PIR Fixing Help - Vaulted Ceiling

[richo106]

Hi All

 

I am planning to start insulating my vaulted ceilings and just ordering all the materials i need for it

 

My plan is to install 120mm PIR between my rafters (rafters are 170mm), they 75mm PIR laid horizontally beneath. I will then either 50mm batten it and plasterboard it or just plasterboard depending on what we decide on lighting

 

I am just concerned about the lengths of screws needed to install the plasterboard or battens....is this normal? I have seen you can get 120mm drywall screws (I am guessing these are ok if i don't batten it)

 

My main question is what screws/fixings are the best to use for installing the 75mm PIR to the rafters? Can i just use 100mm wood screws with penny washers or is that a bit cowboy-ish?

 

I will be taping the 75mm PIR joints...do i need to tape the 120mm jointers over the rafters or not?

 

Many Thanks

 

[crispy_wafer]

decent length screws and washers like these (others are available)

 

TIMco MID35 Metal Insulation Disc 35mm - Zinc (Bag of 100), Silver : Amazon.co.uk: DIY & Tools

[James94]

That’s all we used, wood screws and Timco washers off Amazon. Make sure they are sunk in enough as we had to remove vcl tape and sink them deeper as dry liners said their screws will pop on their plaster boards.

Regared. James

[Thorfun]

if you're going the battens route then 100mm through the PIR will be fine as it's just to hold it until the battens go up and they'll REALLY hold the PIR to the rafters. i made sure i had at least 50mm of screw in to the rafters. so for our 100mm PIR i held them in place with 120mm screws and washers and then used 180mm screws through the 25mm battens which gave us a 55mm hold in the rafters.

 

if you're not using battens then i don't think 25mm hold is enough for 75mm PIR personally. i'd be looking at 120mm screws at a minimum but would probably get 150mm screws. can be had online for good prices.

[eandg]

We did 120mm Diall drywall screws through 100mm PIR. Battens then on with 150mm Timcos which we're hoping will hold the plasterboard when it goes on - if not there's a job redoing it with longer screws! 120mm drywall screws best/cheapest from B&Q (limited stock though so I bought up every box at 3 separate stores) and I found Sitebox cheapest for 150s.

[Iceverge]

Your current proposal.  I'll be ghrd to fit PIR between rafters and lots of foaming and waste. Long term my confidence I don't have much confidence in PIR not to shrink and leave gaps between the rafters. You'll need 175mm screws for 50mm into the rafters.  

 

 

 

I would put flexible insulation between rafters, far quicker and cheaper and more forgiving of any movement in the timber. Better for noise and fire too. A 22mm service cavity with 3x1 strapping @600mm cc would work well for 15mm plasterboard. 175mm screws again. 

 

 

 

 

 

What kind of membrane have you used above the rafters? Full fill may be an option. If you could do this with woodfiber you'd have a far superior decrement delay. You could go to 150mm screws too to save some money and make it easier to fix through the PIR.

 

 

Edited April 14, 2023 by Iceverge

[Thorfun]

+1 for mineral/glass wool between the rafters. it wasn't a nice job but a lot better than it would've been compared to fitting PIR between the rafters!

[nod]

Just a point

50 mil batons are to tight for plasterboard Better with 100 x 25 

[Thorfun]

1 minute ago, nod said:

Just a point

50 mil batons are to tight for plasterboard Better with 100 x 25 

way too late for me! there's no way i'm taking mine all down and replacing them. 😂

[crispy_wafer]

12 minutes ago, Thorfun said:

way too late for me! there's no way i'm taking mine all down and replacing them. 😂

 nor me!

[richo106]

1 hour ago, Iceverge said:

Your current proposal.  I'll be ghrd to fit PIR between rafters and lots of foaming and waste. Long term my confidence I don't have much confidence in PIR not to shrink and leave gaps between the rafters. You'll need 175mm screws for 50mm into the rafters.  

 

 

 

 

I would put flexible insulation between rafters, far quicker and cheaper and more forgiving of any movement in the timber. Better for noise and fire too. A 22mm service cavity with 3x1 strapping @600mm cc would work well for 15mm plasterboard. 175mm screws again. 

 

 

Just to confirm you would use 120mm Rockwool between the rafters and then 100mm PIR over the top instead of 75mm PIR? 

 

 

 

 

 

[richo106]

Just to confirm you guys would use 120mm Rockwool between the rafters and then 100mm PIR over the top instead of 75mm PIR? 

 

Rockwool would be a lot easier to fit and provide a nice tight seal around brackets etc

[PeterW]

Rockwool between, PIR over the top.

 

Done a few like that and run the battens horizontal to hold back the fibre and then infill with 25mm insulation, tape the joints and board over the top

 

 

[Iceverge]

There's lots of types of fluffy insulation.  I would encourage you to make sure it's designed for purpose. Trying to put attic roll in this application for example can be done but it's not worth the hassle. I did it recently with 150mm rockwool attic roll between studs because it was the only thing I could get easily. 

 

 

Like @nod says narrow battens won't be the best. They're constantly splitting if nothing else. 

