Andy Hall wrote:

OK. In rough numbers, assuming you didn't insulate the floor, and

The floor consists of 12" concrete poured onto about 12" of crush (can
you believe I had the concrete pumped through the house, the full 300ft
to the end of the garden... that sure was an epic day). Once the low
level wall was built, it and the floor was coated with liquid rubber,
blinded with sand and then covered with polythene sheeting. 4"
insulation boards were then laid to the whole area and the whole
lot was screeded. The timber frame was then constructed inside the brick
built wall, leaving a cavity which was breached and dressed with roofing
fillets (arris rail) and lead flashing.

your walls are about 2.5m high, if you use 50mm Celotex and divide
equally,

Sorry, you've lost me here... divide equally?

you're going to need about 4-5kW in the office bit and 3-4kW
in the workshop bit if you are happy with 16-18 degrees in that part.
These are round numbers based on my own calculations.

It's easy enough to do the sums. Just look up the R value of the
thickness of Celotex that you plan to use and calculate the U value
from the reciprocal of it. Then measure the surfaces in square metres
and multiply these by the U value and the temperature difference
through the walls - usually -3 degrees is used for worst case outside.
This will give you heatlosses in watts. For the floors you can get
figures from radiator calculation programs on the web sites of Barlo
or Myson radiators.

Maths was never my strong point!
My walls are 8ft - height of a sheet of plasterboard - so near enough
2.5 metres. The two longest walls are 14 metres and the ends are 4
metres. The front wall has a standard single door and the back wall has
french doors centred within it, opening onto a decked area. The two long
walls have double-glazed casement windows with two openers to each
window... the left wall has one 6'x 4' casement window, the right has 2!
To calculate the area of the roof elevations, I need to recall my
trigonometry from school... correct me if I get this wrong:

Overall height of lodge = 4metres
minus wall height of 2.4384m (8') = 1561.6m
Therefore from end elevation of roof:
Adjacent = 2000mm (half of overall width)
Opposite = 1561.6mm (overall height minus wall height)
Hypotenuse = 2537.44mm (I cheated this by using a scaled drawing in
Adobe Illustrator as I had no angle values to hand apart from the 90°).

Therefore total area =

Long wall A = 14m x 2.4384m = 34.1376m
- 1 window ((6')1.8288m x (4') 1.2192) = 2.718m
= 31.4196m/sq

plus

Long wall B = 14m x 2.4384m = 34.1376m
- 2 windows @ 5.436m
= 28.7016m/sq

plus

End wall C = 4m x 2.4384m = 9.7536m
- single door (2.032 x 0.813m) = 1.6520m
= 8.1016m/sq

plus

End wall D = 4m x 2.4384m = 9.7536m
- french doors @ 3.304m
= 6.4496m/sq

plus

Gable end E = 1.5616m x 2m (opp x adj)
= 3.1232m (calculated by rotating triangles to form a rectangle)

plus

Gable end F = 1.5616m x 2m (opp x adj)
= 3.1232m/sq (calculated by rotating triangles to form a rectangle)

plus

Roof elevation (pitch) G = 14m x 2.53744m (hyp)
= 35.5241m/sq

plus

Roof elevation (pitch) H = 14m x 2.53744m (hyp)
= 35.5241m/sq

Therefore total internal area of construction in
square metres would be: 151.967m/sq

Sorry to show all my workings out, but as I said
I've never been confident with maths and could do
with my results being confirmed!

So, how do I now go about calculating the insulation
and heating requirements using the result above?


In terms of the heating arrangement, you are presumably going to run
electricity anyway, and with insulation, this is not getting too
outrageous to heat with electricity. Alternatively, if you have
mains gas you could run that and have a small boiler and radiators, or
perhaps LPG into a small boiler. I wouldn't use portable LPG
heaters though because of water vapour that will be produced.

I spoke to my local electrical retailer (high street not chain)
and his opninion was that running gas (and water supply) underground
and meeting all current regs, would be more expensive than employing
a solution offered by one of the more modern electical heating products,
currently on the market... I have the dimplex brochure to browse at my
leisure!


If you look on the Celotex web site there are application notes which
cover the options that you will need closely enough. I assume
where you say 4x2, you mean that the 4" side is the total depth
available? If so, you could perhaps go with 70mm Celotex and have
a 20mm gap behind. You need to check actual measurements.

Understood... the minimum ventilation gap is what I was after... thanks.

If you want something fancier with exposed beams in the roof then you
might have to go for something thinner for that - it really depends on
whether you want to clad it. For my workshop I didn't bother because
I actually fitted the Celotex over the rafters leaving the foil
showing. The joists were boarded anyway and the area above is used
for storage so I didn't care about the foil showing.

For a cabin that I completed last year, I did care more about the
appearance, so I put small wooden laths brad nailed to the rafters to
make 20mm spacers, then fitted the Celotex against them. The ceiling
was then clad over the top of the rafters, hiding them and the
Celotex.

I think I understand the above! I want to leave a vaulted ceiling and
plasterboard over the rafters (just like the walls) this would again
give me my 4" of space behind the plasterboard to fit the celotex and
leave a ventilation void... is this what you obtained by installing the
laths? i.e. where did you put the laths - between the rafters or on
their internal edge?

As far as the interior design is concerned, I did consider the exposed
beams route, but later rejected this as it would create a period look
and my house is definately not, inside nor out, a period dwelling!
As a graphic designer, anything I can do to make the space more
minimalist and simply defined, the better! This is so my eye will not be
distracted while working in the lodge... it's also the reason why many
designers wear black!


Have you bought all the other materials yet? I found that by hawking
a complete project list around the merchants - I faxed each a shopping
list and asked for a quote, it wasn't hard to get reasonable prices.
I just picked the lowest overall quote but one. I didn't trust the
cheapest not to cock up the delivery.

Apart from the celotex and plasterboard, I can't see what other
materials I would need (in bulk) that I could obtain from a single
source! If you believe otherwise, please suggest!

.andy

Thanks very much for your time, advice and suggestions.

deano.