Fridge Insulation

[theotter]

I have refriguated an existing inbuilt cold/ice box, however I would like to improve the insulation if I can. Can anyone recommennd a suitable material, perhaps even space age type, that I could line the box with internally.

Thanks for any advice.

 

[Stemar]

There's a variety of posh insulating materials, of whose existence and exotic price I am aware, but am otherwise entirely ignorant.

However, is it not possible to put the insulation on the outside? Inside means a smaller volume (and they're never big enough anyway!) & having to line the fridge with an easy-clean, waterproof. On the outside you just slap it on.

 

[pyrojames]

Tri iso 9 or 10, 25 mm thick and equiv to 200 mm rockwall.

 

[tobble]

Hmm, just looked at tri iso site andnot convinced by the sales pitch. "equivalent to 210mm mineral wool when installed in a pitched roof" which is quite a different scenario to a fridge, even if you belive those figures. Foil type insulation works by reflecting RADIANT heat back, but it has to be reflected back TO somewhere - in an attic this is useful as it goes back inside the thermal envelope of the house, but in your fridge box it will be hard up against either the box or liner, so no radiant heat transfer occurs. on the contrary, it actually conducts the heat through very effectively, which is the opposite of what you want... think high school physics!

the best material in terms of thermal resitance per thickness is gennerally thought to be extruded poyurethane (note extruded not expanded - this is good but not as good as extruded) and happily will suffer no ill effects from the marine environment.

I think some of the kingspan products are based on this, and are of course readily available from builders merchants.

 

[pyrojames]

The physics is the same in either case, In the fridge you are trying to reflect heat away from the fridge and back in to the boat envelope. The tri-ISO still has insulating layers bewteen the reflective ones. I suspect that the foil layers are mylar which has one of the lowest thermal conductivities of any material at 0.0001 W/mK, while the intermediate separating layers will have a more typical 0.02-0.04 W/mK thermal conductivity. Triso gets a thermal conductivity of 0.0057W/mK, so around 4 times better than Celotex/extruded polyurethane.

Products such as Celotex will fall in the 0.03 range, and draws its insulating benefits from additional thickness.

 

[tobble]

I don't quite belive the numbers quoted on thier data sheet because they have used a different test proceedure to the usual one, by thier own admission. If they were shining a radiant heat source at it, yes it probably has a very good insulation value. however, in a fridge, radiation doesn't come into it, unless you have a shiny surface on the outside of the box, where the heat can be reflected back into 'free', ventialted space. but radiation can't pass through opaque stuff, i.e. the 'fluffy' insulating layers, and so it is pointless having internal foil layers (or am I missing something?). Even if that did work, it's pointless havng multiple foils because they will reflect the heatback in just as much as they reflect it out once the heat is between the foils...

 

[pyrojames]

Most of the heat transfer through something like rock wool or anything with an air pocket is though radiation. There is little conduction due to the long convoluted, and extremely narrow fibres that form the material and the low conduction coefficients for gases. There is no convection due to the lack of fluid movement. It appears to me that the layer in the Triso are of a similar open cell foam type material. The foil will act as a vapour barrier and anti convection agent, while also having an extremely low conductivity and acting as a radiation barrier. Any heat that gets through one layer is further trapped in the next layer by the foil, giving a step change in temperature profile through the material.

The 0.0057 value I quoted was obtained by independant testing as far as I could ascertain. I doubt that the developers of Triso have gone to the expense of a multi-layered product (and Triso is v expensive) if there is no purpose to it.

I came across it when it was recommended as a space saving insulator that would maintain the insulation levels for building regs. I was going to use it in my fridge installation, but then I found I wasn't that tight on space in the new boat and the cost put me off.