Insulation Questions

15 Mar 2017
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United Kingdom
Hi All,

Just a quick questions with regards to some insulation. I have had my foundations laid for my self build conservatory. I am now reading about the insulation under the concrete slab, and also the cavity insulation. Do i need 100mm insulation beneath the concrete floor slab? Or could i use 75mm or 50mm instead? Can anyone help with which to use, Jablite seems cheapest but is this good enough for what i need?

I am thinking about using a 50mm cavity for my walls so will use a 50mm cavity insulation, can i use the rock wool type for this or is it best to use the sheet type ones?

I am building under permitted development so don't need to build to regs if this helps with your recommendations?

Sorry for all the questions but i am trying to build to a budget :)


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You can use whatever thickness insulation you like. Bear in mind with a conservatory most of the heat is going out through the roof, the rest through the windows. If this was an extension floor it would be 80mm PIR foil board or 150mm polystyrene - which is the cheapest type of insualtion you can get

Likewise for the walls. Rockwool can be used and is 50% as efficient as PIR foil board.
Woody is spot on as always.
However the topic of floor insulation is an interesting one.
For saving energy you need to maximise roof insulation and not floor insulation. The reason is that heat rises, so the ceiling always has the maximum temperature difference, and the floor the lowest. However for comfort you would want the floor to be warm as no one likes cold feet. Increasing the floor insulation won't save money but it will keep your feet warmer!
Also, supposing you use under floor heating the temperature difference will be highest at floor level so you'd need better insulation to be most efficient. This is partly why some people say that under floor heating is expensive to run.
Thanks Both,

I am thinking about underfloor heating, so would you suggest the 100mm insulation for the floor?


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You should be thinking of foil backed insulation with UH, not any old cheap stuff.
Look up how many watts per sq m UFH puts out. I think you'll struggle to heat a conservatory with UFH.

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