Wall insulation

[Phil Pascoe]

If i insulate a wall with 100mm - 120mm foam sheets, surely any advantage in using foil covered ones is so small as to be insignificant? I can see the value if using 25mm or so, but not enough at that thickness to justify the expense. Does anyone know the actual figures? The boards will be clad with 18mm OSB. Is there any other reason for using foil backed sheets?

 

[Random Orbital Bob]

Its got squat to do with thickness. If you remember your school physics...heat travels in 3 ways:

conduction, convection and radiation. Both conduction and convection are substantially slowed by what we commonly understand to be insulating materials. Specifically, materials that trap small pockets of air like rockwool, polystyrene, foam board etc. (So thickness is all important here).

Radiated heat on the other hand is not slowed by those materials. It is slowed almost to the point of stopping by reflective surfaces like the shiny side of aluminium foil. That's why you should always wrap your hot turkey shiny side in....to reflect back the radiated heat while it rests! Then you pile towels on top to stop conducted or convected heat escape. Its also why commercially available insulation products have design features like an outer foil strip, that's for the radiated heat. So a typical piece of Celotex for example has a foam inner with lots of air filled pockets for the conduction and convection AND a foil outer layer for the radiation.

Its also why a thermos flask works incidentally.

 

[treeturner123]

Bob

Hate to disagree, but the main reason that thermos flasks work is, as their other name implies, Vacuum. Heat does not transfer across a vacuum, or so I learnt long long ago at school.

Phil

 

[KevM]

Bob

Hate to disagree, but the main reason that thermos flasks work is, as their other name implies, Vacuum. Heat does not transfer across a vacuum, or so I learnt long long ago at school.

Phil

bzzzt! wrong answer! If you were right we wouldn't feel the radiated heat from the sun.

 

[Craigus]

The cost is usually insignificant between the two. Helps with cooling in the summer as well.

My brother built a 'working' shed that was insulated, he used 25mm of that space agy thin layered foil stuff. It helped with the heat in the winter, but it was like a sauna in the summer.

 

[Random Orbital Bob]

Good point....but nothing to do with the OP's question and regrettably also wrong. In fact, it proves my point Its the vacuum (the total lack of any molecules) that completely stops convection and conduction. The radiation in a thermos flask is substantially reduced by it's mirror finished spaces inside. You'll notice when you dismantle a thermos flask it has highly mirrored surfaces....that's to stop the radiated heat.

If radiated heat can't travel through a vacuum, how is it we feel the sun's heat? (Space being a vacuum)

The vacuum stops non radiated heat because there are no moecules to transfer the vibrations from one to t'other.

Edit: blimey you two were quick off the mark...I was just typing my refutation to Phil's load of poppycock when 2 posts beat me to it!!

 

[KevM]

Is there any other reason for using foil backed sheets?

The foil backing also serves as a vapour barrier.

 

[RobinBHM]

Im not sure the foil has any impact on thermal quality, Ive always thought that it is used as a vapour barrier when joints are foil taped.

Of course there multi layer foil insulation products such as multiquilt which are using foil layers to reduce radiated heat passing through and are supposed to be able to offer u value rating with thinners walls. There seems to be some disagreement as to whether they actually work.

Also taping joints fully will also help with air tightness and that has a bearing on u value.

I guess you are talikng about EPS sheets as PIR sheets all seem to have foil faces

 

[Craigus]

Yes they can act as a vapor barrier if they are tight fitted and joints are taped with foil tape. When I do site inspections I've rarely seen workmanship good enough to achieve this without the need for and additional barrier.

Not that that has much to do with the OP though...

 

[Phil Pascoe]

My point was more to do with the practicalities than the physics. The boards are indoors, so dry (or if not technically dry at least the same humidity on both sides), so a vapour barrier is neither here nor there. I'm just wondering if with boards of that thickness (a 4" foiled board obviously being far more efficient than a 1" foiled board) there is a measurable difference - there can be quite a large price difference (or there is where I'm looking).

 

[Craigus]

Where exactly are the boards going? Are you lining an existing masonry wall with them?

You say a vapour barrier makes no difference but it is more to do with where the dew point is in the build up, and therefore where you may get condensation and damp problems, rather than it protecting from water ingress.

 

[Phil Pascoe]

A four inch block wall between my bedroom and a garage.

 

[RobinBHM]

I thought that manufacturers of insulation included foil faces for use as a vapour barrier not as any relevance to insulation value -which was the point I was asking.But unfortunately dont the answer for sure, its just what ive assumed. It certainly isnt obvious on the manufacturers websites.

Plasterboard with a vapour check has foil, not for any insulation reason as far as I can understand.

A vapour barrier is fitted on the inside, ie the warm side, to prevent moisture migrating through to the cold side and then condensing into water, effect known as interstitial condensation.

 

[kostello]

Foil faced insulation needs an air space between it and the internal wall finish for the radiated heat to be reflected back into the room efficiently.....

 

[kostello]

If plasterboard is fixed directly to the foil face of the insulation it doesn't reflect the radiated heat

 

[blackrodd]

I thought that manufacturers of insulation included foil faces for use as a vapour barrier not as any relevance to insulation value -which was the point I was asking.But unfortunately dont the answer for sure, its just what ive assumed. It certainly isnt obvious on the manufacturers websites.

Plasterboard with a vapour check has foil, not for any insulation reason as far as I can understand.

A vapour barrier is fitted on the inside, ie the warm side, to prevent moisture migrating through to the cold side and then condensing into water, effect known as interstitial condensation.

In my dry lining and suspended ceiling days, I went to British Gypsum,couple of times, to their training school at Erith, which, someone told me, is now in "mothballs".
This led to a walk about the factory when, at that time of my first visit, , they were making 12.7 Tapered edge plasterboard.
Many things were explained including the manufacture of moisture resistant plaster board using a "chrome" like finished plastic sheet, that acts as both a moisture resistant sheet and also reflective mambrane. the main advantage was for them was one product used for two, sometimes different uses and the storage and handling, of it.
The aluminium foil, on the back of the earlier sheets was a bit delicate and not very robust, it easily peeled off by just the handling of it.
I thought I would be bored, but, found these visits very interesting, Regards Rodders