Download the spreadsheet. Join our
linked In group and request the password.
This beta version spreadsheet can be used to
calculate the area weighted thermal performance of a wood frame
wall based on the percentage of studs and window area in
relationship to the total insulated wall area. You can see from
the graph that as the framing factor increases and/or window
area increases the actual wall R -value rating goes down.
Message: A R-20 wall is only a R-20 Wall if
there are no studs and no windows. As soon as you add the
framing and windows that R-20 wall is no longer an R-20 wall -
Final note: Even if you account for thermal
bridging to obtain the adjusted "real" R value, it still is only
an estimate because the enclosure and its R-values are also
affected by air flow, temperature, density, moisture content and
homogenous characteristics of the material.
See overview of
housing performance categories in North America.