- Underfloor insulation
Been giving this some thought.
We are about to have some building work done on the house and asked for a quote. It came back as being £2000k + VAT for the floor so we probably won’t be going ahead.
It is for the Kitchen (6mx4m) Dining room (5mx5m) and the Hall (1mx6m) with 100mm Kingspan. Lots of space underneath, 4ft in places. Don’t want the floor lifted as many of the boards won’t survive and it would generally be an absolute pain. Victorian floor so the joists are not evenly spaced and each piece wuld need to be cut to size specificially. It is Oine floor boards stripped back and varnished. Underneath is crawl space then bare earth
I was wondering if anyone had done this themselves cheaply. What did you use, how much did it cost and how long did it take? (Area done might also be useful)
Using glass wool underneath would be cheap but grim. Is there something similar I could use that is easier and safer. I’ve seen the space blanket and thinner encapsulated insulation blankets in DIY places but do they actually work? If we could use those then It seems it would be a simple case of going down there with a torch, roll of insulation, scissors and staple gun.
Floor itself isn’t massively cold but it is draughty. So would some intensive filling and draught excluding be just as good. I know that filling floorboard gaps is abit like painting the Forth Road Bridge so will need to be maintained but it does seem cheaper and easier.
Thoughts, advice or offers to do it for <£500 appreciated.Posted 5 years agostumpy01Member
Could you not staple/pin some kind of material to the underside of the boards to reduce the draughts coming up? Guessing you wouldn’t want to stop airflow completely with a plastic membrane as you may get damp/condensation issues?Posted 5 years ago
You could then as you mention fit insulation beneath this material to insulate further? You can get that insulation wrapped in silver foil that is very easy to handle.
Rockwool is going to be best fixed in place with net or chicken wire and super cheap too. Just get a paper suit, mask and goggles. An insulation that allows passage of moisture is going to be a better bet as there is far less chance of moisture build up and rotting. Glueing insulation board to the underside of the floor boards sounds like a great way of causing a damp problem.Posted 5 years agotrail_ratMember
ajc – what about creation of a dew point and the rock wool ?
my sanitary space dirt is not wet as such – but if you sat on it youd get a damp patch on your trews – due to to natural water levels around – field across the road same level floods …
Im a bit weary of sticking rockwool up in a net (despite having all the kit ready to do it) for fear of it just getting damp , rotting and smellingPosted 5 years ago
The rockwool shouldn’t get damp and smell but it is important that there is an airflow under your building. If you have a breather membrane as in the article linked above it will prevent any draughts coming in to the building. You can then have a massive air flow under the flooring if the dirt is very damp and it won’t cause a draught in the room above. Rockwool will help cut out most draughts but will not make the building air tight. As in the article there is difference between draughts and air tightness.Posted 5 years agotrail_ratMember
we have plenty airflow underneeth and no drafts above – its a sectional concrete reinforced floor which has also got spaceblanket insulation over the top.
but you reckon im still better with rockwool than insulation board then ….. ill do some more reading but thats good news since the garage allready has the stuff i need to do it in it :dPosted 5 years agothe00Member
I had wet rot and woodworm in the floor at the bay window. I thought it would just be a couple of floorboards to lift, but chasing the rot back I ended up lifting the whole floor. This meant I could do a proper job of the floor, and I cut insulation board held on battens to lay between the joists.
I have some left over insulation board, but don’t much fancy doing another room without lifting the whole floor. Once you get a few boards up you’ll be surprised how easy it is to get the whole floor up.Posted 5 years ago
taking the floor up is no issue, down again is what concerns me bit like stripping bike components. The area is large, the floor boards (115y) old and nailed down.
I wasn’t going to use a vapour barrier. I figured that simple insulation would prevent the air blowing through to a large extent coupled with me sealing some of the holes from above.
I was also concerned with impermeable insulation as to what happens if you spill water. It will pool and not drain away?
I’ve seen recycled polyester insulation that can be stapled up – similar to rockwool and glass fibres but without the itchiness and unpleasant ness when working in confined space.
The stopgap thing looks good as does draughtex but the demo pictures seem to be nice neat floor boards, no pre existing filling, damage, cracks, uneven ness, prelifted and cut boards, wonky edges. The floor boards have character and are definately not pristine so solutions like that become really fiddly.Posted 5 years agomk1fanMember
Rigid PIR insulation (such as Celotex or Kingspan) is the best for a suspended timber floor. 100mm should be sufficient. The trickiest bits will be the gap between the outer walls and the first joists. I would suggest packing mineral wool insulation into these areas.
If you have an accessible crawl space then fitting the boards from underneath is sensible. Personally, I’d fit 140mm deep PIR seconds. Leave them flush with the underside of the joists. Tape over each joint with aluminium tape. The fit a timber cross batten to provide back up support. Boards should fit tightly between the joists.
