Suspended timber underfloor spray insulation?

I live in a bungalow with a suspended timber floor and a crawl space varying from 60 - 90 cms. We’re on the edge of the Pennines and are exposed to Westerly winds and it is quite noticeable how much cooler the house is on windy days and how much more energy is needed to heat the house. While there maybe other contributory factors to heat loss, I think a large part is due to air movement under the floor and leakage at the floor/wall interface.

The joists run across the width of the bungalow and there are two sleeper walls. Insulating from above is not possible because of hardwood floors, tiles or fitted furniture in practically every room. I’ve looked at the Green Building Store solution of Solitex Fronta WA membrane and insulation applied from underneath, but would think that it would be extremely difficult to work in the crawl space and achieve a satisfactory installation with the necessary sealing to the perimeter walls and the sleeper walls.

An easier solution would seem to be spray foam applied by robot, such as the q-bot solution. Has anyone any thoughts on using a sprayed foam underfloor insulation? My main concern is how it would affect existing underfloor plumbing/wiring and also how difficult it would make future plumbing modifications for, say, the installation of an ASHP?

I have encountered Qbot myself and even contacted them about my house. They declined that job as there was some insulation already in place and they only spray to the wood and masonry.
I liked that all gaps are sealed, making the ground floor airtight.

The spray is marginally vapour open but certainly not free draining. For reasons of damp ingress they don’t spray from wall to wall below the DPC but will give depth under the timbers tapering to just above the DPC. I was told that if circumstances permit they can do a second pass with the robot after the first has set to give insulation to EnerPHit standard.

Ensure that the joists are insulated as well as the floorboards or they will become the cold spots that attract condensation and thereby mould and rot.

Regarding underfloor services, they effectively become fossilised. No change is possible without destroying the insulation. Any unused services such a gas pipes after electrification would have to remain in place and become useless thermal bridges. If you are rewiring/plumbing the old wires/pipes will have to be cut off and any new ones would need to be above floor level as with a solid floor. Block the ends of any pipes with spray foam. In the case of gas pipes allow free flow for several days before blocking.

Sofie Pelsmaker did a couple of papers from her thesis on filling the sub-floor void with polystyrene beads. It would be good to know if this idea was ever implemented and how it has performed since. This is potentially a quick and cheap solution given the low material composition and high content of trapped air. Well worth looking into with the floor dimensions mentioned.

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This might be a completely useless suggestion , but how about insulating from above and making sure of airtightness with membrane layers.
I might be missing a trick or two because I’m not tech minded, yet I know above the floor is currently more accessible for us in the short term for our main living room.
Draught proofing at skirting boards, breathable layer on top of floor boards then some sheepwool underlay(10mm thick) and wool carpet. My thinking is at least I can roll this up again for reuse when I’m ready to get under the floor boards to insulate.
Perhaps this might be an option for you in preventing those westerly winds coming up through the floor.

I have wondered about having old gas piping removed from our subfloor.
Indeed could it become piping for other types of who knows what in the future :thinking:

Air sealing from underneath is difficult. I’m having similar problems insulating from above to be honest. Spray foam is often the only choice because the labour costs of other strategies are just too high. If anyone has seen good attempts at creating a good continuous air control layer between sleeper walls and joist from beneath, I’d love to see them.

However, in the medium term, it’s my opinion that you can justify replacing the whole floor system, since it allows the floor to be lowered to allow for a hydronic underfloor heating system. This needn’t be for the whole house.

A 65mm build up, from the sub-floor to finished surface, is recommended for this. Underfloor heating is valuable for utilizing low temperature heat to achieve high COPs, and for general comfort levels. There may also be a problem reaching peak heating power output without underfloor heating - especially if the rest of the envelope is not fully retrofitted as part of a longer term phased development.

Furthermore, where are your ducts going? Uncontrolled air exchange across the enclosure dominates thermal losses and comfort complaints. Tightening up the building for high indoor air quality and general efficiency requires mechanical ventilation, often from a central MHRV.

Without an intermediate floor system, attic or crawlspace duct runs are often the only way to get horizontal distribution. Which one will you choose to condition? Because it is just a bad idea to have ducts passing through unconditioned spaces.

The roof requires much more insulation to reach the same levels of performance as the crawl space, because of the larger surface area of the pitched roof and due to radiative cooling at night. However, conditioning the roof is easier to access and has the additional benefit of not needing to air seal the attic ceiling - which is a significant challenge by itself.

However, replacing the base floor system for underfloor heating gives you access to the ground to add the drained granular pad, insulation, membrane and thin screed, all to create what is essentially a mini basement. Which will allow you to run mechanical systems under the floor, including ducts, and you don’t need to worry about insulating between the joists (infact, it’s better for the underfloor heating). This need not be for the whole floor area, but could be applied to a few commonly used rooms, such a living room/dining/kitchen.

