[Greenbuilding] The dynamic R-value scam...

[John Straube]

While I heartily agree that thermal is oversold, the description of climate effects is wrong.

There is a thermal benefit in heating season in cold weather, even if the temperature rises above 40 or 50F. This is due to the fact that interior-coupled thermal mass can store the bursts of solar energy (or interior gains in office buildings) that would otherwise be wasted.  Basically, I am talking about passive solar heating mass, which does not have to be in the enclosure, but could be in the interior partitions floor, etc. Wood frame buildings are quite low on interior-coupled mass, and can benefit, in Oregon, or Minnesota, as well as Georgia and Arizona, from more mass. 
The problem is that the effect is small unless 1. the house has a really good R-value (the real kind) 2. the house is airtight and 3. there is enough south facing glass to collect an appreciable amount of cold weather solar gain (it does not take much glass area if conditions 1 and 2 are met.

This does not mean the mass needs to be in the walls, just that it would be helpful (read lower energy bills and more comfort) to have mass in heating seasons.


Kat wrote:

> Climate
> 
> While it is often useful to account for the thermal mass, or thermal 
> capacitance, of a building assembly, in Oregon’s climate, the heat loss 
> for a building during the heating season is determined by the whole-wall 
> U-value. In order for an assembly’s thermal capacitance to be of any 
> benefit in slowing heat loss, diurnal (daily) outdoor temperatures must 
> be above indoor temperatures for part of the day and below indoor 
> temperatures for another part of the day (typically nighttime). During 
> our heating season, these conditions are often found in the desert 
> southwest; they are almost never found in Oregon.
> During the cooling season, there are times when diurnal temperature 
> patterns enable some benefit from good thermal capacitance. But because 
> building cooling loads in Oregon are quite small compared to heating 
> loads, code specifications are based on reducing heat loss in the winter 
> months. Preliminary data from a mass wall assembly being monitored for 
> performance suggest that this is appropriate.
> 
> 
> Nick Pine wrote:
>  > "Jeannie Babb Taylor" <jeannie at babb.com>
>  >
>  >
>  >> ... the 1.2 per inch does not take thermal mass into account.
>  >>
>  >
>  > Ignoring thermal mass is the honest way to do it.
>  >
>  > Nick
> 
-- 
Dr John Straube, P.Eng.
Associate Professor
Dept of Civil Engineering & School of Architecture
University of Waterloo
Waterloo, ON Canada