I’m going to be sealing and sanding the kitchen floors this weekend and would like to insulate under them first. I have a 2′ (well ventilated) crawl space and can’t take up all the boards.

Looking for least painful way of doing it rather than most amazing thermally.

Plan – Space between joists is about 330mm so use Knauf mineral wools rolls which come in plastic sausage. It means not cutting of slabs while under the floor. Wedge them in and staple sides of plastic to joists. Could also staple some webbing under insulation so it can’t drop out.

https://www.wickes.co.uk/Knauf-Space-Blanket-Encapsulation-Loft-Roll-Insulation—200mm-x-370mm-x-4m/p/166858

Seems like a plan?

I did look at the foil blanket stuff, running it under the joists, but I’d have to deal with sealing it around the heating and water pipes.

I did it about 10 years ago under most of our house. Crawl space was smaller than yours and I used non-encapsulated roll so on the whole a fairly unpleasant job shuffling around on your back….. It was worth doing and solved some damp / mould issues under furniture as warm moist air was no longer hitting a cold surface inside the house.

As you said it has to be well ventilated or you just transfer the moisture problem under the floor. I held in place with stapled up netting from the local builders merchant (like cheap black garden netting). Also made / stapled together little trays from DPC where the insulation meets any walls / sub walls so it can’t track any moisture (probably less of an issue if your insulation is already enclosed).

I’m doing some heating pipe mods underneath at the moment and everything looks good (no moisture, mould, rot, mildew etc).

Horrible job, need to finish mine as well.
Problems I faced were joists not being uniformly spaced!
So the width was more than the width of the insulation roll.
I tried stuffing off cuts of insulation in to make up the space, it was a total hassle.
Ended up buying a roll of orange plastic barrier mesh and stapling that onto the joist once the insulation was in position.
I used the recycled plastic cup insulation from B&Q, less itchy than the glass wool stuff.

Good idea using barrier netting.

https://www.wickes.co.uk/Wickes-Barrier-Fencing-Orange—1m-x-50m/p/114496

It is a horrible, horrible, unpleasant, sometimes painful (depending on your sub-floor) job. It does make a difference though.

I did pretty much exactly what you described, plus a respirator and some mtb goggles for all the dusty, fibrous horrible-ness.

I actually forgot my stapler and stanley knife and left them underneath the living room on finishing – 3 years later they are still in situ as there is no chance I am going back under there unless I absolutely have to.

looked at doing this as ours is freezing.

decided that as its just over a foot heigh at its furthest point on brick and black lime i would give it a miss. Really fancy getting it done but really dont want to.

Get small kids to do it for you*

*not really…..but that’s how my dad rewired our house in 1979.

I stapled a vapour barrier directly under the floorboards then cut rolls of insulation to the width and held up by wire on the bottom of the joists.

I need to do it again in my new house and not sure if the vapour barrier is necessary or not?

I may have to do this in the future 🙁 doesn’t using rockwool create a very inviting and cosy space for rodents?

I stapled a vapour barrier

Doesn’t putting holes in it defeat the point?

Wool is what you want, as its just such a nice product to use. You do such a good job because its so nice to use.
Wool

Did our loft with it and its fab .

We used wool in an internal wall in our last house. Nice stuff until we got Clothes Moths in it.

I stapled a vapour barrier

Doesn’t putting holes in it defeat the point?

Ha ha, maybe so. Perhaps buying the proper tape would’ve been a better option. Wasn’t sure if it was actually required at all.

Rip out and concrete. That’s what I’m going to do eventually……

It’s a pig of a job as said, I had wonky joists, saggy netting and ended up wishing I’d just put in foam whilst I had the floor up. I also have a job to do and can’t be bothered going under but know it will need done soon.

It did make a difference though, despite the cack handedness of it all.

