re: insulation techniques on new home
22 aug 2000
>...my point is that the material provides a cost effective
>alternative to insulated frame construction.
seems to me that sips are more cost-effective,
and i'm suspicious of these "dynamic r-values."
>...dynamic (effective) r-values for rastra® walls of three different
>wall-thicknesses are shown in the following table:
>location rdynamic wall thickness
> (hft2f/btu) 10 inch...
let's look at a 10" wall in minneapolis...
>steady state r-value 2) 11.0
>1) computer simulation has been based on a steady state r-value
>of 7.6 and a 10" thick wall.
>2) steady state r-values are based on tests issued by two independent,
>accredited laboratories in usa and europe.
first, is the steady-state (ie ordinary) r-value 11.0 or 7.6?
let's assume 7.6, as used in the simulation.
now, here are nrel's avg daily min, 24 hour avg, and avg daily max temps
for each month in minneapolis:
jan feb mar apr may jun jul aug sep oct nov dec
2.8 9.2 22.7 36.2 47.6 57.6 63.1 60.3 50.3 38.8 25.2 10.2
11.8 17.9 31.0 46.4 58.5 68.2 73.6 70.5 60.5 48.8 33.2 17.9
20.7 26.6 39.2 56.5 69.4 78.8 84.0 80.7 70.7 58.8 41.0 25.5
let's say a mythical massless standard house has 100 btu/h of r7.6 walls
with no thermal mass or internal heat gain or air leaks. if it's 68 f for
24 hours per day, adding mass to the external walls does not decrease the
heating bill from october through april, when the outdoor temp is always
less than 68 f. the massless house furnace needs to supply more heat when
it's 2.8 outdoors and less when it's 20.7, but adding thermal mass to the
walls doesn't change that: with lots of wall mass, the furnace needs to
supply the same amount of heat over 24 hours a day to the equivalent of
a constant outdoor temp of 11.8. from october through april, each house
needs 30dx24h(7x68-48.8-33.2-17.9-11.8-17.9-31.0-46.4)100 = 13,968k btu.
so the concrete doesn't do any good at all thermally, over this period.
it just lowers the r-value and raises the price of the wall.
in may, suppose the massless house furnace needs to supply 12h(68-47.6)100
= 24,480 btu/day of heat and 12h(69.4-68)100 = 1,680 btu of cooling, a total
of 26,080 btu/day. add lots of thermal mass to the walls, and the house only
needs 24h(68-58.5)100 = 22,800 btu of heat; 26,080/22,800 = 1.14, so we might
say the "dynamic r-value" is 1.14xr7.6 = r8.69 in may, at best. or we might
say the massless house needed 14,750,400 btu from october through may, and
the one with lots of mass in the walls needed 14,652,000, ie 0.7% less.
the massless house needs 12h(68-57.6+78.8-68)100 = 25,440 btu/day in june,
vs 24h(68.2-68)100 = 480, for a dynamic r-value of 25,440/480xr7.6 = r402.8.
wow... then again, the massless version needs 15,513,600 btu from october
through june, and the massy one needs 14,666,400, ie 5.5% less.
in july, and august and september, the houses need 752,400 and 403,200,
734,400 and 180,000, and 734,400 and 540,000 btu, bringing the yearly
totals to 17,734,800 and 15,789,600 btu, if i did that right, for a
"seasonal dynamic r-value" of 17,734,800/15,789,600xr7.6 = r8.5 vs r22.6.
but there's no such thing as a massless house, or a wall with infinite
thermal mass, so the difference would be less in the real world, no?
and a house with inherent internal mass and purely resistive walls might
store lots of coolth with night ventilation in may through september in
minneapolis, given the daily min temps, with no need for air conditioning.
may jun jul aug sep
47.6 57.6 63.1 60.3 50.3
58.5 68.2 73.6 70.5 60.5
69.4 78.8 84.0 80.7 70.7