[Greenbuilding] skepticism about mass wall values

[Topher]

On 4/24/2013 7:05 AM, Alan Abrams wrote:
> <The 'time to heat' a high mass house can be an order of magnitude 
> longer than a light house, and its return to ambient is similarly long.>
>
> to what extent is that a factor, assuming more or less continuous 
> occupancy and forced air heating?  if it were a troublesome issue, it 
> would be a disincentive for moody climates.

Forced air heating wouldn't be my first choice; one would need to 
transport a /lot/ of air to heat the building should it cool down.  Time 
to heat is only a concern if the temperature is setback for some 
extended time.  With continuous occupancy, the issues diminish, as the 
house is always close to optimum temperature.  I am not sure what you 
are implying with a 'moody climate'.  If inhabitants like to adjust the 
temperature based on weather outside, high mass is not recommended.
>
> but the subtext of all this is whether an AAC wall is sufficient on 
> its own, as its proponent put forth.  my position was simply that in a 
> 4000-5000 HDD climate, with long periods of low temps and no sunshine, 
> additional exterior insulation is advised.

I would suspect that it would depend on the details of that AAC wall.  
The steady-state R-value touted seem to vary wildly, from around 1.1 per 
inch to 3.9 per inch, I would want a pretty thick wall of the former.  A 
12 inch thick wall giving R-13 would be R-20 with the dynamic benefit of 
massive wall (from ORNL study for Minnesota),  given my experience, I 
would want to discount that for my climate, to perhaps R-18.  Just not 
enough, in other words.

Adding insulation on the outside not only increases the R-value, but 
also puts the wall into the more advantageous category of 'exterior 
insulated' massive wall.

I have no data on it, but I suspect that the massive wall benefit is not 
linear with respect to R-value.  Once you get to the point where loses 
are a few degrees over the course of a cold dark day, any heat input is 
getting stretched out over more time.

> Under such extreme circumstances, peak load would still be a function 
> of "steady state" R-values, even though it might take hours or even a 
> day to reach that point.  the lag provided by the thermal mass might 
> reduce annual demand; then again, this "time to heat" might actually 
> increase annual demand.

I don't see how 'time to heat' can possibly increase annual demand in a 
more or less continuously occupied building (intermittent is another 
thing altogether, of course).  Maintaining 68°F 24/365 is just a matter 
of heat loss * heating degree days.  High mass means that one is 
computing that on daily or even multi-day average HDD, as opposed to 
hourly computed HDD.  Which is/always/ going to be lower.

Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
topher at greenfret.com

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