Another insulation question...

[^woody^]

Double glazing, a combi, a few LED bulbs and a bit of loft quilt will get a B or C rating. No need to bother with the walls.

 

[Jim Gap]

Hello chaps. Back again.. Bloody water board disturbed the phone lines and put the street out.

Anyway. I know what's happening now. It looks like polystyrene sheets to the wall, battens then more polystyrene. Not 100% sure what kind of insulation that will provide but that's what's happening.

Think someone mentioned loft insulation. I've been trying for about 7 months going from Nest Wales to The Energy Saving Trust to energy supplier like a bloody tennis ball with absolutely not joy at all. It's been a bit going from pillar to post and of the 2 companies to actually contact me in those 7 months all they were interested in was replacing a 2 year old boiler and nothing else? Got CAB on the case so fingers crossed.

The EPC which was done in 2013 shows the house at a E (42) but could achieve a C (76)

 

[Jim Gap]

Hello all, just a quick one. After the installation of insulating boards is there any benefits to applying Radflek behind the rads?

Thanks..

 

[endecotp]

After the installation of insulating boards is there any benefits to applying Radflek behind the rads?

As with all insulation it is a cost/benefit tradeoff. If the reflectors are free then it's probably worth doing.

But radiator reflectors, like "multi-foil" insulation, verge a bit on snake-oil territory i.e. it's easy to find hyperbolic claims about their benefits. The point is that, despite the name, radiators don't actually radiate much heat. They convect. Radiation comes from heat sources that are much much hotter than a radiator, i.e. the sun, the red-hot wires in a toaster, a gas flame etc. Avoiding heat loss from the back of a radiator into the wall is best achieved with conventional insulation, which it sounds like you now have.

Some of the products that go behind radiators are not just flat foil stuck to the wall; Radflek seems to be mounted in front of the wall with a gap. Presumably the idea is that the rising hot air current (convection) on the back of the radiator is confined in front of this barrier, so it transfers less heat to the wall. That's an interesting idea, but to be honest you could make your own.

 

[Doggit]

I use the polystyrene reflector products are I felt how hot the wall behind a radiator got. If I have to do any flushing or maintenance on a rad, or install a new one, then I put it on as a matter of course.

 

[endecotp]

I felt how hot the wall behind a radiator got.

It doesn't matter how hot the inside surface of the wall gets IF the wall is well insulated. In fact a hot inside surface probably indicates a better-insulated wall.

What I see most often that probably indicates inefficient radiators is curtains, either just above the radiator so that half the convection is going behind the curtain, or with the radiators entirely behind the curtains.

 

[Doggit]

It doesn't matter how hot the inside surface of the wall gets IF the wall is well insulated

And what if it isn't?

In fact a hot inside surface probably indicates a better-insulated wall

Or it could indicate heat from the rad that could go into the room, just being wasted into the wall.

 

[endecotp]

Or it could indicate heat from the rad that could go into the room, just being wasted into the wall.

If the heat were going into the wall, the wall would feel cooler.

 

[John D v2.0]

Gosh, physics please!
heat loss through a wall in watts is a product of :
wall area in metres squared,
heat conductivity (u value) in watts per metre squared Kelvin, and
temperature difference in Kelvin

Therefore the temperature of the inside being higher will increase heat loss a lot, also the insulation being worse will do the same.

Regarding wall surface temperatures, If the wall insulation is worse than the air gap between the rad and the wall, the wall will be closer to outside temp than in. But even if not, a lot of heat will be being lost. And the insulation between rad and wall is very poor indeed, consisting of only an air gap. So the wall will be warm.

 

[endecotp]

the insulation between rad and wall is very poor indeed, consisting of only an air gap.

Air gaps are actually some of the best insulators that we have - hence cavity walls and double glazing.
But convection makes them much more complex to model than solid materials.

More to follow.

 

[John D v2.0]

Ait gaps are actually some of the best insulators that we have - hence cavity walls and double glazing.

Has to be a low emissivity air gap as well as the convenction issue.
Radiators aren't that low unless they're chrome towel rails, and the wall isn't (unless you put the rad foil on the wall)
Even then, a decent double glazed window isn't much better than a solid wall.

 

[endecotp]

The temperature of the inner face of an uninsulated wall will be colder than a well-insulated wall, all other things being equal. This seems intuitive to me - if you touch a wall and it feels cold, surely you don't think "that's a well-insulated wall!"? This is just as true for the wall behind a radiator as it is for any other wall, so when you put your hand on the wall behind a radiator and it feels surprisingly warm you should think "good, it's well-insulated" - though perhaps that is less intuitive.

But first I think it's interesting to consider the worst case losses through the wall behind a radiator, which may be less than you think.

As an example I'll use this radiator which I happen to have in front of me: 500 x 1200 mm so area = 0.6 m^2, rated 1400 W.

