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Airtightness approach for suspended floor PIR insulation?

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Cheapest breathable membrane I can find to go over the joists and hold the mineral wool in place is https://www.fixdirect.co.uk/product/breathable-membrane-100gsm/ at £35.50 for 50m.


Will that do the job or at 100gsm tear too easily? They also do 140gsm which is still a hell of a lot cheaper than Pro Clima Solitex Plus (170gsm).

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Posted (edited)

A few bits which i'm not quite sure I'd agree with here...


Marmox is in no way strong enough to go on top of the joists and under the chipboard/OSB layer, it would get crushed in the areas of contact, once a lot of weight is put on the floor. You would have to put this layer on top of chipboard/OSB layer and either top with 10mm self leveller compound if you are to use carpet, or by using a wood flooring/engineered wood/LVT/Laminate, and use a thin 2mm EPS under the solid flooring to prevent it touching the rough cement layer on the Marmox. I'm using Marmox under underfloor heating in bathroom, and me just kneeling on it has left a few dents, so it definitely needs something else to spread load.


On 09/05/2024 at 08:23, Sparrowhawk said:


I like this idea.


Is Marmox strong enough to replace chipboard in the floor build up, or needs that on top of it?


Longer term the floor level will be raised in this room by ~20mm; it's a half/half mix of old suspended wood floor and (we think) beam and block. Which is probably uninsulated. I thought lifting the floorboards would show me a cross section of the beam and block but they whacked 2 joists across the join so going to have to try harder to determine its make-up and if there's any insulation in it.


No, do not use Marmox for this purpose, it is only XPS insulation sandwiched between two 1mm reinforced cement layers, it is not designed for point loads such as sitting on joists, and needs to be fully supported.


On 11/05/2024 at 11:42, Sparrowhawk said:

Where this laps up the wall and is taped, you can't plaster onto polythene right? So it needs to be a small lap and plaster onto the Tescon Vana tape - or the wall replastered first and then this has to be done after?


You need to use specific tape which is 'plaster-in', such as https://www.airstop.co.uk/product-page/airstop-flex-fee You will need a primer to put on the block/brick work first, and then tape your membrane down with this style tape. You are then ready to plaster in. Most primers are paint on, however I have just discovered Tescon's SPrimer, which is spray primer, though i've yet to use it so can't comment on how good/bad it is.

Edited by MikeGrahamT21
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Sorry I've been skimming this thread, dropping in and out. 


Missed the bit about going back to mineral wool. 


Pragmatic decision I say. 


16 hours ago, Sparrowhawk said:

the timbers should be fine, it's whether to make the glass mineral wool able to breathe.


Sound like Frankenstein's insulation monster. 


Throw "breathing" out the window I say. Replace it with materials of differing "vapour permanbility". It more accurately reflects reality.


Some backyard experiments may help you to get a feeling for this. Get some leaves/grass clippings and sandwich them between two layers of your various materials and tape the edges. Put them in a dry environment and wait. 


Anything that looses it's moisture means the material has an appropriate level of permanbility. Anything that doesn't, eg plastic DPM will see the organic material rot and go manky. AVOID 2 LAYERS OF THIS TYPE OF LOW PERMANBILITY MATERIAL AT ALL COSTS!!! Even right next to each other as moisture will find it's way in there and be unable to get out. 


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Posted (edited)

TBH I think going with the very first pic from environmental building supplies is the one to peg for, done well it'll be absolutely fine. A U value of 0.35 is still 3 times better than the very best 3g windows once installed so pick your battles. 




Material wise....




I really like Glidevales VP400 breather membrane for below the insulation. £140 from Screwfix. In any case get something with a good g/M2 figure (>140g/m²), good tear resistance (>200N)  and a low SD value ( <0.03m)




It is airtight so you could use it above too rather than buying a separate airtight membrane. It would be a good choice given its pretty tough.




If you do want a separate airtight membrane look for a mass of >100g/m² and a tear value of >200N. I like the semi transparent ones as you can see the studs + insulation etc behind it. Something like Powerlon VCL170 can be bough for about £1.15 per M2 and will work every bit as well as in this application as Proclima DA at £2.66/M2. 




As for insulation. As well as the K value look for the density of mineral wool. 10kg/M3 is much harder to cut and fit accurately

than 35kg/M3.  






Edited by Iceverge
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8 hours ago, Sparrowhawk said:

Which side of Glidevale VP400 lets moisture out - the printed side? Or it doesn't matter which way up it's installed?

always printed side to where you want the moisture to go, think of how it’s used on a roof

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