My insulated, air tight coffee mug

If you’ve ever met me you’ve likely seen my coffee mug. If you’ve been to an A-HEET event you’ve likely seen me use that coffee mug as an example of insulation vs. weatherization.

The two are very different, but go hand in hand.

If your home is insulated but leaky, you’ve got a big problem because the cold air coming into your home defeats any amount of insulation you may have.

If your home is air-tight (weatherized) but doesn’t have any insulation you’re well prepared to insulate, but you’re still letting all the warm air out through radiation conduction.

My coffee mug, purchased from a large well-known coffee purveyor, is both insulated and air-sealed. If I put hot coffee in there, close the lid and snap the thing shut, it will stay warm for close to three hours.

My insulated, NOT air tight coffee mug, making me unhappy.

However, if I pop just that little lid so I can take a sip of coffee and leave it open, that coffee will be cold in 30 minutes. Why? Because while the insulated cup is preventing the heat from escaping through radiation conduction, the hole in the top of the mug is letting all the heat out, the cold in, and making me very unhappy.

If, instead of my mug, I use one of those paper cups and snap a plastic lid on top and leave it shut, it will still get cold quickly. Even though that paper cup is now well sealed, that paper has almost no insulative value whatsoever.

Same thing goes for your house. If you’ve got good insulation in most locations but a few well-placed holes, you’re still going to feel cold all the time. This is why the two go hand-in-hand.

One other note: The existing insulation in most homes is fiberglass batts, which do not air-seal at all, despite what most people think. As an example, the filter on most furnaces is made from fiberglass. They’re great as dust filters because they trap all the dirt but let the air go through into the furnace where it is needed for combustion. Fiberglass batts in your attic aren’t stopping air flow, they are slightly slowing down the heat loss due to radiation conduction.

In fact, if you peel back a fiberglass batt located next to an air leak you’ll find it is filthy. That’s not the batt disintegrating, it is all the dust coming through a hole getting trapped by the fiberglass. Fiberglass batts are almost never installed properly, leading some to, jokingly, suggest it should be outlawed.

UPDATED 12/28/2010 – As Jason, from Cambridge HEET politely pointed out, I confused “conduction” and “radiation” in the original post. Most insulation slows conduction, not radiation. Thanks for the correction and my apologies for the error.