Loft office insulation

monty77uk · 2 Jul 2015

Roof has wooden slats, no felt/membrane.

For the new floor I was thinking of using 100mm celotex cut in between the new noggins. Partly because it's better then the blown fibre stuff I have and partly because I'm assuming this means I don't have to lay it over chicken wire. Whilst BCO aren't involved I am trying to do things as by the book as possible.

Roof has 100mm rafters, I'm aware that I need a 50mm gap before I fit any Celotex into the gaps but I was thinking of not doing the between rafters part and just insulating at 90 degrees to them as frankly it's less work/timber/weight on the existing rafters. Again was thinking of using 100mm Celotex for that.

How close would that get me towards a decently warm/cool loft? Or do I need to stop being lazy and put 50mm in the rafters too?

Cheers!

tony1851 · 4 Jul 2015

1. For the floor, the insulation is intended to increase the fire resistance; 100mm fibreglass on netting stapled to the joists is the accepted, effective way. Doubt sticking rigid board between the joists would enhance fire-resistance.

2. For the roof, ensure you put some sort of vapour control layer immediately above the insulation (eg 500g polythene sheet stapled to the rafters and joints taped). Otherwise during winter you will see brown stains down your nice new plasterboard ceiling.

monty77uk · 6 Jul 2015

tony1851 said:
1. For the floor, the insulation is intended to increase the fire resistance; 100mm fibreglass on netting stapled to the joists is the accepted, effective way. Doubt sticking rigid board between the joists would enhance fire-resistance.

2. For the roof, ensure you put some sort of vapour control layer immediately above the insulation (eg 500g polythene sheet stapled to the rafters and joints taped). Otherwise during winter you will see brown stains down your nice new plasterboard ceiling.

1. Fair enough. PITA to do this with noggins in place already but that's my fault for bad planning

2. So your recommendation would be to staple vapour control directly to rafters (nothing between them) and then insulation board at 90 degrees to the rafters (100mm). Everything taped.

Thanks!

geraldthehamster · 7 Jul 2015

The vapour control layer should be on the warm (room) side of the insulation, not the cold (roof) side.

If you're using foil-backed Celotex, the foil provides the VCL.

Cheers
Richard

monty77uk · 7 Jul 2015

geraldthehamster said:
The vapour control layer should be on the warm (room) side of the insulation, not the cold (roof) side.

If you're using foil-backed Celotex, the foil provides the VCL.

Cheers
Richard

Awesome, thanks. Latest idea for the insulation seems to be.

- Leave the 100mm (4") gap between rafters and roof boards
- 100mm celotex at 90 degrees to the rafters
- Battens (not sure on size yet, 20mm or so)
- Supa quilt foil stuff (or alternative) over the battens (taped)
- Plasterboard

..for the stud walls around the perimeter (hipped roof on all sides) was going to go for rock wool blocks between the studs then plasterboard. Behind the stud in the eaves I'm not going to use much, just the foil bubble wrap stuff stapled to rafters to keep the area dust free. Rafter/wall ends will be open for ventilation.

Any issues with the above? If I'm using supa quilt (or equiv) do I need the foil backed stuff at all? Could save a few ££ there

Cheers!

geraldthehamster · 8 Jul 2015

monty77uk said:
- Supa quilt foil stuff (or alternative) over the battens (taped)

This isn't going to add anything worthwhile.

for the stud walls around the perimeter (hipped roof on all sides) was going to go for rock wool blocks between the studs then plasterboard.

You need a VCL here as well, between the rockwool and the plasterboard.

I've just noticed, on re-reading, that your roof isn't felted, so I don't think your 50mm air gap is crucial. In any case, I'd be tempted to save headroom by putting 50mm (or 70mm) between the rafters (cut undersize and fix with expanding foam), and another 50mm over it, on the loft side. When I last did something similar, Building Control wanted 125mm - I don't know whether this has changed.

Cheers
Richard

Cheers
Richard

monty77uk · 8 Jul 2015

geraldthehamster said:
monty77uk said:

- Supa quilt foil stuff (or alternative) over the battens (taped)

Click to expand...

This isn't going to add anything worthwhile.

for the stud walls around the perimeter (hipped roof on all sides) was going to go for rock wool blocks between the studs then plasterboard.

Click to expand...

You need a VCL here as well, between the rockwool and the plasterboard.

I've just noticed, on re-reading, that your roof isn't felted, so I don't think your 50mm air gap is crucial. In any case, I'd be tempted to save headroom by putting 50mm (or 70mm) between the rafters (cut undersize and fix with expanding foam), and another 50mm over it, on the loft side. When I last did something similar, Building Control wanted 125mm - I don't know whether this has changed.

Cheers
Richard

Cheers
Richard

Interesting, seems fairly good going on specs, certainly above the standard cheap tin foil stuff.

http://www.ybsinsulation.com/products_roof_superquilt.htm

OK so revised plan would be:

- 50mm between rafters w/50mm gap
- 50mm at 90 degrees to the rafters
- Plasterboard
- Use the foil backed insulation board, no need for the quilty stuff

Or use foil backed 100mm at 90 degrees to rafters (headroom not an issue) to save knobbing about going in between rafters/foam/spacers etc...

And for the studs I'll do as you suggest and add a VCL of some kind.

Additional question. I wasn't planning on continuing the celotex right down into the hip of the roof (behind the studs). Partly to save labour, partly to save costs. Is that OK? The cheapo foil bubble stuff stapled to rafters behind the stud is purely to keep things clean.

Cheers a million, very helpful chat !!

monty77uk · 28 Aug 2015

Been a while but finally about ready to start with the insulation part .. floor is down and everything nice and solid

For the roof new plan is:

30mm battens on top of 100mm existing rafters
Hang 80mm insulation board (non-foil) between them using rafter clips
Vapour barrier over the top
Battens at 90 degrees to rafters
Plasterboard screwed into battens

...I'm aware that this doesn't meet regs insulation-wise but seeing as BR are coming nowhere near this I'm not fussed and I'm sure it'll be adequate for it's usage. In the future if we do want to BR this mofo then I can put celotex backed plasterboard over the top to bring me up to scratch but it's highly unlikely I'll go there.

As I said before the roof has no felt, it's close boarded, so do I really not need the 50mm? That would do away with the need for the first lot of battens which saves £100 or so in materials.

Cheers!

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Loft office insulation

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