Doing it this way will leave a gap above the insulation and below the floorboards to run services or enable ufh to be added at a later date.
Taping the joints is important to stop draughts and control airflow.
If you do lift the floorboards (at any point) I’d also tape up upper joints too.
Mineral wool would be easier and cheaper to do – as a homeowner you can still buy rolls at £10 a go in your local DIY store – trades pay £35 at buulder’s merchants – however, controling draughts is harder and the chance of animal infestation is high.Posted 5 years ago
For insulation of our suspended ground floor, I’ve used 100mm thick B&Q insulation roll (partially recycled plastic) held up with cheap garden netting and staples.
The crawl space is only ~knee height, so it is quite awkward to do and very dusty. I wear a head torch, overalls, goggles, dust mask, elbow and knee pads.
So far I’ve done the dining room (which had a fairly cold, laminated wood floor) and wrapped the under-floor central heating pipes.
The living room will be next. This is carpeted over thick underlay so the benefits may be less obvious, although it can occasionally feel cool underfoot.
however, controling draughts is harder and the chance of animal infestation is high.
I packed the insulation in quite tightly (plus there is foam under the laminate and the skirtings were sealed prior to the floor being laid).
There was no evidence of animals in the crawl space.Posted 5 years agomk1fanMember
I packed the insulation in quite tightly
So negating the majority of it’s insulative properties. That aside, leaving exposed mineral wool insulation provides excellent conditions for rodents to set up home. I’ve lost count of the number of times I’ve found rodent nests in mineral wool insulation during surveys and whilst doing works.Posted 5 years ago
I don’t mean that I squashed it flat. It was nice and ‘fluffed’.
100mm is the nominal thickness of the roll and also depth of the joists. The width of the insulation does provide a snug fit between the joists.
I agree about mice, but I couldn’t find/smell any signs of activity or chewing of the old pipe lagging.
Anyhow, I’m not claiming to have done an optimal job, but it’s not expensive and possibly preferable to measuring-up and cutting kingspan boards …and crawling back, forth and through the small access hatch repeatedly to do so.
It’s not a pleasant job, but doing an evening here and there is tolerable.Posted 5 years agoluketSubscriber
There was another thread on this recently. Might be worth a search.
In ajc’s link they quote
The temperature difference between an internal space to ground is significantly smaller than the temperature difference between the internal space to outside air.
While this is of course true, but how much depends on how deep and well ventilated the void is.. Well ventilated 3 foot cavities are going to enable quite a bit of heat transfer out . We did ours in similar circumstances using Rockwool Flexi, installed from beneath. Horrible job but made a massive difference. Some of the difference will be from the reduced draught as well as reducing conduction. Cost £1000 in materials alone for a quite large terrace house. I guess 5mx15m. £200 in labour plus maybe a day of my own time.
That was 3 or 4 years ago. Also worth noting that our house just had an EPC done on it and this made a difference. Again, I don’t know how much of a difference but the end result was certainly better than I’d expected.Posted 5 years agoRockape63Member
might sound a bit unusual, but a couple of years ago we had to insulate a huge partition wall separating a heated and non-heated area in a warehouse, to comply with regs. As we’d built the wall already, we needed a solution that avoided taking it apart and installing it.
I brought in a company who pumped it full of polystyrene balls about 1mm diam. I think it costs about £600 which was a bargain compared to the alternative.
It sound like that would work well under your floors.Posted 5 years agospooky_b329Member
I don’t think anyone is suggesting filling the whole void, but boxing in the joists and filling the cavity. Recycled newspaper stuff could also be an option.
Another possibility would be two pack foam you can get that is used to insulate warehouse roofs and (to the dismay of many) the inside of domestic roofs. You just warm the bottles up, take the hose and trigger under the floor and spray it on. Very quick and no worries about the overspray down there, the downsides are it will make a mess when the floorboards do need to be raised as it sets rigid and will be glued to the underside of the boards. It costs around £400 for a DIY kit that will 25sq meters (from memory), but the price per sq meter will come down with the larger sizes.
Rigid board such as Kingspan will be a nightmare especially if the joists are irregular, even more so if there isn’t enough room to cut it whilst under the floor. It will also create a fair amount of wastage in offcuts.Posted 5 years ago
Rigid board such as Kingspan will be a nightmare especially if the joists are irregular, even more so if there isn’t enough room to cut it whilst under the floor. It will also create a fair amount of wastage in offcuts.
Indeed. That was my thinking.
-Great in a nice cellar with plenty of headroom and perfectly aligned joists.
-Awful in an old house with a 50cm void, a dirt base and a tiny access hatch.Posted 5 years ago
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