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

[Black Timber]

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

 

[fernicarry]

If air can move freely through the gap then that kind of renders the outer layer of insulation pointless.

If its not able to move then air on its own is quite a good insulator, that is after all how wool type materials work, by trapping little pockets of air, but its not as good as PIR. So yeah maybe you get a little more bang for your buck with an air sandwich but it will depend on how air tight the void is. In a roof space the air is going to go through some fairly drastic thermal cycles and I wonder whether air tightness tapes could cope with that. None of this could be described as reflection however.

Do you have a source for this theory that explains the principle?

 

[Black Timber]

Thanks for the reply @fernicarry

In the example I found, the air would not be able to move in the airgap. The between rafter PIR was fitted tightly at the wall plate and ridge board.

Do you have a source for this theory that explains the principle?

I think I read it in passing a couple of times but can't be sure nor remember where. But the main source of the idea was a Skill Builder (Robin Clevett) video on YouTube. He discusses it in one of the videos. Don't think he discusses it in the other, but I noticed the gap and then saw that it had been noticed by others and discussed in the comments section.

In my situation space is tight, so the best I could really do is a 5mm-10mm airgap between the top/bottom PIR boards. But that would eat into the ventilation gap behind the PIR, so on reflection I think it would be best to keep the vent gap at 50mm and just butt up the two layers of PIR.

Cheers

 

[noj]

Any unventilated air gap is insulating. Reduces conduction, allows radiation and convection.

Foil reflects radiation, does conduct, no convection involved. Foil backed PIR takes care of a lot of the conduction.

Logically an air gap between layers of PIR will enhance the level of insulation. It's hard to know by how much, but if you're not going to compromise the under roof ventilation then it seem like a zero cost improvement.

 

[fernicarry]

So the idea is that because your insulating material is fitted in such an incredibly air tight manner thanks to the gapotape you can afford to put a deliberate void in between layers for an extra bit of free insulation. The void introduced in the videos isn't that big, just a couple of inches at most so the effect may be very small.

I'd be more worried about the fact that the gapotape will exacerbate the non-breathable nature of PIR since its even more tightly in contact with the side of the rafter. You better be very certain that you don't have any defects in your roof structure allowing water ingress. Something I've just discovered is not the case in my roof thanks to a very poorly installed over flow pipe...