Reflective Radiation - Install PIR with air gaps to increase effectiveness?

[Oxbow16]

Hi 

 

Back again with another insulation question.  I've came across a couple of examples of people pushing PIR further into the void between rafters (but not so far as to impede the airgap behind for ventilation) rather than flush with the rafter face.  Looking into it, I found this was done on purpose to create a gap between those between rafter pieces of insulation and the perpendicular insulation installed beneath to the underside of the rafters.  These airgaps apparently create reflective radiation and increase the effectiveness.  

 

Just wondered what people think of this principle and whether it is right or not? 

 

And if the idea does have merit, what would the minimum amount of air gap needed be to have any worthwhile effect?  Would 5mm/10mm do anything?  

 

Many thanks

[JohnMo]

I have often though about this (sad I know).  When you compare thermal properties of PIR, those covered in foil and those not (other materials not aluminium) the not reflective coating PIR, always seems to have a worse figure for thermal conductivity.

 

So does the 0.022 thermal conductivity already make the assumption there will be an air gap? I am assuming so, as the aluminium foil is actually a heat conductor if its a reflective surface.

[Oxbow16]

Thanks for the reply and for sharing your thoughts

 

[MikeGrahamT21]

I remember reading somewhere a long time ago that without the gap the foil face becomes conductive rather than reflective, how true that is im not sure, but it sounded reasonable.

[Iceverge]

Your scheme is to try to emulate the effect of trapping a layer of gas between two planes like a double or triple glazed window. 

 

This only works if there is no convection. In your case you want to actively promote convection above the insulation to ensure the roof can dry out. 

 

In reality anyway you will find it impossible to create a layer of air between two materials without any convection and a layer of simple mineral wool would aid the insulative effect more than your DIY cavity. 

 

On 16/03/2023 at 12:45, Oxbow16 said:

These airgaps apparently create reflective radiation and increase the effectiveness.  

 

I don't quite understand this. 

 

There is a case for a low emissivity (shiny) barrier for heat protection in very sunny climates with no ventilation below the roof covering but it makes almost zero difference in a heat retention setting.