Emissivity and air gap for insulating an enclosure

Thread Starter

wazman

Joined Oct 31, 2016
20
Hello, I am building small refrigerated enclosure with an aluminium outer housing/case and aluminium inner container (where items to be cooled will be held), separated by XPS insulating foam.
Aluminium has been chosen for its low emissivity – as the exterior will be frequently exposed to the sun – as well as its light weight.
I have been reading up on insulation, and in a housing situation, you would have an aluminium/foil barrier facing the 'hot' side (ie outside wall or roof cavity), then an air gap, then insulating material, then the ceiling or wall.
This air gap is to prevent thermal bridging ie heat being directly conducted from the aluminium barrier into the insulating material and the inner wall/ceiling.
In the case of my cooler, would it be beneficial to include an air gap between the outer housing and the insulation? For example, would 5mm (or even less) of air gap be more beneficial than an equivalent thickness of insulation? If I was going to have 20mm of insulation material, should I instead make it 15mm with a 5mm air gap?
Some thoughts
- If I leave a gap, the air inside it is unlikely to be stationary, and there will be air exchange with the "outside world", which presumably reduces the effectiveness of the air gap by convection etc.
- But having no gap, ie insulation in contact with the outer housing, would presumably introduce some thermal bridging, ie heat from the outer housing being transferred by conduction to the insulation
I note that metal-foam-metal panels used for freezer rooms and roofing of outdoor entertainment areas do not have air gaps, so I am very curious about the pros and cons.
Apologies if this question is not ideal for an electronics forum, but there is an electronics aspect to my project and I always find All About Circuits to be the best place to find knowledgeable people!
 

Tonyr1084

Joined Sep 24, 2015
7,755
NOT an expert here - but what I know about air gaps pertains a lot to roofing or under flooring. You expect a roof to get hot and you want to vent that hot air out via the air gap. Insufficient gapping means lower air flow and longer heat saturation times for the insulation. In my home and garage the rafters contain blown insulation leaving the roof well exposed to open air space to vent out heat.

What you're building sounds a lot different. If your foam boards are going to have venting then you need sufficient air gap to allow for sufficient air flow. Simply having an air gap will add no benefits. But if you want an air gap then cut 1" square spacers to space out the gap you want. Since I don't know the thickness of your boards nor do I know the dimensions - even if I did it wouldn't help me calculate any air gaps - I can't say for sure how thick you should make these spacers. But the spacers will hold the foam and the inner core in place without shifting about. As for your thinking about
For example, would 5mm (or even less) of air gap be more beneficial than an equivalent thickness of insulation? If I was going to have 20mm of insulation material, should I instead make it 15mm with a 5mm air gap?
AND
having no gap, ie insulation in contact with the outer housing, would presumably introduce some thermal bridging, ie heat from the outer housing being transferred by conduction to the insulation
Insulation is designed to be a barrier to thermal bridging. I would guess that use of an air space would be unnecessary. But if you want to go 20mm no gap or 15mm + 5mm air gap - that's entirely up to you. But with the 5mm gap you still need to support your inner container somehow. I would think no gap would be the best way to go. I have Igloo coolers that can keep things cold for days with 20 pounds of ice. That's in the sun or shade. Of course in shade they go longer. But if you're actively cooling the contents inside - my opinion is no air gap.
 

Thread Starter

wazman

Joined Oct 31, 2016
20
NOT an expert here - but what I know about air gaps pertains a lot to roofing or under flooring. You expect a roof to get hot and you want to vent that hot air out via the air gap.
Thanks Tonyr1084, actually the air gapping I’m referring to is not about venting - it’s about creating a gap that heat would have to radiate through to reach the inside. It’s so the radiative barrier (shiny foil/metal) doesn’t end up defeating its own purpose by blocking radiative heat but then conducting heat to the inside. You actually want still air rather than vented or moving air.

However you have made some useful points elsewhere in your post and i will put you down as a vote for ‘no air gap’.
 

MisterBill2

Joined Jan 23, 2018
17,725
Based on the fairly poor heat conducting properties of air, all of the poor conducting materials in the path will reduce heat flow. So an air gap in addition to the foam is a good choice. A reflective outer surface will also help, and with a second outer layer with an air gap and an absorbtive surface on the inward facing side the insulation will be even more effective.
 

Thread Starter

wazman

Joined Oct 31, 2016
20
Based on the fairly poor heat conducting properties of air, all of the poor conducting materials in the path will reduce heat flow. So an air gap in addition to the foam is a good choice. A reflective outer surface will also help, and with a second outer layer with an air gap and an absorbtive surface on the inward facing side the insulation will be even more effective.
Thanks for the contribution. From what I have read, for an air gap to be most effective it needs to be sealed up, as in a double-glazed window. If air can circulate in the gap, heat gets convected into it. I don't think it will be possible to create such a seal in my application, ie outer casing -> air gap -> XPS insulation.

I am tending towards the conclusion that no air gap is the way to do it. But some experimentation might be beneficial.
 

LowQCab

Joined Nov 6, 2012
3,922
Air-Gaps used in Construction are usually for Moisture-Control,
as most homes are made of materials that will deteriorate if they remain damp or wet.

An Air-Gap, in your application would only create the benefit of
putting your box "in-the-shade", as opposed to having direct Sunlight exposure.
If a "gap" is used in this manner it should have unimpeded air-flow so that
the "Sun-Heated-Air" in the "gap" can easily be replaced with cooler ambient Air.

If the Box is always indoors,
adding an additional sealed "gap" will result in
very slightly worse performance than if
additional Foam-Insulation was added in the "gap" Air-space.
.
.
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