insulating with air

[ajrobb]

There are products, like Kingspan K18, that can insulate the inside of a solid brick wall or roof space. My question is, given phenolic foam seems to have about the same K value (conductivity) as air, can I not use trapped air instead of foam?

This is a DIY site and we want to save money. Sometimes we choose cheaper materials at the expense of more labour. Materials don't get much cheaper than air.

I was thinking of multiple layers of polythene separated by 25 mm battens. If the air gaps are blocked at the top and sides but left open to the cold side at the bottom, the air won't move but any vapour from small imperfections can escape before it condenses. The inner surface could be foil backed plasterboard attached with screws long enough to go through several battens. Obviously the air gaps must all be sealed from the warm room and only open to the cold side. The air gap on the cold side should probably be left open at the top as well as the bottom to allow penetrating damp in the bricks to dry.

 

[mointainwalker]

Where do you get this definition of the K figure from ? It is at odds with U values and in normal heat loss calculations, K is the temperature difference in degrees.

If we use U values and commonly quoted examples, your proposed 25 mm air-gap would have a U value of 5.0 whereas PIR would be 1.0 so much better

No literature I have seen talks about multiple air-gaps being cumulable, although that would seem reasonable.

 

[ajrobb]

Where do you get this definition of the K figure from ? It is at odds with U values and in normal heat loss calculations, K is the temperature difference in degrees.

If we use U values and commonly quoted examples, your proposed 25 mm air-gap would have a U value of 5.0 whereas PIR would be 1.0 so much better

No literature I have seen talks about multiple air-gaps being cumulable, although that would seem reasonable.

Using a k-value of 0.024 for air, the R-value for 25 mm would 1.04 and the U-value would be 0.96. However, I assume that is for still air between silvered surfaces. I hope that foil backed plasterboard would provide one silvered (low E) surface.

Other k-values include 0.04 for mineral wool and 0.025 for foam.

 

[ajrobb]

from http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=3653&page=1

"The NRC has indicated that the thermal resistance of a 20-mm airspace with two reflective surfaces would be RSI 0.61 (R 3.46) for wall
applications."

OK, so 0.61 (U-value of 1.64) is not as good a 0.833 (U-value of 1.2) from the k-value alone. But it is a darn sight better that 0.2 (U-value of 5). As I suspected though, it needs 2 reflective surfaces.

 

[mointainwalker]

Who is the NRC ? Since somebody you quote says a 25 mm air gap is worth R 1 ( these things are not reproducible and usable to two decimal places ) and I quote somebody who says it is worth R 0.2 , then obviously there is room for clarification.

If you feel happy with your results, then please go ahead and use your methods and tell us the results.

They may save the country - nay the world - millions of tons of whatever gas you care to name as we won't need insulation materials , just air.

If you would however, could you answer my queries on the NRC and the definition of K that you raised in the preceding post ?

Thanks

 

[ColJack]

the problem with using air is not the thermal conductivity of it, it's the convection of it..
it moves, and transfers heat faster that way..
that's why insulation products were made..
all they really are is ways to trap air..
the foam traps air ( or other gases ) in little bubbles, rockwool and similar breaks up the airflow.

if you compress the foam and rockwool stuff it's insulating proerties decrease a lot..

 

[ajrobb]

If you would however, could you answer my queries on the NRC and the definition of K that you raised in the preceding post ?

Natural Resources Canada

Given conductivity, k, and thickness, t: U = k/t

If the NRC's data is to be believed, multiple air gaps between reflective films could perform better than mineral wool but worse than the best foams. I suspect the cost of multiple reflective films would be prohibitive though.

the problem with using air is not the thermal conductivity of it, it's the convection of it..

Yes, the worrying thing is that double glazing with intermediate films insulates better than ordinary double glazing for the same overall thickness. Very quickly, as you increase the gap thickness, you cease to get the full benefit of the poor conductivity.

 

[noseall]

the problem with using air is not the thermal conductivity of it, it's the convection of it..
it moves, and transfers heat faster that way..
that's why insulation products were made..
all they really are is ways to trap air..

I agree

 

[mointainwalker]

I don\t know if this is the case but is it possible you have been misled by conversion of numbers ?

I know that the USA uses R values but because they are Imperial R values their relevance to what we have is nil ( without conversion factor )

e.g. 200 mm = R 5 in Europe = R 20 in USA ( guesstimate )

????

 

[ajrobb]

I don\t know if this is the case but is it possible you have been misled by conversion of numbers ?

I know that the USA uses R values but because they are Imperial R values their relevance to what we have is nil ( without conversion factor )

e.g. 200 mm = R 5 in Europe = R 20 in USA ( guesstimate )

????

I assume the two R values quoted "RSI 0.61 (R 3.46)" are metric and US respectively - the ratio looks right. Canada is a metric country after all.

 

[ajrobb]

From Proctor Group's "Reflective Technology" brochure:

"8 Reflectatherm Cavity - timber straps 19.00 - 0.644"

i.e. a 19 mm cavity with a reflective VCL foil on one side, has a thermal resistance of 0.644 m2K/W

I take brochure figures with a pinch of salt, but it is encouraging all the same.[/img]

 

[mointainwalker]

How can you give a cavity without specifying both sides ? I would take an extra slug of salt with this one

 

[ajrobb]

How can you give a cavity without specifying both sides ? I would take an extra slug of salt with this one

It was in the list of layers calculating the total R value. There was mineral wool on the other side of the reflective foil and plasterboard on the other side of the cavity (service void).

I have asked Proctor Group if a layer of Reflectatherm between 2 cavities of 19 mm would have a combined R value of 1.3 (0.644 + 0.644). That would be as good as 50 mm of mineral wool in just 38 mm! I'll let you know if they dare reply.

 

[mointainwalker]

Thanks, that would be interesting !

 

[ajrobb]

Thanks, that would be interesting !

I have also asked Celotex for R values to use for sealed cavities next to CW3000 boards. If you can use 0.644 for those too, that implies each 19 mm cavity is equivalent to 15 mm of Celotex - great if you want a service cavity on the warm side of the VPC!