Does foil behind radiators really work?

[John D v2.0]

Very interesting question, if you're up for a laugh check out the replies in the comments. I especially liked the one saying they just crank up the heating, not very guardian!
https://www.theguardian.com/money/blog/2016/oct/22/does-foil-behind-radiators-really-work
However it did get me wondering as a theoretical point how much heat would be reflected, as a percentage of the total heat output of a rad at say 55c. Say 1m² rad on a solid wall. Any thoughts?

 

[Agile]

It all depends on what the wall behind the radiator is.

If it is a solid wall then perhaps it might save a very few %. Say 1-3 % ?

But if the wall is the inner leaf of an insulated cavity wall then the saving is going to be minimal.

Its one of those things which look good but really does very little.

Guess what! You could get the same effect by using common kitchen foil!

Tony

 

[John D v2.0]

I'm not sure it does look good! We're putting 3 short but very wide radiators on the outside solid wall (to run at low temperature) and I wondered if it's worth the ugliness of foil. Sounds like not.
I just had a think, did you work it out by estimating the heat loss of solid walls at 2w per degree and given the temperature difference would be 60 rather then 20 we would be losing an extra 80w for a radiator of about 800w. That would be 10%, then take into account the wall wouldn't be as hot as the rad and the foil wouldn't be perfect to get the 1-3%?

 

[slippyr4]

Don't (the misnamed) radiators convect most of their heat, rather than radiate?

 

[John D v2.0]

Yes indeed, basically because they're not that hot compared with the room. But if you go close you can feel it on your face.

 

[phod]

It's rubbish - the rashers don't stay on the foil nor do they get sizzling hot.

 

[Doggit]

I always fit radiator insulation on all walls as matter of course, as radiators work on booth a convection, and a radiant method. One day I'll do a test and see whether the bubble foil or the polystyrene stuff works better.

If you feel the wall behind radiator you'll find it's warm, and this heat is just wasted. For the minimal cost of the rad foil, I feel it's prudent to use it. If you mark the four corners of the rad, and then set the rad foil about half inch inside these marks, then it won't show much, but you need to use ready mixed paste (or even stronger) to keep it on the wall.

Guess what! You could get the same effect by using common kitchen foil!

Sorry Tony, but as kitchen foil is just thin aluminium, it'll transfer the heat straight through to the wall.

But in your case John, I suspect it won't do much good with low temperature rads.

 

[John D v2.0]

Surely ally foil would be fine, it's the reflection we're stopping not the conduction. We already have an extremely thick wall to reduce the conduction.

 

[Doggit]

Ally foil being metal, is a perfect heat conductor; it's used in cooking because it'll transfer the heat into the food, but it'll keep the moisture inside. As such, it's completely useless for putting behind radiators. Ally foils only good for reflecting visible light, nothing more.

And a thick wall is great for absorbing even more heat than a thin one.

 

[slippyr4]

Ally foils only good for reflecting visible light, nothing more.

Nonsense. A 30 second experiment with a remote control and a TV demonstrates that is completely untrue.

What metal do you think is in your bubble foil????

 

[motorbiking]

Depending on the construction of the wall the foil may conduct heat rather than reflect it. It can only act as a radiant barrier. If anyone disagrees, please tell me the R value of Foil.

 

[John D v2.0]

Yes it's true if the wall construction is bricks then foil then radiator, but I don't think many people have their radiators touching the wall! Foil only works with an air gap otherwise it can't reflect radiant heat anywhere.
http://blog.kingspaninsulation.co.uk/use-foil-faced-insulation/

 

[Doggit]

What metal do you think is in your bubble foil

It's not the metal that's important, it's the air bubbles that acts as the insulator.

Foil only works with an air gap otherwise it can't reflect radiant heat anywhere

Exactly my point. And having read the Kingspan article, when they start quoting that radiation can travel across a vacuum, their credibility goes out the window. Infra red radiation can, but not radiant heat from a radiator, that's partially why double glazing works, because there's static air space that the heat can't travel across. Radiant heat works by heating the air space nearest to it, which then allows that air it to move due to a temperature differential.

And whilst most rads are a about an 5cm from a wall, the heat is being transferred into them radiantly, it's the air bubble or polystyrene backing that stops the heat warming the wall, thus allowing more to be transferred into the room. An outside wall will be colder than an inside one, and therefore be able to absorb more heat, thus reducing the potential efficiency of the rad - okay, I have no idea how much will get wasted, and this could all be just an exercise in nonsense, but I despair a bit when people say that ally foil will cut down the heat wastage of a rad; it won't without a supplementary insulator behind it.

 

[slippyr4]

You're really getting confused with radiation and conduction.

Radiation can absolutely cross a vacuum. There's nothing special with the heat radiated from a radiator. Radiated heat may be reflected away.

The air between the radiator and the wall also conducts heat.

The air bubble is a poor conductor and thus littl heat conducts across it.

 

[JohnD]

Infra red radiation can, but not radiant heat from a radiator

that's a joke, right?

Radiant heat works by heating the air space nearest to it, which then allows that air it to move due to a temperature differential.

You cannot be serious.