In bungalows, due to poor form factors/ SV ratios, it’s either: hydronic under floor, ducted heating with a central return, or actively recirculating wall mounted radiators, which are needed to get good COPs and power output from your heating system. Unless you manage to go all out and get passive house levels of performance out of your building enclosure to reduce demand so much it doesn’t matter - replacing windows, doors, avg 200 mm insulation <1ACH etc.

These are not easy decisions to make at the moment. However, once building surveyors and mortgage providers start taking this seriously, and if very low interest, if not subsidized, loans are made available, this will be the norm. I honestly think we need way more publicly backed financial options to support doing these things properly.

Not sure if this helps either!

I have some old cast iron gas pipes under my floor and 2 copper ones, one of which is in use. When I had underfloor insulation fitted in 2005 I asked the builder to remove the iron ones to improve the laying of the insulation. He refused as the house is still on the gas grid. I now know that the insulation is poorly fitted and too thin so I have the job of redoing it.

As I mentioned, q-bot refused the job as my thin insulation gets in the way.

I would replace the existing floor structure with a tanking layer, insulation, screed and wet underfloor heating. All other plumbing would be removed and wiring rerouted to accessible conduit around the perimeter.

I have room for 300mm of insulation, so the UFH should only heat upwards and the floor would contribute to an EnerPHit like standard.
Compared with the cost of moving everything on the ground floor into storage and rebuilding the floor the cost of extra insulation is negligible. There would be a slight saving in other materials.

There are a few lucky people who can insulate from above and it is relatively common with solid floors. Your doors will obviously need cutting back to avoid snagging in the new floor. If done as a more than short term solution the doors might need to be raised and even the stairs.

Thanks for comments so far. I can’t see another way of achieving underfloor insulation without major above floor disruption. Q-Bot surveyor coming tomorrow, so I’ll see what they say.

I went under the floor for the first time in about 7 years to remind myself of where crawl holes had been knocked through. It’s further complicated by one cross-wall which goes down to foundations, effectively splitting the underfloor void into 8 areas. Fortunately, there are 3 crawl holes already but I’ll either need more, or will have to have lift multiple floorboards to create further access from above.

If additional ventilation is required that can be provided from above as, being a bungalow, the roof space is accessible.


Spraying under the floor and sealing any chimneys should get rid of most of your airtightness issues. Some additional work would be needed if you were going for ultra efficiency/certification. In any case I would get two quotes from Qbot, one for a normal job and the other for a double application. Hopefully there is enough difference between floor level and DPC to fit two spray applications.

You are getting into territory that the current EPC system is unable to evaluate, so don’t get upset if a future EPC doesn’t reflect the true energy efficiency of your home.

I had looked into qbot for our floor, the combination of draught proofing and insulation in an easy application sounded amazing. However, I got drawn down an internet rabbit hole of horror stories about spray foam applications in roof spaces. There are loads of stories out there (I don’t know how reliable/ sensationalised they are) of spray foam applications in the roof causing problems for people selling their house and mortgages being refused. Seems that there are concerns about damp in the roof timbers. Are the same concerns there for application on a suspended floor? I’ve not seen the same stories about problems with mortgage companies but it made me nervous of future issues arising.

One reason there aren’t stories about underfloor insulation is that there are far fewer installations and the mortgage companies are less aware. However there is a fundamental difference. The roof insulation is enclosed and restricts ventilation whereas the underfloor is open to the normal ventilation below. I would suggest checking that the underfloor ventilation is adequate to current regulations and adding airbricks if required. Maybe even exceed regulations if you live in a humid area.

It is this constant external air contact that encouraged me above to suggest two coats of insulation.

Any update or comments?

Two smart young chaps in a Q-bot logo van and clothing spent about 1.5 hours with a tablet doing the survey. The tablet app was obviously designed to make sure they didn’t miss anything and they took photos of everything such as extractor fans, gaps under doors, heating appliances, floor coverings and, of course, the underfloor void. They also measured and did a plan of the house from above .

I had lifted carpets and floorboards in two locations giving them access (theoretically) to 5 of my 8 voids however, they didn’t actually go under the floor, just took moisture readings of the joists and looked through the holes with a torch. They did not think there would be any problem applying the insulation as my lowest void depth is ~300mm and the Q-bot can go down to 200mm. They did say they might have to do a manual spray at the back of the house where it is ~500mm as the robot doesn’t work so well in deep voids. I will, of course have to provide further access hatches or crawl holes to the other 3 voids.

