Fixing insulation under rafters

[ToughButterCup]

I have already put 200mm of insulation between my rafters, and now I need to fix 80mm of Celotex under those rafters. 

Since I should be able to do that by attaching the insulation to the bottom of the rafters, I am guessing that I don't need to use thermally broken fixings.

 

But I have been known to be wrong before - strangely. So I just thought I'd check.

 

Anyone got a suggestion as to which fixings I should use for 80mm Celotex?

 

 

 

[scottishjohn]

then plasterboard after that?

If you not bought cellotex it yet --why not 92.5mm insulted plaster board 

job done in one go and its simple long screws then 

Edited February 4, 2019 by scottishjohn

[ToughButterCup]

9 minutes ago, scottishjohn said:

(...)

why not 92.5mm insulted plaster board 

job done in one go and its simple long screws then 

 

Because I did not know that insulated plasterboard exists. The inexperienced self builder strikes again 

[scottishjohn]

oops

 

what ever yo do you going to need to mark all your rafters well so you can find them with the drywall fixings

maybe something like this

https://www.twistfix.co.uk/insulation-fixings?msclkid=41076a7a775a19e9c0c23622113a79f6&utm_source=bing&utm_medium=cpc&utm_campaign

then you will need  110-120mm drywall screws for plasterboard

Edited February 4, 2019 by scottishjohn

[Carrerahill]

20 minutes ago, recoveringacademic said:

 

Because I did not know that insulated plasterboard exists. The inexperienced self builder strikes again 

 

Neither did I until last year, then looking at my building warrant drawings I saw "40mm insulated plasterboard" - I thought this was just poorly written and meant some insulation then PB - but no, they make it right up to about 150mm I think! So that is what I am doing my ceiling with.

 

https://www.roofingsuperstore.co.uk/product/celotex-pl4025-insulated-plasterboard-1-2m-x-2-4m-37-5mm.html?gclid=CjwKCAiA4t_iBRApEiwAn-vt-2Se-YUgDUcezZG-USHSv1-qocyATnDPBL_jlSLNG0f6SJfaNEc3ChoCyngQAvD_BwE

 

 

[Mr Punter]

I think you may struggle to get drywall screws longer than 100mm.  If you go down to 60mm insulation it will be far easier. Hold the insulation in place with the drywall screws plus penny washers and drive them beyond the face of the insulation. Do all the insulation first and tape the joins with alu foil tape. Make sure you carefully mark the rafter locations at the top and bottom. I prefer 15mm plasterboard.

[Declan52]

I used 100mm nails  to  hold mine up.  About 6 per sheet till I got the  plasterboard up and then used 125mm plasterboard screws. But I done mine room by room so the insulation was only held on by the nails for about 3hrs till I got the plasterboard up. 

[ToughButterCup]

Thanks @Mr Punter

I'm using 80mm of insulation under the rafters.

So, it's drywall screws and penny washers  (and alu tape) I need then?

[ToughButterCup]

1 minute ago, Declan52 said:

I used 100mm nails  to  hold mine up.  About 6 per sheet till I got the  plasterboard up and then used 125mm plasterboard screws. But I done mine room by room so the insulation was only held on by the nails for about 3hrs till I got the plasterboard up. 

 

Interesting..... yes @Declan52, the difference is that you know what you are doing , and I - well, dont.

Did you not tape the little gaps between the sheets of insulation?

[Declan52]

Yep.  I bought the  alu tape from Amazon and tried to use it.  Think it was about half as thick as cling film and impossible to peel back.  Got rolls from  screwfix which where much better. 

As above marking the joists with a black marker is the important bit.  

Nails or screws is the option.  They aren't there to hold it in place for ever that will be the  job of the  50 odd screws you put in the plasterboard. Just make sure they don't stick out proud or you will bust the plasterboard. 

[Cpd]

1 hour ago, recoveringacademic said:

200mm of insulation between my rafters

Did you foam this in > off course you did. 

