Are colloquial R-value recommendations based on nominal or true R-values?
andyfrog
| Posted in Energy Efficiency and Durability on
https://www.buildingscience.com/documents/insights/bsi-081-zeroing-in
In this article, Lstiburek says “Here is the list of metrics for the enclosure and equipment for an ultra efficient house in a cold climate like Boston or Chicago or Toronto (as defined by me4):
- R-5 windows
- R-10 slab insulation
- R-20 basement insulation
- R-40 wall insulation
- R-60 roof insulation”
Are these nominal or true R-values?
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Replies
Andy, those are generally considered to be nominal R-values, though to achieve them it's usually easy to incorporate thermal breaks so they are also close to whole-assembly R-values.
OK, thanks! What about the 'framing factor'? (which I gather is distinct from thermal bridging and merely represents the amount of insulation displaced by framing members, although I'm not sure)
Both. The framing causes the thermal bridging because its R-value is lower than the insulated wall sections. The amount of thermal bridging is proportional to the percentage of wall framing compared to insulation. So the higher the framing factor, the more thermal bridging causes increased heat flow and the more the whole-wall R-value deviates from the nominal R-value.