On Thu, 28 Oct 2004 23:24:03 +0000 (UTC), DeanoH
wrote:

Andy Hall wrote:

OK, so you can pretty much use the U value of the insulating material
on the floor for that surface. Heat loss is not going to worse than
represented by that. Strictly, there is a formula taking into
account the perimeter and area of the floor, but in this case it is
not going to make a lot of difference as the floor loss will be small
anyway.

OK, I'll have to find that out, as I didn't buy or install that, but
it shouldn't be too hard to get the U value.

If you were looking at Building Regulation compliance, it would be a
different story, but for this it's good enough.



I meant if the building is divided equally in two - one half workshop
other for office etc. and with different temperatures. Obviously if
it is divided differently or temperatures are different you can do the
sums accordingly.

Ah! I see what you mean. Seeing as the two areas will have different
use, it would be beneficial to calculate the insulation requirements
of each? By the way, is the purpose of carrying out these calculations
to establish the minimum thickness of Celotex required, in order that
a standard level of insulation is achieved, or is it to obtain 'values'
which would indicate which heating solution should be implemented?

You may want to think about what temperatures and patterns of use as
well.

If you were constructing the building to meet Building Regulations,
there are guidelines for each element and the approach is called the
elemental method. You can read more about it in the Approved
Document to Part L1 on www.odpm.gov.uk

Basically there are maxima for each component that are allowed if you
are in a situation where the building, by size, location, use etc. is
controlled by the Building Regulations. In that case you would have
had to make an application to the local authority and they would
inspect your work.

If you are not required to work to Building Regulations because the
building is exempt, (you can look on any local authority web site for
information on this or again on the ODPM site), then insulation is up
to you.

Since you are paying, then you have to do the trade off between
insulation capital cost and the cost of running the heating. If you
are using a more expensive fuel like electricity, then you will
probably want to spend more on insulation.

I would sit down and think about expected use patterns the year round
- for example will you want to trudge down there when it is ****ing
with rain horizontally? The go from there.

If in doubr, it probably pays to err towards more insulation now,
because it will be a pain to add later.




Sorry if I sound dumb, but this is all new to me and while I certainly
appreciate that a bit of time spent now, calculating temperature
parameters, could save me a lot of time and money, it's very involved
compared to the opinions I have so far been offered by various friends
and associates in the building industry! In summary, is the purpose of
all this so that I can make an informed decision on purchasing the
minimum of spec and amount of Celotex, while not setting myself up for
huge bills resulting from having to overcompensate on a heating solution?


Exactly. You can pick a given size of Celotex and work out the heat
loss for the building at given temperatures.

One part of the exercise is to ensure that you put in enough heating
to reach the temperatures you want inside for the coldest outside.

The other is to do the cost trade off. To do that one, you can look
at average monthly temperatures, take the predicted use pattern for
that month and hence the energy use and cost based on that outside
temperature. You have to decide on what the pattern of use is,
though.



OK, so you have to look at the heat loss through each element of each
surface.

In other words for each side wall, take the total area and subtract
the window areas, for the end wall the doors and so on as you have
done.

Strictly speaking you should take into account all the components of
the wall, e.g. the outer wood, the Celotex and the inner cladding, but
you will find that the Celotex is the dominant one by far (as you
would hope) so for this purpose you can just use that.

45mm Celotex has a U value of 0.5 W/m^2.K
for 55mm it's 0.42

For other thicknesses you can use the table in
http://www.celotex.co.uk/appl/PDF/SOL_PRS.pdf

Just take the reciprocal of the R value.

You can also combine if you want to use multiple thinner sheets. Add
the R values and take the reciprocal of that.

Heat loss for a surface is then given by

Area in sq.m x U value x temperature difference.

Usually you work with -3 degrees outside.


So for an area of 5 sqm, 45mm Celotex and 21 degrees inside it becomes

5 x 0.5 x 18 = 45W.

So for the Celotex covered areas just calculate each piece.

Standard double glazing in wooden frames has a U value of 2.8 W/m^2.K,
or 2.1 if it has low-emissivity glass.

If the doors are substantially glass, then it;'s reasonable for this
purpose to treat them as windows.

So again take the areas and do the sums.

OK, time for the maths bit again...

to convert R to U... U=1/R

So, for 55mm Celotex, R=2.35; U=0.42

Wall A (minus window) = 31.4196m/sq x 0.42 x 18 = 237.5W
Wall B (minus windows) = 28.7016m/sq x 0.42 x 18 = 216.9W
Wall C (minus door) = 8.1016m/sq x 0.42 x 18 = 61.2W
Wall D (minus 2 doors) = 6.4496m/sq x 0.42 x 18 = 48.7W
Gable end E and F = 6.2464m/sq x 0.42 x 18 = 47.2W
Roof elevations G & H = 71.0482m/sq x 0.42 x 18 = 537.1W

For windows and mainly glass doors, U=2.8

3 casement windows = 8.154m/sq x 2.80 x 18 = 410.9W
3 single doors = 4.956m/sq x 2.80 x 18 = 249.7W

Total = 1809.2W

Does this mean I need a heating solution that can produce
1.8kW of heat? Per hour? per day? etc etc!

