Yet another loft insulation thread

juraj.kecso

Hi All,

After about 3 years of living in our house, I have finally managed to get around looking into insulating the loft. Current insulation is only the usual 100mm sitting between the joists. Part of it is boarded and used for storage. I know that the go-to approach would be to top up either with 170mm or 200mm mineral wool laid perpendicular. Boarding over can be achieved by using appropriate loft legs and chipboard.

The problem I have is that this would naturally lower the headroom by as much not leaving a lot of space for storage. I'm 178cm tall and can feel that there is not a lot of headroom already, let alone if the floor is raised by another 170mm to 200mm.

Instead of doing this and to achieve the same thermal value whilst not losing as much headroom, is it possible to use something like the 100mm Celotex or Kingspan (or similar PIR) simply laid directly on the joists (perpendicular) on top of any existing mineral wool, at least in an area that I'd like to use for storage and chipboard or OSB sitting on top? How would I fix the PIR board to joists to prevent movement? How would I fix the chipboard to the PIR board? Is there something I'm missing?

Areas that are too small for storage would be fixed by the 200mm top-up.

grumbler

juraj.kecso said:

How would I fix the PIR board to joists to prevent movement?

I'd use some silicone as weak adhesive. Or double-sided foam tape.  Plus strips of duct tape on the top to hold the boards together.

How would I fix the chipboard to the PIR board?

Use bigger boards. If you don't care how it looks you can use small nail plates with few short screws and leave the boards floating.

Small T&G boards you can glue together.

juraj.kecso

Thanks. Will this be as thermally efficient as 200mm of mineral wool?

Also, can the PIR boards be placed directly on joists? Won't those boards break if stepped on?

grumbler

juraj.kecso said:

Will this be as thermally efficient as 200mm of mineral wool?

I don't know, but I think this will be efficient enough.

IIRC, it was 10cm rockwool + 5cm celotex specified for the roof of converted loft.

Also, can the PIR boards be placed directly on joists? Won't those boards break if stepped on?

They can break - while without boards on the top.

Not sure about the small contact area with the joist, but I think it'll be OK. Normally, if it's floor, PIR boards are laid on flat surface, not on joists.

juraj.kecso

I found this B&q guide, which specifically mentions insulated loft boards. The principle is the same, lay rigid insulation on joists with chipboard on top. I have existing chipboard flooring, which I'd like to reuse, so will only need PIR boards.

juraj.kecso

https://www.diy.com/ideas-advice/how-to-insulate-your-loft/CC_npcart_400221.art

the_lunatic_is_in_my_head

juraj.kecso said:

 How would I fix the PIR board to joists to prevent movement? How would I fix the chipboard to the PIR board? Is there something I'm missing?

Areas that are too small for storage would be fixed by the 200mm top-up.

You can use washers to pin the PIR board which prevent the screw driving through the board

https://www.diy.com/departments/q-board-stainless-steel-washer-pack-of-20/1919340_BQ.prd

With 100mm insulation you'd probably need 130mm to 150mm screws. The flooring boards would also be fixed to the joists so you'd need screws to go through the boards, the PIR and into the joists. Usually you'd use flooring screws which have an extra bit of thread under the head but I don't know if they come in longer lengths. Other option is timber at right angles to the joists, squeeze the PIR in-between and screw the boards to the new timber. 

Not sure if it could all float or pin the insulation and stick the flooring boards with adhesive, perhaps someone more knowledgeable will advise. 

Worth considering PIR boards are usually 8 by 4 sheets and presumably the only access is the loft hatch so you have to cut the boards to fit through the opening. I think PIR will be more expensive than the rolls of loft insulation but then again the loft legs aren't that cheap so it might balance out. 

It does sound like a lot of effort to save 70mm in head room though.

FreeBear

juraj.kecso said: lay rigid insulation on joists with chipboard on top.

Two things to bear in mind using rigid insulation boards.

1. It is not vapour permeable, so any condensation on the underside will have nowhere to go, This could lead to rot in the joists.
2. There will be a gap between the rigid boards and ceiling that will need to be filled. If you don't, there will be a cold draught blowing through which will render the insulation pointless.

If you must use rigid boards, they should be cut to fit tightly between the joists and any gaps sealed. I'd also suggest just doing the central area of the loft space only, and using fibreglass around the perimeter.

If you are wanting to use the space for anything more than storage, then you need to look at having a proper loft conversion that is compliant with Building Regulations and has planning permission. The last two will save no end of trouble when you come to sell and will add value.

Teapot55

My solution wouldn’t be right for everyone, but . . .

. . . at age 24 I ran up the stepladder into our loft and laid 100mm insulation really quickly.

Thirty years later I couldn’t believe how much stuff had accumulated in the loft and it was a devil of a job getting it all out in order to bring the insulation up to more modern standards (200mm?). Also couldn’t believe how much more difficult everything was now I was older!

Nowadays we keep nothing at all in the loft but might consider hiring storage in the big place down the road if necessary. At least our kids won’t get a nasty shock one day in the future like my sister did when she ventured into our parents’ loft after they passed on. 

EssexExile

I just put some multi-layer foil insulation under the boards, I'm taller than you so need the headroom.

As Teapot says, I'm reducing the amount of junk up there but still need to be able to get to water tanks etc, so the boarding is staying.

ashe

use loft legs