Apparently, all the gathered information goes to head office and I should get formal quote within a week. Expected installation time: 2 days. It is recommended, in the case of a bungalow, that you vacate the property during the installation although the fumes should dissipate with 24 hours. For a house they just suggest you stay upstairs.

I don’t have any concerns about the process. To quote UKGBC: "Q-Bot is the only BBA Certified installer of underfloor insulation, being PAS accredited and TrustMark registered, and qualify for a 25-year guarantee for installs. Q-Bot’s UFI measure has also been awarded the Queen’s Award for Innovation and been independently verified by the Energy Savings Trust (EST). "

The price is another matter! I don’t expect reduced energy costs will cover the installation cost within my lifetime, but I do hope to reduce my carbon emissions and potentially be closer to being able to move from gas boiler to an ASHP.

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Legend of the US building science community and founding director of the Building Science Corporation, Joe Lstiburek, says ‘don’t insulate the floor; seal the perimeter and insulate it (externally, probably, in an existing building, but the underfloor space can also be internally insulated) down to the foundations, and thereby bring the underfloor space into the thermal envelope of the house. The UK Green Building Forum has lots on this, championed especially by Tom Foster (FosterTom) there. Over several seasons, the theory goes, a column of warm subsoil (and rock if present) builds up, extending from the fairly constant 10 degrees or so temperature at around 10’ down, up to your floors. It is difficult to model I gather, but where done it seems to have worked well, and is the approach of which I am taking the first steps. Some have insulated an area extending outward from their external walls, near-horizontally but with a slope to shed the rain, with XPS or similar, a foot or so underground I believe, to create a larger warm column than the footprint of the building, as an optional extra. There is a huge resource of expertise on the UK Green Building forum, on all topics discussed here anyway, including this, so well worth a few hours study.

I get the idea, but not sure about the implementation? If you’re going to bring your underfloor space into the thermal envelope of the house then you have to block up all your air bricks. And, unless you completely seal the walls and the concrete sub-floor below DPC, you would be creating a warm damp space under the floor which would invite wood rot. Or am I missing something?

I’ll try and find some discussion of this topic…

I have seen a short video series from New England about underfloor insulation and ventilation. The area is far hotter and more humid than anywhere in the UK during the summer so may not be entirely correct for our climate but there were basically two ideal solutions, depending on the structure of the house.
Either incorporate the sub floor into the thermal and ventilation envelope or very thoroughly insulate under the floor and open the subfloor space to the elements. This is the most common solution given that most of the housed were timber grime and panels could be replaced with trellis to keep out larger animals. Hence my comments earlier about ensuring there are enough airbricks.

The AECB preference is to do as I am planning, to replace the suspended floor with water proofing, insulation and screed.

Yes, Mike, you seal the perimeter, as I mentioned, so closing off all the air bricks (as long as air is not needed for other purposes such as boilers, stoves or fires), using a vapour barrier such as 250 micron polythene or thicker. White polythene is recommended, taped at the joints. Not a reinforced membrane of any sort, as these have been known to produce unwanted smells, in time. White poly keeps it nice and well lit down there. (I didn’t see this suggestion until after I had used black, unfortunately). I can’t personally vouch for the results yet as I am only part way through. I have first taped the wall-floor junction with Compego tape and then applied a parge coat of cement mortar to the exterior underfloor walls and over the Compego, for airtightness. To avoid bridging the wall’s DPM I first taped a 1" extension piece to the DPM, with Compego tape above and below, which happily takes the parge coat over it and separates the parge coat layer above and below the DPM, avoiding any bridging. I have added 100 mm of mineral wool insulation inside/above the polythene around the perimeter, extending for a metre or so inwards to insulate against the cold-in-winter subsoil outside. I am planning to install a few temperature and humidity sensors to monitor conditions down there, and if necessary I will cut a hole in the floor and install a small fan from an old desktop computer CPU to blow a bit of conditioned air down from the sitting room above. I will probably screw the tongue and groove floorboards back down rather than nailing them, so I can get access if necessary again in the future. I haven’t considered what you would have to do if your floor joists are inserted into the walls; mine were sitting on small, loose wooden blocks, sitting in turn on blue bricks cantilevered out from the walls, so I just levered each joist end up and pulled the polythene under each, between the wooden blocks and the bricks. All very inexpensive so far, and with the intention of insulating externally down to the foundations before too long, probably with a ‘french drain’ of Leca (DIY and relatively inexpensive).

Lstiburek recommends this for all the climate zones of the US, including the pacific north-west (Washington state), which has pretty much an identical climate to ours. For new-builds in that area suspended timber floors also work well I gather, but for existing buildings like ours, and where floor insulation is non-existent, the perimeter sealing and insulating is more cost-effective and rewarding it seems.