We did one room last winter as a proof of concept with considerable improvement. Checked in spring and like new, just finishing the rest of the house.

nteresting and super detailed - thank you. One point - you refer to seeking to avoid condensation. My thinking is that the main concern is not condensation but sustained elevated Equilibrium Relative Humidity (ERH) values, which can lead to sustained high levels of Equilibrium Moisture Content (EMC) in hygroscopic materials, such as joists. In other words, problems can arise even when the temperature remains above the dew point, and even if, therefore, condensation does not occur. With the reduction of heat leakage into the subfloor, the average temperature will now be lower and so the RH will be slightly higher, on average. The vapor control layer will prevent water vapor from moving into the sub-floor environment from the occupied spaces (it will prevent this moisture moving to the cold side of the insulation). However, there are other sources of moisture, especially from a slab laid on soil, or from bare soil, which could lead, with the now lower temperatures, to sustained elevated RH values. If these exceed well over 85% RH then the EMC of the joists could remain elevated well above 20%, to a level sufficient to promote fungi growth. It would be interesting to monitor the moisture levels of the joists where this system is installed with soil or slab-on-soil subfloor. It would take many years for rot to arise and cause structural damage. Another way this could be predicted would be if the RH was monitored within the sub-floor space perhaps by a data logger (or IoT RH sensor) put into the void through an air brick. Just a thought...

What a fantastic video, with the clarity of your explanations and comprehensive guidance and the way you’ve addressed all the queries you have inspired faith that this is the way to proceed, I’m looking forward to the project, thanks

Temp profile last winter showed suspended floor was 8 degree Celsius below room temp. So about to start two floors so video gave excellent instruction on build up. Also reminded me I forgot to order pipe grommets for rad pipes, oops! Will let you know how I get on.

Thank you for this video and the guide on your website, from much research this is the method we are going to go for when we replace our floors in the coming year

Ordered all the materials after seeing the video! Would be interested in what's the best approach when dealing with vents that go near the joists.

that's what in US is called a crawl space. Another solution is being suggested to incapsulate the crawl space with a membrane to prevent moisture from the ground and more importantly the radon gas reaching the living space. The wall of the crawl space are insulated, as well as, in cold climates, the floor joints, just like in this video. Any moisture is also removed with a dehumidifier.

I am assuming we can install under floor heating over this form of insulation. We are hoping for UFH between joists to keep floor height low so I’m assuming we therefore just start the insulation level a little lower to leave space for UFH

Great video can u use breathable roof felt underneath insulation?

This is fantastic. Can you do one for method 2? We went the original boards and its freezing. I'd like to tackle it from the cellar

Great video and solution! Could this system be further enhanced by adding an additional layer of wood fiber insulation with screw strips, such as Gutex Thermosafe NF 40mm, on top of the inner floorboards or plywood? Where would the airtight vapor control layer need to be placed?

If the underfloor space is unheated, should there not be a Thermal Break somewhere? Great video. Thanks for posting.

Awesome video. Can we apply it to our suspended floor (ground level) house in Melbourne, Australia?

Great video. It mentions at the start of the video that heating pipes under the floor should be lagged - therefore shouldn't the pipe in this scenario need lagging? When the floor boards are then re-laid, how are the membranes prevented from being penetrated by the fixings?

great explication. thanks is the crawl space ventilated in winter as well?

I'm thinking of insulating from below, without removing floorboards. The method (2) you describe involves fixing strips of vapour permeable membrane under the floorboards and sealing them to the sides of the joists with Orcon F. Would it be wrong to instead run a continuous, wider length of the Proclima DA membrane across the underside of adjacent joists (and up against the floorboards), effectively placing the joists on the internal side of a continuous vapour permeable membrane, together with the floorboards (then installing the bulk insulation, then the Solitex Plus)? It just seems technically simpler, but I wouldn't want to compromise the joists with excessive moisture, for example.

Fantastic video, thank you very much for sharing! I will be sharing with my own network. Is doing this DIY the only option really? Do you have(or are you building) a competent installer network that has been trained to install a floor retrofit in this manner using these products? Thanks again.

Is there any issue with nails and screws from fixing the floorboards going through the vapour barrier?

Assuming that is a roofing membrane (blue), shouldn't the membrane be installed (flipped over) (patterned side facing external)?

Can you do this insulation with kooltherm k103 floorboard? Does it do the same job as this wool insulation?

How similar / disimilar are the membranes from those used in roofing? And then, in theory, could you apply a similar system between roof rafters for further house insulation? (Currently in the process of trying a renovate and sensibly modernise a cold 1940s house, thank in advance for any advice!)

Unfortunately none of the local builder where I live (South East London) provide underfloor insulation. Some provide spray foam insulation. Are you able to put me in touch with someone who provides underfloor insulation using the same method as the video?

Thankyou. Great explanation. Can you use the breather membrane as described in your video and PIR board between these joists sit on the breather? The reason is to allow underfloor heating pipes to clip onto the pir board.

A little confused when you described the Solitex Plus to be a "breather membrane" that is also "airtight". 🤔 Can you tell me if is it Breathable?.... Or Airtight? aka a "Vapor Barrier". Cheers

What’s the u value of this floor buildup assuming 100mm joists?

If not fully taking the floor apart is it ok to work from the bottom space, reaching up tucking insulation in fresh where it may need it then attaching a permeable layer to the underside of the joists?

Would it be possible to staple the permeable membrane to the underside of the joist . It would use less material but would enclose the joist. Is that bad? Would it reduce air flow around the joist and therefore result in rot? Or would it help maintain the temp of the joist in a dryer environment?

Surely a plastic DPM can be used instead of the green membrane or is this a big no? Thanks

Does adding a vapour barrier on top hinder the ability of an older property to release internal moisture and encourage damp/mould issues. Can you add a second wind tight breather membrane instead of the vapour barrier to encourage less internal moisture build up?

I like the concept and thought of it myself, but there is controversy over a water tight membrane next to the subfloor acting as a means to hold water or condensation here in the USA ( location North vs South ). Have you experienced any issues due occupants creating a source

What other types of insulation can you use? Can you use Steico Flex 036 wood batts?

Great detail, unfortunately the only item not addressed was the thermal losses through the floor joists. This can be done using a thin layer of cork board or wood fibre board before the floor finishes. It dose mean doors will need to be altered nut no point in doing all this and falling at the end.

What sort of u value does this achieve?

Can I use tescon no1 tape instead of tescon vana tape for sealing membrane overlaps?

Our 20th century meddling/improvements to our Victorian houses led to unintended problems.. and still do.. Back then when coal was king (and cheap) we actually need air coming through the carpets to aid combustion.. We made the best with what was available.. When gas was introduced I believe it became mandatory to have a wall vent. The Victorians used to put newspaper under their carpets or horsehair underlay.. They had lots of horses then.. It seems every material has its benefits and down sides.. Perhaps rodents like Hemp.. Eating.. not smoking.. who knows.. I would have thought a MultiFoil type would have been easier to install and less absorbant..

4.18 6 materials in shot, and the flooring 2 or 3 just out of shot = this is another part of industry thats needs to get its act together and innovate 'an engineered lay 1 item on the foundations' way of building.