For those considering whether to insulate a suspended timber floor, this article describes our experiences installing PIR thermal insulation board from underneath the floor using the crawl space. For details on insulating from above the floor see Insulating A Suspended Timber Floor From Above Floor.
We chose to insulate from underneath the floor as pulling floorboards up was not an option; we either had vinyl flooring (posh lino) laid over floorboards or original 1930s oak flooring.
Insulating from below involves crawling in the cavity under the suspended floor. You obviously need enough crawl room to work; my experience is that you need a minimum of 5 and ideally 6 bricks below the level of the joists (38-45cm or 15″-18″ of space). It is possible to work in smaller gaps but it becomes very difficult and I would only consider it for small areas of floor. Whilst working under a floor can be quite claustrophobic it can also be quite relaxing as well; I guess that’s why some people enjoy potholing as a hobby!
You also need easy access to the crawl space.We had already installed an access hatch in our lounge which could easily be lifted using a screwdriver to lever it up. Below the hatch was a removable section of PIR thermal insulation board.
The hatch size was one joist width by about 6 floorboards wide (about 650mm). 650mm is more than generous and in fact we could have probably reduced this to about 450mm and still had easy access for people and materials.
From under the lounge most other areas of the ground floor could be accessed through holes in sleeper and load bearing walls. These holes had been cut some decades before, presumably to allow the installation of central heating pipes; more recently they had been used to rewire the house and to install a newer central heating system.
For safety reasons this work really needs two people to be present at all times; if you did have an accident underneath the floor then it is essential that someone else knows you are there. They don’t actually have to do any work (although it would certainly help) but should be on hand to call for help if necessary and at least make a brew from time to time for the serving worker. You are likely to find gas pipes, electricity cables and central heating pipes under the floor area so do consider what risks these could create.
The work area was lit with an inspection light, fed from an extension reel plugged in “up top”. This light was topped up with a hand held battery operated LED inspection light which provided a bit more light on a specific area.
It’s dusty down in the cavity, particularly if you trim any PIR thermal insulation board down there. Overalls came in handy and some kind of dust mask was essential. The disposable dust mask worked well but with hindsight I would have invested in a full face dust mask which covered the eyes as well. Lying upside down there was a high likelihood of getting things falling in your eyes; I wear glasses so it is less of a problem but a full face mask would have been better.
The ground under the floor can be quite bumpy – it may be a soil or a concrete base – so it is worth having some form of foam pad to lie on when working (especially if you encounter pieces of discarded wires and pipes).
Surveying The Area
It can be quite wearing to move around a lot under the floor and some work is best done above the floor so it is wise to plan an effective way of working.
I decided to do the entire measuring task first as it also gave me the opportunity to assess what I needed and how easy specific areas of the floor would be to access and insulate. At the same time I could assess where different thicknesses of insulation were required, where air bricks needed clearing and how I was going to handle cables and pipes from existing services.
Measuring was done with me underground with a tape measure and with an assistant armed with pen and paper above. Each area for insulation was given a letter and a joist number so I knew where I was measuring. So the first gap between the joists in the kitchen was location K1, the next K2 etc. The key dimension to measure is the gap between the joists which is hopefully consistent for each two joists. In my case there were no perfectly parallel joists so I measured 3 or 4 points and recorded the maximum and minimum over a 1.2m length (the length of the rigid insulation I was using). So K1 was 36-37cm width and 260cm length. The length of insulation was generally the same for most joists in an area.
All the measurements were yelled through the floorboards to the assistant above along with any other observations on work that needed doing.
Installing The Insulation
Above ground with the measurements the work to cut insulation boards could start. I was using 2.4m x 1.2m PIR thermal insulation boards which are faced with aluminium foil. For ease of working I cut 1.2m lengths of various widths out of the board. The cut insulation needs to be manageable in a tight space and a larger size than this would be difficult to get through the twists and turns in the sub floor area.
