expanding foam to fix picnic box

[AronSearle]

So I have a picnic box that turns out to have lots of air space in it rather than insulation, it does however easily dismantle into the inner and outer shell.

I want to insulate it with a can of expanding foam, but haven't used this stuff before.......

1. What is the working time, I want to squirt it all over the inside of the outer shell then press the inner shell into it, I imagine it stays soft and malleable for several minutes, is this correct?

2. The space between the inner and outer shell is about 20-25mm, I imagine I want to create lines of insulation "so far" apart, so that when I fit it all together those lines expand and fill the gaps up, to much squirty insulation and I guess it will push the thing apart as it expands? Is that right, how far should the lines be apart from each other?

 

[Burnerman]

Different makes have differing expansion rates, I'm afraid, although they all achieve the same thing in the end!
One thing is for sure, it continues to expand even if you think it has stopped, and it's therefore vital to allow the stuff to escape somewhere......leaving it be will almost certainly split your picnic box.
Have a bit of practice with a can.....warm it in warm water if it's cold and shake well. Lay a strip, and see how far it goes......there's a good chance you'll have plenty of the stuff left anyway.You may like to consider drilling a hole or two on the lid underside, and allowing the foam to expand through there if it needs to. The hole can be closed later with a plastic grommet.
John

 

[foxhole]

It has air in the space because it's a more efficient insulator than foam, so you will reduce it's efficiency.

 

[Nozzle]

It has air in the space because it's a more efficient insulator than foam, so you will reduce it's efficiency.

Only if that air is trapped - if it can circulate or leak/replenish then it's no good as an insulator - having closed cell foam would prevent that circulation. If air is a good insulator, why do thermos flasks have a vacuum...

Nozzle

 

[Burnerman]

That is absolutely correct.......next to nowt in it though, thermal conductivity wise, so long as the air is dry and trapped.
John

 

[AronSearle]

The box un-clips apart fairly easily, so the airspace between the two shells is far from a sealed air space (you can see gaps all around).

The thing is next to useless as it is, put a few ice-packs in it, and condensation forms on the outside.

I had not considered the thermal conductivity of air at all, maybe instead I will try foil lining the inside face of the inner shell and outside face of the box to reflect heat out, and tape the edges to stop air movement.

 

[foxhole]

New box , no use at all unless sealed tight.

 

[AronSearle]

New box , no use at all unless sealed tight.

I am too cheap to buy a new one, and I like bodging things together.

 

[Nozzle]

Foil? Made from Aluminium? GREAT conductor!

Nozzle

 

[AronSearle]

My understanding is foil works as an insulator (or heat reflector) if it has an airspace in front of it, I was thinking of foil+bubble-wrap+foil+bubble-wrap+foil.

 

[foxhole]

It has air in the space because it's a more efficient insulator than foam, so you will reduce it's efficiency.

Only if that air is trapped - if it can circulate or leak/replenish then it's no good as an insulator - having closed cell foam would prevent that circulation. If air is a good insulator, why do thermos flasks have a vacuum...

Nozzle

Air is used as it's far cheaper and more practical than a vacuum in a large box with a large lid, not comparable to a Thermos.

 

[Nozzle]

Air IS cheap, you're dead right about that. Doesn't mean it's any good. In fact, cheap things are very often the opposite of good.

Nozzle

 

[EightyTwo]

You could use expanded polystyrene sheet as found in packaging... I've made a "free", cheap-skate cool box this way.

Heat will get into the box three ways - conduction, convection and radiation. An air space will minimise conduction but convection and radiation will still be avtive. Aluminium foil will help to stop radiation but conduction is high (no convection in foil). Foam (polyurethane, polystyrene, etc.) will stop convection and minimise conduction. If you add foil, you'll reduce radiation too... Oh yes, that's how you insulate buildings!!