"Rick Dipper" wrote in message
...
On Fri, 20 Aug 2004 21:06:11 +0100, Andy Hall
wrote:

On Fri, 20 Aug 2004 20:24:48 GMT, Rick Dipper
wrote:

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.

No, sorry. I did mean that the wall is on the outside as you describe
but did not describe it as well as I should have done.

What I meant was that if you treated the outside wall as not existing
as you were for insulation calculation, you have outside temperature.
If you add the wall back in, you have not only its U value (which will
make things better), but more importantly a huge thermal mass. The
effect of that will be to stabilise the temperature to some extent at
the point where the insulation meets the outer wall.
Therefore, looking at your model of effectively ignoring the wall, the
effect will be to give a much more stable "outside" temperature - in
other words you won't need to account for the extremities of
temperature unless they last a long time.
It is hard to say whether this has a big overall effect on energy use
averaged over time, but if the temperature can be maintained in a more
stable way inside, you are less likely to want to reach for the
thermostat.



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.


There are tables of U values for roof structures in the building
regulations. To a first approximation, you can simply factor in
the U value of the insulation. By comparing the relative areas and
losses of the walls roof and windows, you can then figure out where
the cost/benefit comes.



Thanks Andy, now I understand. The building your own house is a good
way to learn how this stuff works. Experience in the bit of the house
we live in now suggest that once its hot inside it stays hot, but
getting it up from cold is a real nightmare.

At a guess,

- the underfloor heating needs massive insulation below it, as its the
hottest place, the area being much smaller than the walls its also
quite cheep to do.

- The windows need to be good, especially the one in the point of the
roof where all the heat rises too.

You can fit a duct that is built into the wall that fans this hot air back
to ground level.

and the sliding door in the master
bedroom. This one needs some thinking
about, windows can eat up *LOTS*
of money.

- The roof itsself will need pleanty, not only to stop loss in winter,
but to stop gain in summer. The ammount of money I can save on the
extra 12% in the wall, will double the insulation in the roof.

It is not a matter of robbing Peter to pay Paul. You insulate properly,
0.1ish for the walls. Buy the book, The Whole House Book, all is there, all
the figures. Don't rely on the advise of inexperienced people making it up.

- Also the money saved can be better
spent insulating the 2 rooms that
we have not rebuilt, that simply have stone walls.

Thanks for all your advice, I feel much happier now. I really did not
want to save a pile of cash now so I could regret it for 25 years to
come.

Spend more on insulation and eliminate a full hearting system.