I chose to cut boards for a whole area (say the kitchen) and then install them as I was working on my own. I cut the maximum width, which in the case of our K1 example was 37cm and labelled the first 1.2m of this K1a, the next K1b and so on until I had sufficient length to fill the whole length between the joists. When I had about a dozen cut pieces it was time to head underground.
The tools and fixings I took underground were:
- dust mask
- floorboard saw to make final adjustments to the insulation width
- cordless screwdriver
- tape measure
- 3.5 x 30mm screws
- 100mm length of 10 x 45 batten
- Aluminium tape
- fire rated foam filler 700ml – for filling any gaps in insulation
- plastic rubbish bag – to collect offcuts of insulation
- angle grinder with masonry cutting disc – used to cut access hole in one sleeper wall and to smooth off the rough edges of another sleeper wall so I could squeeze past more easily.
Moving the labelled pieces of insulation is not without its challenges, especially if you are working 5 or 6 metres from the access hatch. You have to decide whether you are going to push them in front of you as you move or pull them along after you. I used both methods; it was a balance between storing the insulation so the pieces were easily accessible when working but also not cramping the work area.
Be careful when using foam filler. It is a nightmare when wet if you get it on your hands or body (easy to do when you are filling above and crawling on your back). The cans are under pressure and can easily be punctured; I put a can down a little carelessly and it hit a sharp brick and punctured; it proceeded to empty itself completely which was a little disconcerting as well as a few quid wasted. If you do get any on you or anything else then let it set; it is far easier to remove when it is fully expanded and set.
When arriving at the location of the next piece to install it was a case of identifying the correctly labelled piece and then rolling on my back to try and fit the board between the joists. As most of the joists were not parallel to each other it was a case of visually assessing how much material to remove to allow it to fit between the joists. This usually involved removing material from one end of the insulation. There were two methods I used for this:
- where there was significant material (more than 10mm) to remove then I would cut this with a saw. This involved rolling onto my front and then finding a suitable place to allow the board to be cut without elbows hitting joists or walls.
- where there was less than 10mm then I tended to use the saw to stroke down the full side of the material, wearing it away to the right size. The benefit of this method is that you could do it lying on your back with repeated re-tries to fit the insulation until it was a good fit. The downside is that the under floor area became very dusty which is why a mask which protects mouth, nose and eyes is essential.
Once the board was the right size it could be gently tapped into position by hitting each side of the insulation until the bottom of the insulation was flush with the joist.
When a few boards had been installed then Aluminium tape was used to seal any joints between boards. I also used tape between the joist and the board so there was no air flow possible. The 100mm battens were then screwed into the 50mm wide joists at right angle to them to ensure no movement of the insulation was possible. For each 1.2m length of insulation I would use 3 battens per side; supporting the middle and both ends.
Small gaps were filled with fire rated foam. These occurred either due to inaccuracy in my cutting or where two joists were butted up against each other and there was no room for insulation.
Improving the under floor ventilation
The under floor area relies on good ventilation from air bricks and a good movement of air. Where we encountered sleeper walls then we switched to 50mm thick insulation to allow air to still flow over the wall between the joists. This gap was in addition to honeycomb gaps left in the walls and the cut access holes. Had we used 100mm insulation then all the air flow would have been blocked. The 50mm insulation was butted up to the floorboards with battens screwed to the inside of the joists.
Insulating central heating pipes
After insulating the floor there should be no air flow between the warm room and the cold under floor area. So central heating pipes are at greater risk of freezing and should be insulated with pipe insulation (with all joints taped).
The pipes should have been already insulated when they were originally installed but this is not always the case and it will also help your gas bills if you are not heating the sub floor with your hot heating pipes.
The end result was we had a suspended timber floor with no draughts and which did not feel cold. We saw a big fall in our overall gas bill (see Ten Steps To Saving £1,000 On Our Gas Bill) as well as having a nice warm and comfortable house on a winter’s day.
I would certainly use this method again if we ever moved into an old house with suspended timber floors and overall thought it was a better and less disruptive approach than taking the floorboards up to insulate. Having two people, one above ground and one below, would have also made the task a lot easier.