[Strawbale] Re: At what point (R value) do we reach 'enough' insulation?

[Rene Dalmeijer]

André,

You are quite right in concluding that this depends on climate and local
circumstances. The amount of insulation required depends on how passive you
want to be. If you accept some supplementry heating like a high mass wood
stove you don't need excessive insulation. At a certain point you lose so
much more heat through windows and ventilation the contribution of super
highly insulated walls does not help lowering heating requirements very
much.

If a house of 100m^2 has 125m^2 walls (windows 10% floor area) exterior
temperature is -10 interior 20 R=6 walls (normal bales and earth stucco) The
the heat loss 5 w/m^2 giving 625 w total heat loss through the walls. The
loss through good quality dubbel glaze windows at 45w/m^2 would be about the
same amount. Ventilation would be about 600w this is at absolute minimum
ventilation value with no heat recovery. Roof and floors would amount to
about 1200w loss. So there you have it doubling the insulation of the walls
is not going to make a big difference on a total of 3000w

Turning back to what is enough. Besides the amount of supplementry heat
required another deciding factor is the inner surface temperature of the
wall. Preferably this is as close to room temperature as possible. This is
the very reason why SB walls are considerd much more cosy then other stone
or earthen wall surfaces. In a cooling periode (ie late evening and night
time) a wall surface will be a little cooler then the room air temperature
this differential is a building physics given in the Netherlands we name it
rli. For a well insulated r=6 wall the differential amounts to about 0,5 C
which is quite acceptable. A difference of about 2.0 C starts to get
noticeable (a little uncomfortable) this is equivalent to about R=3 which is
equivelent to the high quality double glasing.

The above is a simplification. But effectively how much insulation depends
on what interior wall surface differential you consider as acceptable at
what outside and interior temperature and how many days per year. The above
is a worst case scenario for the Netherlands occuring maybe 2-3 days in an
extreme year. This means that there is very little purpose and use to
increase the insulation from a comfort point of view above R=6 here in the
Netherlands.

The next thing to consider is how much supplementry heating is considerd
acceptable. This decides how many heating days are required per year. If you
are aiming for no supplementry heating then you have to know how much heat
is coming in due to Sun(640w), people (3x85w), cooking and lighting(200w)
Total 855w. Then subtract losses like ventilation (300w). So you are allowed
to lose the rest (795w) through walls, floor roof and glasing Let say you
can improve the insulation of the windows with shutters to R=5 (very good)
the loss for glasing =100w. This leaves 695w allowable loss through the
walls, floor and roof meaning R16 average for the steady state daytime
situation which is a ridicules value ie you need to use some supplementary
heating for these very cold days. If you accept that you start heating on
days below 0 C outside temperature the required R=8 becomes which is much
more acceptable. In the Netherlands this occurs about 10 days a year. Ie you
have to use supplementry heating on 10 days in the year. The preceding is a
gross simplification because it takes no account of thermal mass and dynamic
effects and no direct sun. The solar heat gain is South facing on overcast
days.

I know from experience that a well designed house with R=6.3 walls, roof and
floor and shuttered normal double glasing required no extra heating on 0C
days. Just cooking already required extra ventilation to avoid too high
temperatures. The maximum required heating capacity is 1000w which equates
to a minute wood stove. The houses involved have a balanced ventilation
system with heat recovery.

Greetings,
Rene