Insulation is good stuff............but how good?

[DIYedboy]

I understand U values and I realise that an R value is the resistance to heat transfer, but that is all very nice if you're in a laboratory.

are we saying that if we have a foot of loft insulation we won't lose any heat at all through that ceiling? or if we use 100mm of celotex instead we also won't lose any heat?

 

[JohnD]

the problem with the rigid foams is that it is very difficult to get a perfect fit between joists. Mineral wool is easy because it will stuff into irregular gaps, and you can snip off an extra bit if needed for a wide gap, or where the joist runs close to a wall.

The foams are better insulators, and very roughly half the thickness will do.

You never stop all heat loss. Each additional layer will reduce a proportion of what was lost before.

For example, if layer 1 reduced loss by 50%, layer 2 would reduce 50% of what was left.

So not 100% of loss (50%+50%)

but 75% (50% + (50%x50%))

 

[cjard]

I understand U values ... [but] ...
are we saying that if we have a foot of loft insulation we won't lose any heat at all through that ceiling?

Contradiction here; if you understood u values you probably wouldn't have asked that question

U is heat loss for a particular thickness of material, per its area, per temperature difference between the two sides. If your material X has a U of 1, and the room is at 20 and the world is at 0 and the insulated area is 50 sqm then the heat loss is 1*20*50 = 1000. Notionally youll have to have a 1 bar electric fire running constantly to keep that room at 20 degrees

If you can use an insulation that is twice as good (half the lambda) you need half the power input

No insulator is perfect so you don't get situations where there is no heat loss from a ceiling. Passivhauses that have no central heating system are not so perfectly insulated that they lose 0 heat, theyre just designed in a lab so that the sum of the heat losses equals the sum of heat received through the windows as sunlight pours in, plus waste heat from the occupants and their activities. They also use clever stuff like systems that recover heat from stale air as it is being extracted to pre-warm the incoming air, representing a lower heat loss

 

[DIYedboy]

The thing I understand about U values is we want a nice low number!
Here's the question then............They say we lose 25% of heat through the ceiling (I assume they mean an uninsulated one), if I bung in 12" of mineral wool , does 25% turn into 1%?
Secondly, they say we lose 33% through the walls, I guess they are basing that on a 9" solid wall, so if I build a wall with a U value of 0.28 (the current requirement) does that mean that 33% has changed to 1%?

 

[theprinceofdarkness]

Insulation obeys the law of diminishing returns, the first bit saves the most money, Take the first example, 20 degs inside and 0 degs out side, first layer of foam of foam, saves £100 per year, the next layer would be £50 per year, the next £25. So would you invest £1000 to save £25 per year or leave it in the bank and pay the £25 out of the interest? Of course if fuel costs double in time then the figures work out differently. This is why loft insulation has got thicker over the years, only 2" in the 1960s.
Frank

 

[DIYedboy]

Yeah, POD, I've got that, BUT that don't answer my question..........12" of loft insulation cuts 25% of heat loss down to what?

 

[theprinceofdarkness]

It all depends on the temperature outside, the temperature inside, the cost of the heating and the heat loss. It is possible to work out exactly what your heat loss was for last year and because you know your fuel costs, you can work out how much you could have saved. As for next year, you don't know how cold it will be or your fuel costs, so the thing to do is to take the advice from the various Government departments and add a bit.
Frank

 

[cjard]

The 0.3 in the current regs is "quite good" but it will fall to lower and lower values. You do get into the realm of diminishing returns and you reach a point where it is better to spend money on draft proofing than insulation

I had the same question a long time ago and there wasn't a really good answer that works for all situations. You're best off doing some measurements of what you have and where your money goes. Eg if your loft has 100mm of wool now and you triple that it'll reduce the heat loss to a quarter of its former value. If you have a 1 bed flat and it costs 1200 a year to heat, then maybe it'll only cost 900 a year to heat. Why didn't it drop to 300(quarter of 1200) you ask? because the ceiling is not the only heat loss. Drafts, walls and windows will soon overtake the ceiling as the biggest losers of heat

If you really want to bring the bills down there are no half measures.. You have to properly insulate the whole place, externally is usually better, and the insulation has to be continuous as a barrier, which also cuts down on drafts. You then should consider the ventilation system, and install a heat recovering device. The process isn't cheap, and buyers don't usually give a damn that your property is EPC A rated when the drafty, cold turd of a house next door has nicer views so don't do this to add resale value to a house. If you want to live there for a long time, and you want to spend many thousands now to have permenently lower energy bills then consider it but chucking a bit of loft wool around (though a good idea because it's cheap) is t going to magically reduce your bills to pence

 

[DIYedboy]

Aint that the truth!.......if the house nextdoor is 5K cheaper, no-one will be buying yours cos you've spent 10K on insulation n heat recovery!

It seems my question doesn't have an easy answer and relies on half of a half n half of that half etc. My question was purely theoretical as I'm already insulated up to me earholes! It's a pity there isn't an easy answer, they tell you these heat loss statistics but don't give you the after insulation statistics just potential financial savings

 

[cjard]

The financial bit is generally the only way to relate it to people..they get the gas bill and do the shock horror bit. Sad but true, end of the day the only thing that really motivated someone to buy something was the price, not how manny whales (or even their own children's future) it saved