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Buildings As Radiators

Until about fifty years ago, U.S. builders and homeowners weren’t terribly worried about insulating our houses. Once we started to care, we still had a lot to learn.

In 1967, my father built a Capp Homes Finish-It-Yourself house on a site in the Iowa countryside. Their marketing pitch was “The House You Want – At The Price You Can Afford”. The house he wanted was well-insulated by the standards of that time – he wanted 5 ½ inches of insulation in the exterior walls instead of 3 ½”. The company refused, saying that much insulation would result in condensation and rot in the walls. He didn’t quite get the house he wanted.

Fast forward to today. We know that an exterior wall assemblies’ ability to control heat loss is key to reducing energy loads and maintaining a comfortable indoor environment. We also know that to some extent the folks at Capp Homes were right. Done incorrectly, highly insulated exterior walls can trap condensation and result in a host of ills from mildew to rot to supporting critters in the walls. Luckily we have several tools they didn’t have in 1967, chief among them fifty-some more years of research and WAY more computing capacity.

One design tool we use in our office is WUFI ®, a moisture design software created by the Oak Ridge National Laboratory.

In thinking about that directive to my father from Capp Homes in 1967, and the results of many a WUFI® analysis, my conclusion is that the state of the art in 1967 was a wall that would keep the temperature of the exterior sheathing above the dew point.

In other words – on an Iowa winter night, with an outdoor temperature of -10°F, that 3 ½ inches of insulation was letting enough heat from the inside of the house through to the outside to keep the exterior wall sheathing above 40 degrees – fifty degrees warmer than its surroundings. Our family home was radiating a LOT of heat to the great outdoors!

Today we still try to keep the inside surface of the exterior wall sheathing above the dew point in heating mode to prevent condensation in the wall. But we’ve moved the face of the exterior sheathing INTO the wall, and put more insulation between the sheathing and the exterior finish. That allows the exterior finish to be much colder, closer to the outdoor temperature, while the interior finish is much warmer, closer to the interior air temperature.

If you’re interested in learning more, check out our upcoming blog about our WUFI analyses of some typical, and some less- typical, new single-family home exterior wall assemblies in the Philadelphia area.

While this analysis focusses on new home construction, the same analysis can be applied to energy retrofits of existing buildings of any type.