Lifted floorboards to Celotex and need some advice please!

[Rodders66]

Hello All,

We may need to convert the lounge downstairs into a bedroom for developing medical issue, and it's breezy at best so I've decided to insulate the floor.

It's a 1910 semi, suspended close board floor (with huge gaps!), 3 air bricks, 4x2 joists sitting on dwarf walls. Room is 4m x 5m.

Lifted of the floorboards in 2 different areas to check whats going on and, and there's the normal crap & rubble, and all is bone dry which was good to see.

looked at various forums about insluating the floor and was going to use 100mm Celotex or Kinkspan, sat on battens between the joists, fit nice and snug, then fully tape joins and up to walls then seal behind a fresh skim of plaster and skirting.

Probably a daft question...how do i use 100mm slab insulation with 100mm joists,?....whats the best way to fit battens to hold the insulation in place?

I've read that it's a must to hold the inslaution in place as the joists will move overtime and not to reply on friction alone.

Any help welcome!

 

[JohnD]

heat loss through floors is mostly by way of draughts, so IMO mineral wool quilt is better because you can easily stuff it into irregular gaps without need for precision cutting. You can staple garden netting to the underside of the joists to prevent it falling or shifting.

If you stuff it into the gap between the last joist and the wall, and between joist ends, draughts and dirt will not come up behind the skirting.

Clean away all the dirt and rubble first, and take away dust and cobwebs obstructing flow of air through airbricks and honeycombs.

Mineral wool does not burn or emit toxic fumes. Unlike foam, which since Grenfell I would not have inside my house.

 

[Rodders66]

...my initial guess was to throw in a few metal 90 degree brackets just to hold them there..?

 

[Rodders66]

Thanks John,
I'm guessing id need a greater depth of mineral wool to match 100mm Celotex?...what about damp issues, that said its dry down there it seems.

Am I needing to get all the sand / rubble out to make it tidy or is there a genuine benefit to things?

 

[JohnD]

draughtproofing a wooden floor is much more important than insulation.

Convection means that the hot air does not lie on the floor or flow downwards under it. There will not be much loss from conducted heat once you have a layer of mineral wool under the boards.

100mm wool is plenty.

Builders rubbish under a floor may contain organic debris that can harbour fungus or woodworm. Sometimes there are dead animals.

 

[Rodders66]

OK thanks.
Just out of curiosity, why do new buildings and extensions etc seem to always use the slab form of insulation under wooden floors?

 

[JohnD]

putting down one big slab is quicker than fitting cut pieces

You can lay a floating floor (even a concrete slab) on top of foam because it supports weight

the insulation is better, but that's not your problem. Your problem is draughts. Put down a carpet and the difference will be massive.

 

[Rodders66]

Using Mineral wool sounds much easier to fit, what are peoples suggestions around moisture barriers etc...as the solid type provides the alumminium covering and once taped up to behind the skirting is all sealed up...

There seemed to be a general feeling that a barrier is a bust...with good ventilation below.

 

[geraldthehamster]

When I did my floors with Celotex, I didn't use battens. I cut the boards fractionally under-size, held them in place temporarily with skew screws into the joists, then filled the gaps with expanding foam. They will never move anywhere, and there are no gaps.

Then you can tape over, if you wish, but I'm not convinced that's crucial.

Celotex-type boards are less messy to install and more efficient. Search for "suspended floor insulation" in the forum, and you will find many threads and descriptions of how to do this.

 

[geraldthehamster]

You can also run Celotex round the perimeter, wherever there is an external wall

 

[foxhole]

If cut tight to timber a few nails near the top of the joist left sticking out 20-30 mm before fitting insulation jam it into position .

 

[Rodders66]

Thanks..

Knocking a few angled nails right at the bottom of the joists, and then pressing the slabs down into position between the joists wresting on the nails, makes sense...but you said top of the joist...!?

 

[Rodders66]

Another question...

I'm also looking to raise another suspended floor (3.5m x 8m) to match the rest of the house.

Its sitting about 140mm lower than the others, is a T&G on decent joists and is very structurally sound. There are 5 air bricks to this part of the house.

Can I simply put down 100mm Celotex directly onto the existing wooden floor as a floating floor, and build up the levels that way ending up with the same Engineered floor throughout the ground floor?

Is there anything to worry about regarding airflow etc?...or regulations?

To my mind it should be fine!

 

[geraldthehamster]

I don't see why that wouldn't work. I'd fit chipboard or ply over the top before fitting your engineered flooring

 

[JP_]

I did a "best of both" method for insulating my floor last year, but only because the local building merchant ran out of 50mm boards and sent 25mm instead. So I put screws in about 75mm down, layed the board on them, taped in, then put 50mm rockwall across the top. I made sure the rockwool was slightly proud so the very snug once floorboard down.

Not sure you really need 100mm board under the floor, seems overkill. In my new extension they only put down 70 or 75mm boards.