I'm trying to work out the losses in my new (1970) bungalow. ATM I have a spreadsheet that tells me that I need 16 kW with an external
-1C. Strangely, the boiler, a Worcester HeatSlave Oil combi, is a
15-19kW model - perhaps the engineer that spec-ed it back in 1991 could add up too :-)
Now I can find U-values for most things, but how do I work out the effect of adding another 150mm to the existing 100mm loft insulation.
Also, my heating pipes run through the loft, and are lagged, but I think I should allow a loss per meter run, any ideas how to do that? The house is L-shapped and some of the runs are very long.
Look up the value for 100mm - call it U1 Look up the value for 150mm - call it U2
Resultant U = 1((1/U1) + (1/U2))
This ignores the surface effects etc. but is good enough for this exercise
Find the U value of the insulation from the manufacturer.
Calculate the internal surface area by multiplying the circuference of the end (pi x Dia) by the length. Use pipe diameters and Make sure you end up with sq. metres.
Treat it as having a temperature drop of 85 degrees and see what you get.
It should not be more than a few tens of watts and can then be ignored for boiler size purposes
If it's more, then thicker lagging is needed.. Thickness equivalent to dia of pipe is typical.
What sort of insulation and how thick is it? If it's old sacking I'd expect it's well worth replacing with thick foam, especially if adding loft insulation makes freezing temps more likely in the loft.
There's a formula for pipe insulation heat loss in the following paper:
Suprisingly 9mm foam insulation gives ~18W/m, so adding lots more gets you well into diminishing returns. But 25mm will freeze almost as twice as slowly...
Although, as Andy said, that will underestimate the combined U value somewhat, since the boundary layer resistances are being included twice.
More accurately: U = 1/((1/U1) + R2) where R2 is the R value of the added insulation. The R value is found from the thickness of insulation (in meters) divided by the thermal conductivity of the material (k).
Example: existing U value say 0.5 W/m^2K, add 150 mm of fibreglass or Rockwool with k value of about 0.034 W/mK. R2 in this case is
0.15/0.034, i.e. about 4.4 m^2K/W and the new U value is 1/(1/0.5 + 4.4) which is about 0.16 W/m^2K.
Many thanks to everyone who helped with this or the previous question about underfloor heating.
My spreadsheet (not guaranteed correct, or even sensible) is on my website, if anyone is interested:
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are three scenarios:
1) the original almost uninsulated house (July 2005)
2) what I hope to have by mid January after loft insulation to 250mm and low-e 28mm double glazing (January 2006)
3) and my plan for the next phase with Cavity Wall insulation and some other improvements (Next Phase)
Of course, the key thing to realise is that U-values have to be added up as reciprocals..... Thanks again.
The other thing, Richard, is that you can look at the Approved Documents (Part L1) for the Building Regulations at
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for information on combining methods and aspects.
However, when you start to add reciprocals, you realise that if you add insulation where there has been none, typically the U value for it dominates the total, almost to the point that the original can be disregarded. The other is that incremental insulation often does not make as much difference as one might think to the total story. For example, for a typical two storey house, adding cavity insulation will normally make more difference to the total situation than adding a lot of loft insulation if there already is some. In a bungalow, this is skewed somewhat because of greater roof vs. wall area.
Notice also that the air exchange losses are significant and much less predictable than surface losses, so the odd 50W here and there in a room requiring 1kW or more for heating, doesn't matter a great deal.
Two other things.
It is normal these days to work on the basis of an outside temperature of -3 instead of -1 degrees.
If you are going to use a condensing boiler (presumably so because you have to unless certain exemption conditions are present), then it makes sense to design for boiler flow and return temperatures of 70 and 50 degrees rather than the conventional 82 and 70 degrees. Doing this will allow the boiler to run more efficiently for most of the time.
However, doing this will reduce the heat output from radiators of a given size, ergo you have to use a larger output radiator than you otherwise would. If you look at radiator data sheets, you will see a correction factor that should be applied. For 82/70, it is normally
0.9 and for 70/50 normally around 0.6 You can obtain increased output by use of double instead of single panels or with fins rather than necessarily increasing the size.
Thanks, that's very useful. The appendixes in particular.
Yes, I noticed this. At the moment I can't add CWI as some cretin built a paved path tight to the wall on one side, above the DPC! I really don't want damp rockwool in the cavity. Moreover, my walls only have 50mm cavities, the minimum needed to get rockwool in. The other issue there is that they are not plastered - bare brick - and the bricks are concrete, not baked clay, the whole rendered with cement on the outside. I've just guessed at the U-value for that in the spreadsheets, but it won't be far off 1.
The big problem with this house right now are the air leaks and the enormous single glazed windows. Unlike most modern or Old houses this one has big glass walls, so the double glazing 'gain' is going to be very high. The loft insulation may appear to be 100mm, but in many places it's been compressed by 'stuff' in the loft - I've teazed this out a bit but it needs topping up. Finally the pipe runs - over 30 meters to the furthest rad - are wrapped in sacking - so poly-pipe insulation is also going to make a big difference.
Yes, I realise that. In fact here in Cumbria I want the system to work well within it's capabilities down to -5, with -10 possible by closing some rooms. I'm expecting the Gulf Stream to stop, and even this won't be enough eventually.
The current boiler will be OK for a bit I think, it's not pressurised, and not condensing. But when I replace it, I'll deal with both those issues, and I may go for Underfloor with a Heatstore, so that I can keep the mains hot water - even though this will mean finding somewhere for the damn tank and a lot of extra plumbing! Underfloor will also mean that I will have to address the issue of insulation of the floor - currently concrete slabs. This will also address:
Thanks very much for all the suggestions. It's really good to have people to talk these ideas over with.
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