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Is there a significant advantage to providing more insulation in north facing walls in heating dominated climate zones?

For example n Santa Fe, NM (CZ 5B), where there is a significant heating load during the heating season but very minimal need for space cooling in the cooling season, would the heat loss through north facing walls be reduced enough to warrant adding more insulation to those north facing walls. I believe that may be the case. I believe because south walls may absorb more solar-generated heat and conduct that heat into the home (or reduce the outward flow of heat from the home) that it may be better to have 'well insulated' south facing wall systems and more highly insulated north facing walls where there is no benefit of heat gain during the heating season due to solar exposure. Is this thinking correct?

Asked by kim shanahan
Posted Aug 9, 2014 11:24 AM ET


2 Answers

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Generally speaking, I think what you are saying is accurate. However, I believe you would have to perform heat loss calculations, including solar gain calculations, to know if this idea is worthwhile.

I would want to compare the cost of the additional insulation (and possibly framing) to the proposed savings. My feeling is that going from something like R 19 to R 22 in the north wall would not net you noticeable results, and may or may not be financially justified.

Strictly speaking, the sun's energy (heat) hits all walls of a house, even the north (via diffuse irradiance). Obviously, the south facing wall (or windows) are hit more often, and directly.

Windows aside, the sun warms your building envelope where it strikes. This would, theoretically, lower the effective delta T, a little. Which in turn may slow down heat loss through the wall in question, again, a little.

Whether this increase in your decrement delay is significant depends on a lot of things, such as density, mass, emissivity, R value and probably most important, your design goals.


Answered by Jason Hyde, Peterborough 6A
Posted Aug 10, 2014 2:00 PM ET
Edited Aug 10, 2014 3:19 PM ET.


Hi Kim, I believe your concept is a sound one, but will require some difficult math to quantify. I would need to go back to some notes from grad school, but the phenomenon you are describing I believe is called the Sol-Air Temperature. When calculating a steady state heat loss for a building, or a room, it is fairly straightforward to determine the rate of heat loss via conduction and infiltration. And many people also calculate the heat gain due to solar radiation transmitted through windows, for south facing walls. However, you can go a step further and use another set of formulas to adjust for insolation lowering the effective delta T between indoor and outdoor. Imagine a cold sunny winter day. For the north facade, using indoor air temperature and exterior air temperature for your delta T would yield accurate results, because the very outside layer of the exterior siding should be pretty close to the exterior air temperature. But on the unshaded southern facade, the solar radiation is partially transmitted through the windows and mostly absorbed by the opaque surfaces. If the sun heats up the siding enough, it can slow down the heat flux due to conduction. Incidentally, the reverse is true for the roof. On a clear night the roof shingles can be colder than the ambient air temperature due to heat loss through radiation. You might have some luck researching with Google, or you can find equations in ASHRAE Handbook: Fundamentals. One final note, the magnitude of this effect will depend on your climate and geography. In the summer, the sun's elevation could be high enough so that it will be hitting the roof much more than the southern walls. And in the winter when it is low, you might not have many sunny days. Good luck!

Answered by Stu Turner
Posted Aug 11, 2014 2:09 PM ET

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