First question - my first stopping place for insulation u values was the DIY Wiki. They don't seem to be there which surprised me - or should I have looked elsewhere other than in the obvious place 'Insulation' ?
The reason for the calculations is convincing a disbeliever that he should insulate his workshop. Single brick structure, to be lined with OSB on a 75mm framing. But how do I calculate the heatloss across such a 75mm cavity without any insulation - do I need to take into account convection ?
1 - Insulate walls AND ceiling AND floor
2 - Not insulating the floor gets calf muscle ache all night long
3 - Vapour barrier is important
U value of wall...
- Find the R value for 1 brick
- Find the R value for various insulation types of various thicknesses
- U =3D 1 / (R1+R2)
So 20mm marmox (R=3D 0.60) on uninsulated cavity wall (R=3D0.73) gives a combined U of 0.75. An uninsulated cavity wall on its own is 1.37 so nearly halves the heat loss. Makes warm up very fast, plus when cavity wall later insulated it jumps further. No point using plaster anymore on external walls, Marmox is quicker and almost anyone can skim 3mm with a bit of effort... belt sander... angle grinder... expanding foam.
Actual wattage differences, for 21oC internal temperature...
- Calculate wall surface area
- Calculate U value of wall
- Heat Loss =3D Area * U * 21 =3D Watts
So for example a 5.31m^2 wall with 50mm celotex (U=3D0.40) has heat loss is 45W, without the celotex a solid 9" brick wall is 235W. That shows how even a little insulation helps hugely - consider four such walls and you've 1kW heat loss BEFORE considering your door, draughts, windows, floor & ceiling. Doing 50mm celotex on those four walls saves
0.8kW.
Do a Google for "R value" and look for single brick. You have to do an estimate for the studding re "cold bridging". Simplest is to stick a piece of 25mm XPS over the studs before the final plywood or hardboard. B&Q do lots of cheap(ish) insulation at the moment.
Thanks John, for the pointer - as you say there ought to be one on the 'Insulation' page.
js - I take it that what you are saying is that a single brick, 75mm stud wall with OSB facing is going to have the same insulation value as a single brick wall with no lining at all ?
I don't believe that. And what's more that's not going to help my discussion with the 'Disbeliever' who is expecting figures from me to satisfy the argument.
Not really. single brick is about 7 U valeue wise. insulation should bring it down to 0.5. Even with windows and a door, overall value should be no worse than one or two. Probably at a rough guess from 200W/sq meter floor area in winter to 50W/sq meter.
No. a single sheet of plasterboard makes a lot of difference.
Cant be arsed to work it out, but likley a 50% reduction in U value. I lived in a single brick plus 2" rockwool and plasterboard, and that is pretty good.
Old cottage I rented, 3mm of oak tiles on exterior kitchen wall completely stopped condensation. single brick is DIRE - almost anything makes it better.
Out of interest, I calculated the thickness of a stone castle wall needed to reach modern U values. It was either 3 meters or 6 meters.
Those ice age people who lived in long barrows, weren't stupid, it seems. Nor are castles necessarily the cold places we think they might have been.
The reality is uninsulated will not have perfectly insulated floor, ceiling & windows.
- 2hrs/day becomes =A342 v =A312 insulated
- 8hrs/day becomes =A3192 v =A348 insulated
Typical figures re wall 30% of heat loss, uninsulated floor/ceiling/ window and insulated floor/ceiling/window to equivalent of 50mm celotex. It comes down to how often it is used (2hrs/day or 8hrs/day) and whether frost protection is required (which really costs with uninsulated). Assuming walls are roughly 30% of heat loss and insulated example copies the 50mm celotex (or equivalent).
There are other considerations in a workshop as well. With a well insulated place its easy to keep a heater in there on a frost stat. That can help ensure you keep the temperature above the dew point, and hence stop all your tools getting damp and going rusty. Alternatively you need to mess about with heated cupboards etc.
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