If you've correctly calculated the U value. You do know that you can't add U values to get the value of the composite wall, but have to add the R values to get a composite R value, then convert that to a U value (R=1/U)
U value gives you the number of watts per square meter at a 1K (1C) difference.
To get the number of average watts you'll need to overcome the heat lost from the walls, you take the average temperature difference (in C), multiply by the area of the walls, multiply this by the U value, and you'r there.
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.
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.
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.
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.
Even at 2.55 the insulation is costing more than the blocks/sand/cement. My outside wall has a lean on it, I intend to fill the whole space with insulation, giving some 8 - 10 inches of the stuff at the top of the wall. 8-10 inches of celotex is mind blowingly expensive.
200mm Rockwall at 0.037 = 5.54
blocks .6666
Comes out at 0.16, getting down to 0.1 is gonna make for some very expensive walls.
The idea of more insulation at the top of the walls feels good, heat rises, so the tempertaure difference is bigger at the top. My room will be 3 meters high of walls, with 2 more meters to the top of the pointed roof.
No he shouldn't ! The L regs require 0.35. Of course doing better than this is a good thing but you are better off spending the money improving other heat losses - windows and doors for example.
but not much - even 2 feet of stone has a very poor insulation value.
This is more important though. However one has to be careful not to end up with a thermal lag so bad that south facing living rooms stay cold all day and need more heat than expected to keep them warm until evening when they suddenly warm up dramatically.
100mm blocks / 0.15 (from makers website) = .6666
50mm celotex / 0.023 = 2.1739
60mm rockwall / 0.037 = 1.6216 All those up = 4.4621
1/4.4621 = 0.224
on a cold winters day, 24 degress difference, 75 square meters of wall
75*24*0.224 = 40.32 watts required of heat input to maintain stable temperature assuming no loss from other means.
That does not sound very much to me, the coolong from wind will be close to zero, the house is well sheltered desined so the windy side has no doors/windws at all, and the insulation is protected with some
600mm of stone walls.
Any idea of the values for a slate/lime mortar stone/rubble wall ?
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.
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.
0.1U would here (on all walls, floor and ceiling) result in overheating most of the time except in sub-zero temps from the ambient electrical load, and human inhabitents, not to mention solar gain.
If going from 0.25 to 0.1 costs around 1500 quid extra (and incidentally knocks 50 cubic meters off the habitable space), saving me no money, unless it goes below 10C outside (heating set to 20C now) and vanishingly small amounts if it does. (currently I reckon the overall insulation is about 900W/K, with the walls having a U of about 2.4)
- Also the money saved can be better spent insulating the 2 rooms that
Having just had to buy loads of foil faced PIR foam, one thing worth doing is shopping around. I saved over 1500 quid on the price of celotex as quoted by a builders merchant! It is still not cheap (about 12 quid for a 8x4 50mm sheet) but a factor of two to three times variation in price between suppliers is not uncommon it seems.
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. 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.
- 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.
0.1ish is the point where a 2000 squ foot house does not require a full heating system. So this extra £1500 is clawed back by not installing a full heating system. As oil is not getting cheaper, it is a worthwhile investment for the future. Also the insulation will keep out heat in the summer. All makes sense.
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