Most effective insulation under carpet – above suspended vented wooden floor

30 Apr 2022
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United Kingdom
My living room is on ground floor. It has a suspended wooden floor over a large vented cavity (several feet deep). It gets pretty cold in winter. I want to get carpet in this room; both for comfort and to increase the warmth/insulation in the colder months. I’m aware that doing under-floor insulation would be the best way to insulate this room, but it’s a large room and this would be expensive and disruptive so I’m unlikely to go down this path at present given my budget.

I want to get the most insulation I can from above-(floor)board insulation/underlay/carpet.

I came across this ‘Eco-Tech floor foam insulation’ stuff online which seemed like a silver bullet for my situation; as a solution to increase insulation above floorboards (but below carpet underlay):

But delving a little deeper I’ve begun to think it may just be snakeoil. I can’t find much in the way of genuine trade advice on its effectiveness and or suitability for this situation. I’m also concerned it would effectively form a waterproof membrane above the floorboards which may ultimately lead to issues of mould/damp under the underlay. Any experience/trade-level advice most welcome!

Whether or not I the above, I’ll be looking at filling gaps in floorboard best I can the getting the max tog carpet/underlay combo. The best rated underlay I seemed to find was Wilsons Plushwalk 12mm:

…but this seems to have a damp proof membrane (DPM) built in – and again I’m concerned that this could cause issues like the above given the nature of the floor underneath (suspended wooden floorboards over vented cavity). Is this a valid concern? Would I be better with a non-DPM underlay?

Any advice on any of the above – or alternative solutions to getting the best insulation I can from above-board (literally!) work in this room would be most welcome!

Many thanks
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One of the most important things you can do is stop draughts through the floor.

Also you could get remote control car access to the void, so it could be insulated without disturbance.
The majority of the water vapour in the household air comes from human activity; breathing is quite common in them:sneaky:
This vapour is ventilated above the floor level, so you should have no qualms about introducing an above floor vapour barrier. Below the floor most vapour comes out of the ground or out of thebrickwork: this is ventilated with cross-ventilation (airbricks).

If you have a large void under this room it would be very easy to install celotex, or similar, rigid insulated sheets between the joists, and even a second layer under the joists. Celotex has a built-in vapour barrier, and the tight fitting will prevent underfloor damp air contacting the joist surface. The hardest part will be relocating wiring and heating/water pipework so it doesn't compromise the insulation. Again your deep void will make this much easier.

So don'tmess around with 'high tech space-age' products which are thin, and likely to be damaged by foot traffic over the years, get underneath that floor and take advantage of your good luck.

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