Imagine that radiator with its back in perfect thermal contact with the wall behind it (i.e. stuck to the wall). Say the temperature of the radiator is 60C and the outside temp is 0C. Say it's an uninsulated solid wall with U = 2 W/m^2K; over the 0.6 m^2 of the radiator that's 1.2 W/K. So heat loss through the wall will be 60 x 1.2 = 72 W. Note that's only 5% of the radiator's total rated output. (Well, you could argue that because it's in perfect contact with the wall the output will only be from the front and hence 700W, so it's 10%.)

Modelling the air gap is difficult, but let's arbitrarily imagine that it has an equivalent conductance of 1.2 W/K. With the uninsulated wall also being 1.2 W/K, the total conductance from the radiator to the outside will be 0.6 W/K and the total heat flow will be 60 * 0.6 = 36 W. The temperature of the inner wall surface wll be 60 - (36/1.2) = 30 C - half way between the radiator and the outside temperature.

Now consider a more insulated wall, say U = 1 W/m^2K so over the area of the radiator its conductance will be 0.6 W/K. Now the total conductance from the radiator to the outside will be 1/ (1/1.2 + 1/0.6) = 0.4 W/K, and the total heat flow will be 60 * 0.4 = 24 W. (Less heat flow as expected now that it is better-insulated). The temperature of the inner wall surface will be 60 - (24/1.2) = 40 C - HOTTER than the uninsulated wall.

 

[John D v2.0]

Note that's only 5% of the radiator's total rated output.

Sounds plausible, but my only comment is you can't use the radiator's rated output, as you have to use the output at delta T 40C for a 20C room, normally they quote the output at delta 50c. So your rated output at 60c would be 1120W.
Another comment is that a delta 20 would still exist even with under floor heating, so we're only really losing the delta 40 to outside rather than the full 60.
Taken together the two factors make little difference to your overall point.

Modelling the air gap is difficult, but let's arbitrarily imagine that it has an equivalent conductance of 1.2 W/K. With the uninsulated wall also being 1.2 W/K, the total conductance from the radiator to the outside will be 0.6 W/K and the total heat flow will be 60 * 0.6 = 36 W. The temperature of the inner wall surface wll be 60 - (36/1.2) = 30 C - half way between the radiator and the outside temperature.

Now consider a more insulated wall, say U = 1 W/m^2K so over the area of the radiator its conductance will be 0.6 W/K. Now the total conductance from the radiator to the outside will be 1/ (1/1.2 + 1/0.6) = 0.4 W/K, and the total heat flow will be 60 * 0.4 = 24 W. (Less heat flow as expected now that it is better-insulated). The temperature of the inner wall surface will be 60 - (24/1.2) = 40 C - HOTTER than the uninsulated wall.

Indeed, but you forgot to complete the illustration.
Now consider that more insulated wall with a rad foil on it, say U = 5 W/m^2K so over the area of the radiator its conductance will be 3 W/K. Now the total conductance from the radiator to the outside will be 1/ (1/1.2 + 1/0.6 + 1/3) = 0.35 W/K, and the total heat flow will be 60 * 0.35 = 21 W. (Less heat flow as expected now that it is better-insulated). The temperature of the inner wall surface will be 21/0.6 = 35.3 C - still hotter than the uninsulated wall, but COOLER than the insulated wall without rad foil. The Cooler in this case means less heat loss, wheras in your second example hotter meant less heat loss.

If you could get the wall temperature down to 20c internally due to extra insulation behind the radiator, that would be the most efficient with only 0.6*20=12W heat loss.

So in summary,
cooling the wall by insulating inboard = more efficient
cooling the wall by removing insulation outboard = less efficient

otherwise summarised as more insulation = better, but has a bigger benefit in the hotspots of your room

 

[endecotp]

Now consider that more insulated wall with a rad foil on it, say U = 5 W/m^2K so over the area of the radiator its conductance will be 3 W/K. Now the total conductance from the radiator to the outside will be 1/ (1/1.2 + 1/0.6 + 1/3) = 0.35 W/K, and the total heat flow will be 60 * 0.35 = 21 W. (Less heat flow as expected now that it is better-insulated). The temperature of the inner wall surface will be 21/0.6 = 35.3 C - still hotter than the uninsulated wall, but COOLER than the insulated wall without rad foil. The Cooler in this case means less heat loss, wheras in your second example hotter meant less heat loss.

You've calculated the temperature at the inaccessible junction between the rad foil and the wall. The temperature that you would feel if you reached behind the radiator and touched the foil would be 42C.

So in summary,
cooling the wall by insulating inboard = more efficient
cooling the wall by removing insulation outboard = less efficient

I don't know what you mean by "inboard" and "outboard" there.

 

[Doggit]

Sorry you two, but just what sort of crap trap have you got yourself into; please cool down and back off. This started off as a very simple scenario, and look where you've both taken it.

It is a pure and simple case of do you put some insulation on the wall to stop heat from the radiator getting wasted by heating the wall. The real discussion here, should be what is the best insulator to keep as much heat going into the room, and not the wall.