My house is detached built about 17 years ago. It is conventional
design with a cavity wall. It does not seem to retain heat and the
walls always feel cold. There is a gap between the plaster board and
the insulating blocks. This seems to be open to the roof void and
there is a strong current of cold air in there (as I found out when
installing some new electric points). This would seem to negate the
effect of a thick wall by reducing the effective insulation to the
thickness of the plasterboard.
What is the purpose of this gap?
Have the builders left something undone?
What can I do about it to save on my heating bills?
The normal cavity is not insulated but it would appear to be a waste
of time with this
On 1 Feb 2004 01:48:49 -0800, email@example.com (J) wrote:
There is a gap behind the plasterboard because the fixing method is
dabs of plaster on the blocks and then the board pushed onto it.
This is the standard way of fixing.
Although there is an airflow, it is not likely to be the major loss of
The real issue is that the cavity is not insulated. In a house of
that age (mine is about the same), I am not sure that it was mandatory
to insulate the cavity as part of building regulations; although some
The difference in doing so is remarkable.
Heat loss through a surface is determined by the area, the temperature
difference and a factor based on the insulating properties of the
surface, also known as U value. For a wall, for example, you measure
the area in square metres, temperature difference in degrees
centigrade and multiply these numbers together with the U value.
For example, a wall of the type you describe you have, will have a U
value of between 1.0 and 1.5 W/m^2.K
If you were to have it filled with cavity insulation, it will drop to
something in the 0.4 to 0.5 region.
For an outside wall, you can count the exterior surface with the full
temperature drop from indoors to outdoors. In your gap behind the
plasterboard, although there is a flow of air, it is not equivalent to
the outside because there is effectively the insulating aspect of the
walls and also a flow of air. Although it is open in the loft, and
you could perhaps seal the gap (not the vents into the loft), there
needs to be a path for the air. You could be rigorous, and go round
and seal up all the electrical outlets as well, but in comparison with
the non-insulated walls, this is going to be a second order effect.
In a detached house, the heatloss through uninsulated cavity walls is
likely to be the largest proportion of the total through surfaces, and
usually exceeds the amount through roof, windows and floor. It can
easily be half of the total surface loss. There are additional
ventilation heat losses, but in a house of this age these are not
normally the major factor until the surfaces are insulated.
In a recent thread on insulation, a complete calculation for a house
of mixed construction was presented. You could do a similar
measuring exercise and see where the factors are for your place.
To email, substitute .nospam with .gl
My feelings exactly, best money I spent, but ....
After the blasting with wind and rain last Weekend I had water dripping
in from above the patio door frame (South facing wall). If you live in
an 'exposed' area (Cornwall, Cumbria, West Scotland) full-fill cavity
insulation _may_ cause water ingress problems. Your local BCO will
advise, but make sure your pointing is sound anyway. Any sign of crumbly
mortar (not unknown even on younger houses) and you might have a
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