Insulation: air gap required for foil Insulation?

[Tim Good]

I’m in the process of insulating my saloon roof.

Initially I have 20mm or closed cell foam push up and stuck to the roof. It also has a foil back. See pic below:



I then added a layer of Super Quilt which is layered foil backed insulation. It has layers of foil with layers of very light foam in the middle. You can see it here:



However I was watching this video which clearly suggests by putting the products together I might be making it worse.

What do you think?

 

[Graham_Wright]

I’m in the process of insulating my saloon roof.

Initially I have 20mm or closed cell foam push up and stuck to the roof. It also has a foil back. See pic below:

View attachment 102756

I then added a layer of Super Quilt which is layered foil backed insulation. It has layers of foil with layers of very light foam in the middle. You can see it here:

View attachment 102757

However I was watching this video which clearly suggests by putting the products together I might be making it worse.

What do you think?

You risk condensation on the foil - it likes hard surfaces.

 

[thinwater]

The backing is there as a vapor barrier; the IR blocking is secondary. The backing goes on the high-humidity side, which in the heating season, is the inside. However, unless it is properly sealed into place, it can also trap moisture.

I suggest reading up on the installation of vapor barriers and insulation. It can be complicated.

 

[cpedw]

The foil added to insulations can be performing up to three functions: Stopping radiation heat loss, vapour barrier and mechanical strength for the insulation.

Th radiation effect is supplied by the innermost layer. It needs to have air (or a vacuum) against one side to be effective. The vapour barrier is significant in the innermost layer too. The mechanical strength is needed to make installation straightforward.

If I understand your installation correctly, the innermost foil layer is doing all three jobs. The other foil layers are only giving mechanical support to the insulation. They are neither helping nor hindering the insulation effect. IMHO the video is a bit misleading as it's describing the effect of a layer of foil only; without the insulation, foil layers are useless after the first layer.

 

[Keith-i]

If you can exclude moist air from getting past the insulation onto a cold surface then condensation should not be an issue. Conduction is not a big issue with the multi-layer product you are using, nor with the closed cell foam you stuck up first. Tape all the open edges up carefully and you will hopefully be ok. It might affect your phone signal and anything else with an internal aerial.

 

[geem]

I would suggest that the addition of a super quilt is unnecessary if you already have 20mm of foil faced closed cell insulation. I would use a good quality aluminium foil tape to seal the edges of your foil backed foam insulation to maintain the vapour seal. We did this on a previous boat and it was very effective. You need to make sure you are using a high temperature adhesive. The deck can get hot in the summer and unless the adhesive is HT it will eventually fail.