[richo106]

Thanks for all your help

 

I am just trying to decide on which way exactly to go

 

120mm Rockwool between rafters (or 70mm + 50mm Knauf onnifit as that's what my supplier has)

 

Do I 25mm batten horizontally with 25mm PIR taped in between and then 75mm PIR over the top?

 

Or just 100mm PIR straight over the top?

 

[Thorfun]

1 hour ago, richo106 said:

Thanks for all your help

 

I am just trying to decide on which way exactly to go

 

120mm Rockwool between rafters (or 70mm + 50mm Knauf onnifit as that's what my supplier has)

 

Do I 25mm batten horizontally with 25mm PIR taped in between and then 75mm PIR over the top?

 

Or just 100mm PIR straight over the top?

 

firstly....there are other suppliers so you're not stuck with what the one supplier has!

 

also, what is the make-up of the roof above the rafters? we fully filled our rafters, made possible by what's above it.

[richo106]

5 minutes ago, Thorfun said:

firstly....there are other suppliers so you're not stuck with what the one supplier has!

 

also, what is the make-up of the roof above the rafters? we fully filled our rafters, made possible by what's above it.

I will be going with 120mm Rockwool over 50 & 70mm option

 

Above the rafter is roofing felt, slats and tiles

 

Cheers

[Thorfun]

49 minutes ago, richo106 said:

roofing felt

?? is this a refurb? i didn't think anyone used felt any more when there are such good breathable membranes out there that will allow the airgap to be above the rafters!

 

As @Iceverge's lovely U-value calculations show to get the same U-value as the PIR between the rafters you'll need 100mm PIR underneath. ultimately these decisions are all about compromise. we used https://www.insulationshop.co/140mm_isover_timber_frame_batt_32.html between our rafters (190mm of it) and 100mm PIR below. if you used mineral wool insulation with a thermal conductivity of 0.032W/mK you might get closer to the U-value with only 75mm PIR. i spent many hours going through different permutations on a U-value calculator to get what i wanted.

[richo106]

7 minutes ago, Thorfun said:

?? is this a refurb? i didn't think anyone used felt any more when there are such good breathable membranes out there that will allow the airgap to be above the rafters!

 

As @Iceverge's lovely U-value calculations show to get the same U-value as the PIR between the rafters you'll need 100mm PIR underneath. ultimately these decisions are all about compromise. we used https://www.insulationshop.co/140mm_isover_timber_frame_batt_32.html between our rafters (190mm of it) and 100mm PIR below. if you used mineral wool insulation with a thermal conductivity of 0.032W/mK you might get closer to the U-value with only 75mm PIR. i spent many hours going through different permutations on a U-value calculator to get what i wanted.

Sorry for the lack of info regarding the ‘felt’ it’s a new build and I don’t actually know what they put on? I will most certainly ask

[Iceverge]

Running some quick costs on this. ( All from insulation4less) 

 

Take 3 examples. All the same U value. Decrement delay ( for heat protection) is worst with all PIR at 7 hrs and best with full fill rock silk at 10hrs.   The cost of tape, screws , 50mmx50mm timbers and 100x25mm timbers are the same in all cases. 

 

Celotex 120mm £20.38/m2

Celotex 75mm £13.12/m2

FM330  foam £/1m2

 

Total £34.50/m2

 

 

Rocksilk 37 batts 2 x 60mm £13.44/m2

Celotex 100mm £15.27

 

Total £28.71/m2

 

 

 

Full fill Rocksilk 180mm £20.16

Celotex 75mm £13.12

 

Total £33.28/m2  

 

 

BUT

Fitting dense mineral wool batts is a pleasure, minimal itchiness and doesn't disintegrate like loft roll. Cut them with a sharp knife and they push right in. Almost zero wastage. 

 

PIR is tricky to cut straight unless you use a table saw or similar and the dust is not pleasant. When you have 4 x 560mm spaces between the rafters you will end up with about a 160mm piece to play with from every board.  Wastage will be higher undoubtedly. 

 

 

 

 

 

 

 

 

 

 

 

[Dave Jones]

allways space the trusses so you have 400 gaps not centres, it may mean an extra truss to complete the run but this will be more than paid for in the un-wasted offcuts.

[ProDave]

36 minutes ago, Dave Jones said:

allways space the trusses so you have 400 gaps not centres, it may mean an extra truss to complete the run but this will be more than paid for in the un-wasted offcuts.

But will then totally screw up the spacing for fixing plasterboard as the spacing will be wrong for that.  

[Dave Jones]

2 hours ago, ProDave said:

But will then totally screw up the spacing for fixing plasterboard as the spacing will be wrong for that.  

irrelevant, boards are cheap and easily cut.

 

wood for the trees etc.

[Pocster]

I put pir between rafters ( I was led to believe it’s less heat loss than the equivalent depth of rock wool ) . Foamed any gaps . Then put 50mm pir over the lot . Foamed gaps and taped all joints .

[richo106]

Thank you very much @Iceverge

I had also carried out these calculations and this just confirmed my decision.

I am going to go with 120mm Rockwall between the rafters then 100mm PIR over the top.

how many screws per PIR would people allow? Would 12 be enough?

Do I have to ‘join’ them on a rafter like plasterboard? 
 

Many Thanks again for all the comments, very much appreciated