OK, so you've used 18 degrees as the temperature difference from
inside to outside.

That would mean that if it drops to -3 outside, you would have +15
inside which might be OK for a workshop but a bit chilly for sitting
around.

The typical inside (lounge) temperature used for design purposes is
+21 degrees, so the temperature difference is then 24 degrees.

If you wanted to achieve that, then you would need to multiply your
figure by 24/18 which is approximately 2.4kW

In other words, when it's -3 outside, a 2.4kW heater would maintain 21
degrees inside.

There is also one more component that should be taken into account.
You won't want to have a hermetically sealed box, so some air change
needs to be included.

To calculate this, you need to take the volume in cubic metres and
multiply by 0.33 and the temperature difference again. This would
give you the heat requirement to cover one air change per hour.
However, the normal design criterion for a living room, games room
etc. is 1.5 air changes per hour, so to do the whole lot it would be

Volume x 0.33 x 1.5 x temperature_difference.

Answer again is in Watts and you add that in. It could be as much
heat as for the loss through the walls or even more.









If you were looking at what is necessary to do Building Regulations
compliance, you ned to be more particular about the other components
because they help a little towards insulation. Here it it doesn't
matter because you are trying to get to a reasonable worst case for
heating requirement. Obviously adjust the outside temperature used
if you are in a colder or exposed position and want to oaccount for
it.

Understood and agreed.

He's probably right. Obviously it would depend on your pattern of
use, how long you want to take into account for the costs (i.e. how
long will you live there) and to some extent taking a punt on energy
costs. You have to run electricity anyway.....

Makes sense, and as I'm hoping to run a profitable business from the
building (usage constraints aside), I would envision that this would
become an overhead of the business and as such would be infinitesimal
compared to accounting for rent (of which I'll have none) when
calculating operating costs and charging such onto clients!


OK. I was trying to understand whether you wanted to leave the rafters
exposed.

Sorry, should have made that clearer.

The laths were in behind purely to make sure that the Celotex was
spaced away from the felt. If you cut the Celotex to be an
interference fit between the rafters you don't really need them.

Interference fit? Held there by friction? Accurate cutting?

Let's say your rafters are 500-600mm apart.

You could cut the Celotex about 3-5mm oversize and it will squash in
with a friction fit. If it's at size or slightly less, you tape them
to the rafters with foil tape.

One other thing is to not to forget to put some vents to the outside
from behind the Celotex. When I did my workshop project, I used
soffit vents (round plastic vents with a grille to keep out insects)
that fitted into holes drilled under the eaves, one between every pair
of rafters each side.

You can put them in a different position, but ventilation is
important.

Another thing that I did was to coat the inside wood surfaces with
clear Cuprinol solvent based wood preservative before putting in the
Celotex and covering over.

Also, at this stage, have a think about the electrics.

If you want to hide them and use flush mount fittings, then a good way
is to put in conduits and outlet back boxes and to then run PVC
singles wiring to connect them together.

I went for function over form and used three compartment trunking
everywhere, surface mounted. This allows me to add things as I go
or if necessary move them without disrupting the walls.



Another technique is to put some between the rafters and some over the
top. This reduces cold bridging - i.e. heat conducted through the
rafters, but I am not sure that I would bother for this application.

That would be guilding the lilly

Oh, I see. I thought that it was the "uniform". :-)

It is I suppose... in an environment with lots of 'marketing
types' who are constantly concerned about being 'en vogue', it's
best not to waste valuable creative energy in the mornings by
dressing to show individuality and personal taste! They would
see this as a statement and would spend all day discussing your
choice of attire for the day rather than focussing on making
the 'big decision' about the concept(s) you've spent the last
3 months trying to get them to approve! It's complete irony that
in order to achieve 'closure' (as the American's put it), creatives
have to work with non-creatives!


Hmm. I think I'd lose patience with that, but I can certainly relate
to keeping customers focussed on the subject in hand.

Personally speaking, I am quite left-brained with the exception of
music. Visually, I know what I like and what I don't, and would far
rather see a limited set of choices and work from those than be
bombarded with 20 choices.



Worth a try even with these, akthough you may still not reach the
price point of Christian's Seconds outfit.

And would you consider this route?

Definitely. It doesn't mattter if the odd sheet is slightly dinged,
even. You can cut that out.

I didn't bother because I was able to get a reasonable price for all
the project materials from Jewsons.






deano.

..andy

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