Celotex

[aerochago]

Our lounge extension built in 1973 has a flat roof and very poor ceiling insulation, its cold in the winter and hot in the summer and you can hear seagulls padding about and rain drops falling. We want the original ceiling replaced anyway so what thickness of Celotex would be suitable. 100cm has been mentioned and possibly and extra 50cm and sandboard?

Regards

Alan

 

[endecotp]

Celotex is not the best material for acoustic insulation.
Is there a limit on how far the roof could rise?

 

[noseall]

120mm thick proprietary roof boards are available for flat roof deck and would satisfy latest b'regs. Kingspan and Ecotherm (and prob's Celotex) supply a 6mm ply bonded sheet specifically for 'warm' flat roofs. Only downside is finding long enough fixings. We used four boxes of Spax 180mm fellas at a cost of £56 per 100, on our last pair of flatties.
Superb screws though.

 

[Notch7]

You could use an online u value calculator to work out u value, each thickness of celetex.

I think buildings regs want a u value of 0.18 for flat roofs, which equates to 120mm / 125mm over joists.

 

[aerochago]

Celotex is not the best material for acoustic insulation.
Is there a limit on how far the roof could rise?

No cant raise the roof, its a black felt roof.Possibly considered lowering the ceiling, there is a thin layer of fibreglass insulation only above the original ceiling.

 

[^woody^]

We used four boxes of Spax 180mm fellas at a cost of £56 per 100, on our last pair of flatties.

I've never rated Spax screws, nor the trend for "Brands" in the screw world. It's a screw.

You can get screws for half the cost of Spax (and thats before quantity discount), which will perform exactly the same.

 

[John D v2.0]

I found wickes "stick fit" ones made by philips are really good without breaking the bank. I was using the torx ones from screwfix for a while, turbo gold I think, but the bits they supply are great for not slipping when driven off centre, the equivalent problem is the screws don't stay on the bit when working one handed.
The builders were using cheaper bulk ones, and a few of them have snapped after I've had the floor up and back down with an impact driver.

 

[^woody^]

Just put as much as you can between the joists and then as much as you like below. It will need a snug fit with no gaps anywhere. Or use a combination of celotex and quilt if you have cables and herringbone struts or suchlike.

200mm of rockwool quilt between the joists (for sound and heat) and 50 celotex below will be a good performer, and anything greater may have theoretical greater insulation qualities, but these may not be actually be perceivable in comfort or realised in lower bills. I dont think its worthwhile lowering ceilings too much as the small gains compared to having to live with a low ceiling are not worth it.

 

[noseall]

I've never rated Spax screws

The ones we used are the ones with the torx heads and the larger washer head type, ideal for those thin ply sheets.
They are incredibly easy to drive in and we encountered zero failure rate.
I guess when you have used others along side you realise the tangible difference in performance, ease and speed of the spax fellas.
I would not big them up unless they were any good.

 

[tomfe]

Toolstation do a 180mm torx screw for about £20 for 100.
I have used the 200mm ones for 150mm board with thin insulation metal washers, no ply just the black torch on boards.
A thermally broken fixing would be much better, with the rise of EWI there are plenty about and at a reasonable price.

 

[Craig Baxter]

For acoustic (sound) insulation you need density, ie weight. None of the PIR foam boards are particulary heavy.
If you’re replacing your ceiling, it would be better to double up on your plasterboard, use something like Rockwool Flexi within the joist, while remembering to leave an air gap, which is key to noise reduction.
If weight allows, it is advisable to use tge denser boards within the construction build up on the outside. However, you need to be aware that although the denser boards are better acoustically and fire prevention wise, you need a much thicker product to match the Celotex type boards thermally.

 

[^woody^]

For acoustic (sound) insulation you need density, ie weight. None of the PIR foam boards are particulary heavy.
If you’re replacing your ceiling, it would be better to double up on your plasterboard, use something like Rockwool Flexi within the joist, while remembering to leave an air gap, which is key to noise reduction.
If weight allows, it is advisable to use tge denser boards within the construction build up on the outside. However, you need to be aware that although the denser boards are better acoustically and fire prevention wise, you need a much thicker product to match the Celotex type boards thermally.

The OP can't leave an air gap in an unventilated roof, and in any case the the probelm with the seagulls pecking and stepping will be more to do with the felt covering and the joists, than anything else. Difficult to stop without an isolation layer directly under the felt.

Dense plasterboard is good to stop sound getting out, but not so good to stop it getting in from the roof surface.