Rip out and concrete. That’s what I’m going to do eventually……

In the house i am refurbish, this has been previously done in the dining room, kitchen and part of the hallway. It looks like it has been done properly, I.e they removed the joists etc.
So what the advantage of doing it ? Just in the process of re-wiring the lounge and installing the pipes for the radiators , and upon taking up the floorboards, there appears to be 2 sleeper walls running across the room. The void is around 500mm deep.
Will need approx 12 cube of concrete so would cost me around a grand (ish) depending on spec. All the joists look in good condition so would need to be a good reason to rip it all out.

Get small kids to do it for you*

I couldn’t get through one of the internal brick wall gaps in our house underfloor. I may have resorted to a more sleekit member of the household. Ahem.

(And ours was awful for spiders, I still shudder)

Yes lots of cobwebs in the face. I now know almost every inch of our house and how it is made, which are structural walls etc.

Employing children didn’t work. In one room we replaced the floorboards and did the insulation at the same time. Instructing an old enough to know better small child not to go in a room ends approximately 30 minutes later – rescuing same child who is now stranded waist deep in netting and insulation……

@revs1972

In the house i am refurbish, this has been previously done in the dining room, kitchen and part of the hallway. It looks like it has been done properly, I.e they removed the joists etc.

It pretty much removes any risk of damp and kills all cold spots. Cases where external paving is breaching u/s of joists etc are very common in older properties. You could also equate a time versus cost exercise where getting a bit of type 1 in say 200mm in your case and a 100mm of jablite will reduce your 12m3 to say less than 5m3. But that means someone has got to barrow 10 tonne in.
Not sure what the max depth of insulation such as cellotex you could go to prior to pouring a specific depth of concrete but so longs as your sub base was sound you could further reduce concrete quarts that way, cost would again be debatable as cellotex is nore **** expensive than ever since the glue factory burnt down.

Instructing an old enough to know better small child not to go in a room ends approximately 30 minutes later – rescuing same child who is now stranded waist deep in netting and insulation……

I’d laugh but it happened to me. Twice. Personally.

I could do with doing this, ours it’s freezing when it’s windy. It’s windy here a lot.

Luckily there’s nearly 1m space at the lowest point and nearly 1.5 m at the highest. It’s still horrible under there though, I’ve spent a fair bit of time under there plumbing and running cables.

Watching thread with interest. This is likely the solution to our cold living room. If a rockwool or wool type insulation is used is it worth having a vapour barrier and or air barrier to stop any wind? Our house has alot of air vents all round the edge of the under floor.

Our house has alot of air vents all round the edge of the under floor.

To stop your floor from rotting.

Watching thread with interest. This is likely the solution to our cold living room. If a rockwool or wool type insulation is used is it worth having a vapour barrier and or air barrier to stop any wind? Our house has alot of air vents all round the edge of the under floor.

Unless your going to lift the floor boards I can’t see how you can install a vapour barrier so it can do its job as it’s meant to. Either you would have to install it in strips under the floor boards which would have lots of gaps for the vapour to breach. If you cover the full underneath of floor the moiture could accumalate on the bottom of the joists as these would be in the cold part of the floor, leading to wet rot etc.
I would go for a breathable membrane fixed to underneath of floor joists and wool inbetween. The membrane would stop drafts etc and wool can absorb moisture up to 30% and then release through membrane or floor.
Doing a job well is more important than thermal value of the product you choose.

Well that was one of the most unpleasant days work I’ve done for a long time.

Had to wash my hair 5 times.

Let’s hope the kitchen is a bit warmer tomorrow morning.

Hope you got all the black snotters out too.

Well that was one of the most unpleasant days work I’ve done for a long time.

Had to wash my hair 5 times.

Should have worn a murder suit 😜

< Our house has alot of air vents all round the edge of the under floor.

To stop your floor from rotting.>

Thanks but that much is obvious and why I havent blocked them off like our neighbours did, but the vents do mean the wind whips through under there. I wouldnt have thought just fibre glass would do much in that situation though as it works best in dead air. Sounds like you have the right plan to do it properly with foam sheets with a damp course and concrete.