Marking top and bottom of the joists is essential, I then line the laser line at it to get bang on centre every time.......

I am sticking 60mm under my joists in the shed so it’s a bit easier, I can only buy 100mm max length drywall screws locally. 

[ToughButterCup]

2 minutes ago, Cpd said:

Did you foam this in > off course you did.

[...]

 

Who's a clever boy then? I am.....

Thanks about the tip to use the laser . I might have worked that out after I'd got it disastrously wrong......

[bissoejosh]

Are you having a service cavity under the 80mm? Won't the battens that create this do the job of fixing the insulation / plasterboard long term - could just be long wood screws I'd have thought. 

[ToughButterCup]

1 minute ago, bissoejosh said:

Are you having a service cavity under the 80mm? [...]

 

No, but only because I haven't thought about that - or even knew that you could do that.

 

To be clear, under the joists, tack a 25mm batten (like a roofing batten) to act as a service cavity. Then attach the 80mm. Insulation to those battens, yes?

 

That's got me thinking now..... interesting.

[Cpd]

3 minutes ago, bissoejosh said:

could just be long wood screws I'd have thought. 

I have found gyprock drywall screws to be the cheapest when buying at 90mm £14.40 for 250 quality screws for sure. 

[bissoejosh]

6 minutes ago, recoveringacademic said:

 

No, but only because I haven't thought about that - or even knew that you could do that.

 

To be clear, under the joists, tack a 25mm batten (like a roofing batten) to act as a service cavity. Then attach the 80mm. Insulation to those battens, yes?

 

That's got me thinking now..... interesting.

 

I'd do it the other way around. Insulation then batten - the screws for which  could be long timber screws to sandwich the whole lot together. The other advantage is this gives a service zone for cable runs / light fittings and also a safety margin for your vapour control / airtight layer when fitting plasterboard screws. 

[bissoejosh]

Also 280mm of PIR is quite a lot of insulation - nothing wrong with it but a thinner layer might make things easier if it can work with your SAP etc?

[Cpd]

Just now, bissoejosh said:

Also 280mm of PIR is quite a lot of insulation - nothing wrong with it but a thinner layer might make things easier if it can work with your SAP etc?

I went with 170mm between rafters and 25mm over the top.... am still tempted to add another 25mm or more onto the top...... 

[bissoejosh]

Just now, Cpd said:

I went with 170mm between rafters and 25mm over the top.... am still tempted to add another 25mm or more onto the top...... 

 

I was thinking a reduction to something like 40/50mm might make fixing easier especially if battens are added to the mix. 

[Declan52]

16 minutes ago, recoveringacademic said:

 

No, but only because I haven't thought about that - or even knew that you could do that.

 

To be clear, under the joists, tack a 25mm batten (like a roofing batten) to act as a service cavity. Then attach the 80mm. Insulation to those battens, yes?

 

That's got me thinking now..... interesting.

Insulation then the batten would mean you don't have to use long plasterboard screws.  Still need long screws for the fixing of the batten though but you would use less of the longer ones plus shorter plasterboard screws so cheaper. 

You can have it insulated and batten then the spark can come in and do his thing.  What kind of lights are you going for as these might need some timber to fix to depending on your choice. 

[scottishjohn]

1 hour ago, Cpd said:

I have found gyprock drywall screws to be the cheapest when buying at 90mm £14.40 for 250 quality screws for sure. 

I looked when i posted before and  i could find 120mm drywall no problem on ebay

 

1 hour ago, bissoejosh said:

 

I'd do it the other way around. Insulation then batten - the screws for which  could be long timber screws to sandwich the whole lot together. The other advantage is this gives a service zone for cable runs / light fittings and also a safety margin for your vapour control / airtight layer when fitting plasterboard screws. 

 vcl will need to go directly to insulation  then batobs then drywall or you will be cutting holes in it for lights