batts you are thinking of placing in the floor joists.

What's more, you will discover that your basement will become more uncomfortable and colder than it is now. If you do any work down there, you may find yourself adding heat so you can work in relative comfort.

Because your basement walls are conducting cold into your basement via the cold ground outside, it might be worthwhile to add insulation over your exposed masonry foundation. You can choose to use closed-cell foam or fiberglass if you choose. But be sure you check with your local building department as some insulations that are flammable - such as closed cell foam - must be covered with drywall or other approved material to prevent rapid fire/flame spread.

I would also inspect the juncture between the wood framing and the top of the concrete foundation. Do this on a windy day and try to feel for air leaks. Air infiltration can be a major drain on your heating budget. Pack insulation in any cracks you discover or caulk them to stop air flow.

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