On Fri, 20 Aug 2004 19:19:22 +0100, Andy Hall
wrote:

On Fri, 20 Aug 2004 17:43:23 GMT, Rick Dipper
wrote:


I have worked out the U values of the insulation I intend to use in my
cavity walls, I have 2 solutions

1) Blocks -celotex-rockwall = 0.224 W/m2K
2) Blocks -2 * rockwall = 0.255 W/m2K

I did not take account of the external wall, plaster, mortar or wall
ties in working out these figures. The external walls are 600mm stone
walls.

Normally I would go for option 1, but its arround twice the cost of
option 2, a whole lot more difficult to install (I am building the
inner wall second) and there is not a big difference in the insulation
values.

Any ideas on how I work out if its worth it ?

Is it as simple as working out the temperature difference, the surface
area of the wall, to work out the loss via the wall, and then mutiply
by the cost of a watt of heat ?

My thoughts right now are to save the money, and use it to put extra
into the roofspace.

Thanks
Rick

taking Ian's point that you have to add the reciprocals of the U
values of the different insulations and then the reciprocal of that to
combine them, and assuming the figures you have above are right, then
the discussion is about the second decimal place and in effect a 12%
difference. This is not significant.. You also have the effect
of the stone. By insulating, you are altering the thermal time
constant of the inside of the house. It will heat up and cool down
quickly. For heat loss purposes, the stone wall inner face will
have an averaging effect on the temperature as seen by the outside of
the insulation

Assuming that you can accomodate the extra thickness of Rockwool and
don't mind working with it

I would do the same heat loss calculations for the roof and compare
the overall figures. Normally the roof space and the totality of the
windows have about the same heat losses as one another and account
together for approximately the other half vis-a-vis the walls. It
does all depend on sizes of course.



Thanks Andy, I am not sure I understand what you are saying, the stone
wall is on the outside, I think you think its on the inside.

Doing the figure for the roof sort of imples that we have designed it,
we have some sketches and strength caculations but have not got much
further, thats for a cold winters night.


.andy

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