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
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 heat.
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 builders did.
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.
Close the gap at the roof void end with expanding foam. Then insulate your cavity. You might even get some beaded insulation into the plasterboard gap, if the gap is of any size.
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 problem.
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