"Ed Sirett" wrote in message
news
On Wed, 10 Dec 2003 18:31:34 +0000, Andy Hall wrote:

On Wed, 10 Dec 2003 17:32:29 -0000, "Christian McArdle"
wrote:

When you add an extension, modern building regulations are going
to require cavity walls and double glazing and the heat losses
in this part are going to be a lot less (could be only a quarter)
of the equivalent size of existing house.

Indeed, I've heard of cases where adding a two storey extension has
actually
resulted in reduced calculated heat losses from the house. This is
because
an uninsulated solid wall was appended by the heavily insulated 2 storey
extension, which had considerably lower heat loss than the original
wall,
despite have many times the surface area.

Christian.


The area doesn't need to be much for there to be an improvement.

The U value of a plastered solid double brick wall is about 2 W/m^2.K

For a new wall it's 0.35 or 0.25, so almost a 10:1 difference.

For old single glazed windows the improvement with double glazed, low
emissivity glass is about 2.5:1, although the areas are smaller of
course.

Not to mention the much reduced air changes which accounts for significant
part of the total heating demand.

Very true. Air leakages account for 42% of heat loss in a modern house. An
old Victorian solid walled house with modern sealed double glazed windows
and doors is much tighter than the modern equivalent, which, with cavity
walls leaks far more. A prime leakage point is the window reveals and where
the joists penetrate the inner wall. This should be fully sealed up, but
never is.


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