Insulating vaulted ceiling

[James94]

Just finished scaffolding for my vaulted ceiling which will be my next job, but before I start buying materials I’d like some advice. I know there are a few posts regarding this subject but I’d like to start this post so it’s specific to my needs.

The rest of the  220mm rafters have all been filled with 150 PIR then 50mm onto rafters then VCL and then counter battened with 25x50mm laths for service cavity.

 

Now this was going to be my initial way of tackling this section to ensure a good u-value but after having such a hassle hand cutting 150mm PIR to get neat cuts and tight fits I can see this is going to be an absolute nightmare for this awkward roof structure.

Anyone got any thoughts?

 

James
 

[tonyshouse]

Fibreglass quilt ?

[Iceverge]

MIneral Wool and then Pir, then VCL then 45mm insulated service cavity. Swap out some of the PIR or mineral wool for woodfiber or hemp if you want better heat protection. 

 

 

 

Edited December 3, 2021 by Iceverge

[Iceverge]

By the way have I assume you have't battened and counter battened the roof? If so you could ventilate above the membrane and full fill the rafters. Depending on the membrane used you may be able to do this anyway. 

[nod]

Mineral wool would be useless in this 

You will have to go with you initial build 

I’ve done loads of these and would recommend if you are struggling with the cuts Is to use a 100 and a 50 pir 

Much easier than handling a 150 

One person measure and the other mark and cut down below 

You can always foam any loose cuts 

[James94]

@Iceverge thanks for those calcs, is the 45mm  insulated service cavity pir or 45mm mineral wool?

 

@nod what’s the reason you say mineral wool would be useless? I don’t mind doing 2 layers of pir rather than a single 150mm layer. Just thinking the mineral wool would fit better?

 

James

[nod]

9 hours ago, James94 said:

@Iceverge thanks for those calcs, is the 45mm  insulated service cavity pir or 45mm mineral wool?

 

@nod what’s the reason you say mineral wool would be useless? I don’t mind doing 2 layers of pir rather than a single 150mm layer. Just thinking the mineral wool would fit better?

 

James

Over the last 30 plus years I’ve lost count of the times we’ve ripped sloping ceilings down due to black spots and sweating 

I do hundreds of m2 of slopes each year 

It would be great to be able to put put rock wool in But the spec always calls for a ridged insulation with a true air gap

 

A possible solution would be to get your boarders to do it If they are doing it all the time They would have in-between the rafters done in a day (two men) 

Your saving £6-8 m2 

 

Its unlikely you will get it past BC with rock wall also 

If you do decide to full fill with Rock Wool I’d run it part BC first

 

Edited December 4, 2021 by nod

[cwr]

"The rest of the  220mm rafters have all been filled with 150 PIR then 50mm onto rafters then VCL and then counter battened with 25x50mm laths for service cavity."

 

That's pretty much what I've done. The builders did the pir between rafters and used a machine to cut it, kinda like a table saw. Quick to do and gave neat square cuts. I'm using 22mm lath batten+ counter batten for service void, is about half the price of 25mm stuff. Will insulate the void once first fix is done.

[Dave Jones]

16 hours ago, Iceverge said:

MIneral Wool and then Pir, then VCL then 45mm insulated service cavity. Swap out some of the PIR or mineral wool for woodfiber or hemp if you want better heat protection. 

 

 

 

 

slightly incorrect you show a gap to the underlay, as you can see from the photo there is none. the counter batten is on the other side of it.

[ProDave]

Reading this thread remands me one of the best design decisions we made was to make it a warm roof, with 100mm of insulation over the top of the rafters and then no need to ventilate and you can full fill the gap between the rafters, we used Frametherm 35 for that, an easy job.

 

I post this for the benefit of anyone at the design stage, it won't help in this case now unfortunately.

[Dave Jones]

warm roof as you say much easier but on a pitched roof can not look good due to the build height. How did you seal it to stop air/moisture from entering externally as tiles wouldnt be sufficient ?

[Iceverge]

20 minutes ago, Dave Jones said:

 

slightly incorrect you show a gap to the underlay, as you can see from the photo there is none. the counter batten is on the other side of it.

 

Have a quick look at the diagram including he labels and dimentions. its just the way i had to draw it in the calculator that it looks like a separate batten.