Measuring Insulation Effectiveness

[jameson_uk]

So I have a small hot tub sat on top of some 100mm PIR insulation boards I managed to get hold of. Now PIR isn't designed for moisture so I wrapped it in a sheet of damp proof membrane (Capital Valley Plastics Ltd Damp-Proof Membrane Black 1000ga 3 x 4m - Screwfix) I had in the garage.

I am now wondering whether the damp proof membrane is actually transferring the heat and bypassing the insulation.

What I can't figure out is a decent way of actually testing this and working out if the insulation is actually efficient?
I have various temperature probes and I can easily knock some more up but I am struggling as to what to measure and how I would interpret the results anyway as it is going to be affected by ambient conditions.

 

[ScareCrow]

I'm definitely no expert on this but I've been reading up on the subject of insulation in preparation for renewing the flat roof on my extension, so not exactly related either.
What makes you think the damp proof membrane is transferring heat? As it's essentially a plastic sheet it should be a pretty poor conductor (to my mind). As long as you're sure the insulation isn't getting wet it should be fine.
A picture of the setup or diagram of the build up would help identify if there is an issue and what it might be.

 

[jameson_uk]

I'm definitely no expert on this but I've been reading up on the subject of insulation in preparation for renewing the flat roof on my extension, so not exactly related either.
What makes you think the damp proof membrane is transferring heat? As it's essentially a plastic sheet it should be a pretty poor conductor (to my mind). As long as you're sure the insulation isn't getting wet it should be fine.
A picture of the setup or diagram of the build up would help identify if there is an issue and what it might be.

It is literally about 2m x 2.4m of board (I think it is IKO Enertherm PIR Cavity Wall Insulation Board - 1200 x 450 x 100mm | Wickes.co.uk) that has been wrapped in the membrane and sealed with with membrane tape (just like wrapping a present really with only a little overlap of the membrane). On top of that are some 10mm rubber mats then the inflatable tub.. This all sat on a brick paved patio.

The primary reason for looking at it is that I don't seem to be getting the same performance as other people who have done the same (but not used any membrane). This is based on turning the heater off and seeing how much the temperature drops by overnight or over a 24 hour period. This is obviously difficult to compare as there will be lots of different things affecting this. It costs a small fortune to heat the bloody thing so trying to ensure I have it running as efficiently as I can.

I guess the second thought was that yes plastic should be a poor conductor but it will be better than the insulation? so I was wondering if it would be like water or electricity and heat would essentially go for the easiest route to the cold brick floor

 

[ScareCrow]

It is literally about 2m x 2.4m of board (I think it is IKO Enertherm PIR Cavity Wall Insulation Board - 1200 x 450 x 100mm | Wickes.co.uk) that has been wrapped in the membrane and sealed with with membrane tape (just like wrapping a present really with only a little overlap of the membrane). On top of that are some 10mm rubber mats then the inflatable tub.. This all sat on a brick paved patio.

I see, it all sounds good. I guess is the position of it more shaded than others getting better results? Their paving may heat up more and radiate heat over night reducing heat loss. As you say there are multiple variables.
Is there a chance that water has got into the insulation and now it's creating a thermal bridge? Or possibly sat on top of the membrane between the pool and insulation, which I guess would allow it to radiate heat.

Not getting to the bottom of your problem but could you add a solar heater to help raise the temp a few degrees so that the heater isn't working so hard? They're easy to make or basic ones are pretty cheap.

 

[jameson_uk]

I see, it all sounds good. I guess is the position of it more shaded than others getting better results? Their paving may heat up more and radiate heat over night reducing heat loss. As you say there are multiple variables.
Is there a chance that water has got into the insulation and now it's creating a thermal bridge? Or possibly sat on top of the membrane between the pool and insulation, which I guess would allow it to radiate heat.

I will open up the membrane to check but I am guessing it should be OK. There are so many variables which is why I wanted to start looking at the efficiency rather than just looking at the rate of change of water temperature. In theory the PIR stuff is a great insulator but it is designed to go inside dry walls and the like so not really the right tool but it should work.

Not getting to the bottom of your problem but could you add a solar heater to help raise the temp a few degrees so that the heater isn't working so hard? They're easy to make or basic ones are pretty cheap.

There is a 2kW heater in there so I guess a solar heater would only help a little. Also the pipework makes things a bit complicated to enhance as there is just an inlet / outlet pipes on the side of the tub and anything that adjusts the flow causes the pump to start throwing errors.... The most cost effective way to heat the water seems to be LPG but I don't really want to go there.