Floor Insulation

So has anyone got any tips about floor insultation - I can’t seem to get my head round how effective it will be (hot air rises, right?) and how it would work with airbricks (I have an odd situation that one of the downstars rooms has 6 airbricks and the other none!).

Any thoughts or comments welcome! It’s such a great place here to be able to disucss stuff like this…


There are some good references/blogs around, like this one. If the airbricks are in the way, there are telescopic extenders to make sure air flows under the insulation, though some recommend at least 200-300mm of clearance between the insulation and ground to allow sufficient ventilation of the timber. If using barriers lapped over the joists, it can be make a big difference if you seal the barriers to the walls, otherwise the cool air just gets pushed up around the skirting and into the room.

@epickering , As you suggest, from an energy conservation point of view, insulating the floor has a low priority. What you do need to do is completely draught proof it, and while you are at it you may as well insulate.

You will see a marked increase in comfort as there should be far less cold feet and ankles.

Hi all, I think you need to work out what type of flooring you have (suspended, or mixed (solid)) then work out how much heat you are losing / how much cooling it is responsible for - (measure temperature difference between rooms and or do an IR survey (take a load of pictures) to see what is happening in real time. This will help you prioritise action. I did most of my house including all the skirting and concrete floor dug up and replaced, and it is the best thing I did. Weak sunlight through the windows warm the floor quickly and it feels like underfloor heating. If you sit on the floor it warms up and stays warm! While I was doing the work I thought I had gone too far but now living with it I am beaming.

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As an alternative to floor insulation, I am considering using these “smart” air bricks - anyone know anything about them? https://www.airex.tech/

If you believe the blurb they seem a must for virtually every home.
I am encouraged that a well known name is involved, by which I mean Dr. Sofie Pelsmakers.
( Sofie Pelsmakers | UCL Energy Institute - UCL – University College London )

Hi, is there any recommendations for fitters of draft proofing in Manchester? We have a crawl space but it looks tight and I doubt I’d get down there…thanks for any advice

What type of floor is it?

*Slab on grade, timber with ventilated crawl space, or conditioned crawl space, slab on an intermediate floor etc. Are there more types I’m missing?

The ideal is to avoid this from happening and to have even temperatures in the room.

However, the base floor doesn’t need to be as well insulated because the ground won’t drop much below 10deg, whereas the outside could drop to -5deg or so during winter.

During the summer, the ground can cool the house a little, which would be beneficial, therefore insulation would counteract this effect.

If you add a hydronic underfloor heating system in, then the insulation is more worthwhile, because the ground will be heated to 28deg, instead of being at room temp of 21deg (or lower if you don’t mid a cooler house on the coldest day <1% of the year).

I think 100-150mm is fine in this climate. There’s a diminishing rate of return on more.

I would not hammer out an existing slab with 50mm of insulation below grade to add 50-100mm of extra insulation.

Conversely, if you have to take the floor up anyway you may as well add generous insulation. The marginal cost of insulation is minimal, the main expenses are lifting and relaying the floor.

@lloydham , I presume that the photo is of the ground floor. If you have a cellar or basement the advice might be different.

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It is a ground floor with a crawl space. The skirting board is rather high so I expect the plaster is finished a long way from the floor - hence the expanse of cold seen in the image. I am intending to use 200mm thick mineral wool plus breather membrane as I know this works well courtesey of Mr Wickes who seem to have the better accessible offer at this time. I also do not believe the copper heating pipes located underfloor are insulated but will know for sure once I get access.

Suspended floor with a ventilated crawl space is essentially outside. So not as mild as slab on grade. In which case this does not apply.

Depending on the depth of the crawl space you can even consider bringing it into the conditioned space by adding:

Filter fabric, gravel drainage pad with perimeter drain/soil gas vent (if needed), all your insulation, a poly membrane tapped to DPC, then a thin screed.

This essentially creates a mini basement which can now be used for your duct runs and other services. Even mechanical systems can be located down there. Which is not uncommon in the US.

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Air tightness is key here. How are you intending on maintaining a continuous air control layer from your floor into you wall system?

Are you also planning to have exterior or interior wall insulation?

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Yes, a good idea but needs a good understanding of the individual house structure and situation.

If it seems ok to proceed my own inclination is to fully tank the underfloor area as per a cellar to basement conversion before insulating but this does add considerably to the cost.

As your plaster probably ends well above floor level I suggest removing the skirting board, tape the airtight membrane to the brickwork and then plaster down over the join to much nearer the floor. DO NOT NAIL OR SCREW back skirting as you will damage the membrane. Use a proprietary glue once the plaster has cured and been primed. As you are using Wickes you can ask what they suggest.

@lloydham, In the end your choice comes down to cost and practicality.

Conversion to airtight insulated “solid” floor is best done in one go in order to avoid additional joins that can lead to weaknesses. Insulating with suspended matting between joists can be done on a piecemeal basis. It is also much easier to do as a DIY task.

My recommendation if you can afford to do the job and put all downstairs furniture that you can’t get up stairs into storage for a week or so is definitely to do a full conversion.

Thanks for the input. I have previously used breather membrane and insulation that works pretty well- < 0.2U-value- see above. I will need to see the state of the wall floor junction then decide on the appropriate action. My intention is to complete in 2-4 days.

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There are friction fit rigid insulation systems which are designed for floor cavities like this one. It’s very important to fill all the voids. It takes time and attention to detail. The manufacturer will have advice on how best to do this. Loft insulation seems a bit dubious to use in this application.

The benefit is that you won’t need to use the membrane to support the insulation. This will be part of the insulation system.

I’m fairly sure there will be a way to put the air control membrane above the joist. Whether it can be below the sub floor idk. That way you won’t need to do the endless detailing around the noggins. Just roll it out, staple it in.

I would ask a chippy what to do with the sub floor if you’re reusing it. You don’t want squeaking floors after all that. Blocking to support the butt ends, lots of the correct screw and lots of glue would be my guess.

Whether you use loft insulation or choose another type you ought to have an airtight membrane over the insulation. This will prevent moisture permeating the insulation and condensing on the cold side. The timbers too will be cooler than they were and with less air movement around them there is higher risk of condensation and mould.

It’s nice to have, but I don’t think it’s necessary here. It would be really hard to do anyway. Again, the insulation manufacturer will have this as part of the specifications.

My understanding is that it’s convective loops which will be your problem. Which is why the insulation needs to be tight fitting against the joist and up against the sub floor. Once air starts moving around it can reduce the thermal resistance by half.

As long as you don’t block the vents there’ll be plenty of airflow to allow any condensation to dry back into the air.

It’s a shame it’s difficult to insulate on the underside of the joist to reduce thermal bridging.

I think the trickiest part of this job is getting good adhesion from the membrane onto the brick without ruining too much of your plasterboard. Safest is to parge coat it flat and keep it dust free, use tape and then mechanically fasten it to the wall. But I don’t know enough about how to do that well and productively!