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

Coralie & Andre de Bouter m.ep at laposte...
Wed Jun 26 14:16:49 CEST 2002

Dear René,

Thanks for your explication making a house comfortable is a complex but
tremendously interesting field.

I know you simpify things in order to keep this mail short (instead of
writing a book on the subject).
So in that simplified line of thought; By what outside temperature do we
start heating an example house 150m²
with standard double glaze windows and walls- & roof insulation at:

? :-)


----- Original Message -----
From: Rene Dalmeijer <rened at cistron...>
To: <strawbale at amper....muni.cz>
Sent: Wednesday, June 26, 2002 12:04 AM
Subject: [Strawbale] Re: At what point (R value) do we reach 'enough'

> André,
> You are quite right in concluding that this depends on climate and local
> circumstances. The amount of insulation required depends on how passive
> 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 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
> 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
> 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
> 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
> 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
> 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
> 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
> 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
> have to use supplementry heating on 10 days in the year. The preceding is
> gross simplification because it takes no account of thermal mass and
> 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
> 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
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