Insulation - foil faced or not?

[Ian H]

Do you think the price of the boards will come back down anytime soon?

I've just offered another guy £300 for 9 sheets of 100mm but if it ever came down to £30 a sheet new from the merchants I'd be well gutted!!!

 

[Doggit]

It'll depend on how long it sits at the current price. Petrol should be be down at about 60p a litre, but we've had high prices for so long, that they've become the norm. They've dropped from the high of £1.40, so the current price of £1.15, suggests the Celotex prices could settle at about £45 a sheet when the chemical factory gets back on line.

On the other hand, they could well come back to £30 a sheet, but unfortunately, you need it now, so what price it drops to in the future, is immaterial.

 

[Ian H]

Back to the non foil.

I was just working near the guy that's selling the non foil 100mm for £15 per 2.4x1.2 board so I thought it rude not to go and look

I took an offcut to compare to the celotex. Although it's unbranded it is the same as the celotex inner, it's fireproof and waterproof. He is a builder and says some building control guys are fine with it but others prefer the foil.

I need loads and I'm tempted to have it between the rafters and in the dormer roof space with 50mm of foiled board under it.

Opinions?

(That's a rough corner that's been out in his yard for a while)

 

[Doggit]

The problem you've got, is that you don't know the true performance of the PIR, but there's a good chance that with 150mm in total, you should be fine. The question is whether you ask the BCO if he'll accept it or not.

100mm underneath, and 50mm of foil faced on top should be okay, just make sure that everything is sealed with aluminum tape to get the VCL, and the BCO shouldn't batter an eyelid.

Now being unbranded, it wouldn't have cost the same as celotex, so it may only have been £20 a board when he bought it - see if he'll come down any further.

 

[John D v2.0]

The foil only really does anything if it's facing into a void. It also acts as a vcl. If you don't need either maybe it's OK!
However as it's unbranded you don't know how they made it.

 

[tomfe]

The foil only really does anything if it's facing into a void

I am not sure if I agree with that.
But the main reason for the foil is to slow the off gassing of the insulation, this main reason is why normally pir with foil face has a higher lambda-value than tissue etc; to make it fall out the forms and for marketing.
The idea is it should reflect radiated heat, so it should face inwards however if you want to cool say a warm loft it may work facing the void.
Lets muddy the water even more with the likes of the thin foil insulation, (super foil etc), which had real trouble getting BRE to take them seriously.

However as it's unbranded you don't know how they made it

To my knowledge there are only a couple of places in the UK which make ridged insulation and they pretty much make all the brands.

 

[John D v2.0]

I feel a little confused by your post

the main reason for the foil is to slow the off gassing of the insulation, this main reason is why normally pir with foil face has a higher lambda-value than tissue etc

Not heard that, surely the simple explanation is the foil is low emissivity therefore it has a higher R value with would give it a higher average lambda value.

The idea is it should reflect radiated heat, so it should face inwards

Low emissivity works both ways equally, so you can have the void on the cold, warm, or both sides. However by the time you took into account thermal bypass and condensation risk, you'll probably have fewer options.

if you want to cool say a warm loft it may work facing the void

Or warm a cold loft, or any other space you want to maintain differing temperatures.

 

[tomfe]

The foil reflects heat in the infra-red spectrum, there will be little if none from the internal space on the outer surface of the board. The idea of cold being reflected is somewhat outside the normal realms of physics.

Look at the story of the 'super foils'.

 

[John D v2.0]

The idea of cold being reflected is somewhat outside the normal realms of physics.

Well I didn't actually use the word reflected, technically it's a low emissivity surface, but either way it's physics not common sense.

A shiny surface is a poor absorber and emitter of radiant heat. A black body is a perfect absorber and emitter of radiant heat.

I'm aware of the controversy surrounding multi foils, but that's more about how they work in practice rather than the physics theory behind them.
Also the air gaps are certainly not as consistent as in rigid insulation.

 

[Ian H]

I'm digging through some Quinntherm today:

I'm going to ask the BCO, if he is happy, I'll be happy.

 

[Ian H]

Seems the foil faced is easy to get cheaply if you work all over in a van.

I've started filling between the rafters but wonder if there is an expanding foam that will fill the tiny gaps or should I foil tape accross the rafters and owner the gaps?

A foam that's applied like silicone would be ideal

 

[endecotp]

A foam that's applied like silicone would be ideal

Do you have a foam applicator gun?

 

[Ian H]

No, the only foam I've used has been the cans with the nozzle that's too fat for a small gap.

 

[garyo]

The gun grade is much much better. You can apply it at a trickle just like silicone, plus a can will last a month if you're tidy. You end up getting much better value from the cans as a result.

 

[Ian H]

I'll give that a go, thanks.