My rear pitched roof (cold) has 75mm PIR between the rafters and I'm going with 75mm overboard.

 

It works out a lot easier (due to velux positions amongst other things) if I overboard with the boards long edge on the vertical, which is against what I'm seeing in most instances where the second layer of PIR is 90° to the rafters.

 

I assume it's ok to do this and BC won't get upset? I'll be battening afterwards for my service gap.

it'll be fine.

2 hours ago, Super_Paulie said:

 

I assume it's ok to do this and BC won't get upset? I'll be battening afterwards for my service gap.

Never mind about upsetting BC.. you should be more concerned about knackering your house and wasting your money.

 

It seems you are suggesting a hybrid roof.. and these are fraught with condensation issues... like it will start raining inside!

 

Can you tell us more about how are proposing to build up the layers of insulation, the membrane if any on the top side of the roof... and the type of plaster board etc you are going to use on the underside..do you have bituminous felt (not breathable) etc. There are a lot of things you need to get your head round before you take this route.

 

Don't be put off as often the solution is simple.. you just need to know how these seemingly simple things work if put together in the right way.

 

 

 

 

 

It's all in hand Gus, it's too late for the dramatic!

 

Cold roof, breathable membrane, 50mm gap, 75mm PIR in rafters, 75mm over rafters, service void with batten, board.

47 minutes ago, Super_Paulie said:

It's all in hand Gus, it's too late for the dramatic!

Pity you wasted all that money then, why did you ask the question if you we just going to do your own thing anyway?

 

Take it that the 50mm air gap is ventilated to keep BC happy? If so the insulation above that 50mm gap is doing nothing.. other than making you feel good. Or have you sealed off the 50mm gap.. seems odd how you can make that work..

 

If that is the case then you have dodge a bullet as what you have is a roof with less thermal perfomance (75mm PIR) but likely to last a lot longer as you won't be getting interstitial condensation.

Edited September 27, 20233 yr by Gus Potter

My question was in regards to the orientation of the second pir layer, horizontal or vertical.

 

There is no insulation above the air gap.

Tiles-batten-membrane-gap-pir-pir-batten-board. Pretty standard install.

 

I guess it makes me feel good. At least as good as a roof can 🤷🏼‍♂️. Don't think I've wasted any money so far.

56 minutes ago, Super_Paulie said:

Cold roof, breathable membrane, 50mm gap, 75mm PIR in rafters, 75mm over rafters, service void with batten, board.

I thought from reading the above you had a hybrid roof working from the top down.

 

If you have tiles, membrane, a gap, insulation between rafters and insulated plasterboard below the rafters then that is a standard install... no drama.

 

But if you had ended up with a hybrid roof then..  when you said part the insulation was over the rafter that is a big game changer..

 

I went to a job last week where the builder has changed one of my designs from a warm dormer roof to a hybrid roof and not asked me if it will work.. if they can't demonstrate to me that it will work by calculation .. it's going  to have to come off and get built the way I designed it... and that is about 15 - 20 k plus down the pan. It's not just the roof it's all the plaster board inside as on a warm roof no need for MR plasterboard in bathrooms and vapour membranes etc.

 

 

 

 

 

 

 

 

 

 

I agree with @Gus Potter  The use of the words “over (the) rafters” made it sound like external insulation. Which is obviously where the confusion came from. 
 

8 hours ago, Super_Paulie said:

Cold roof, breathable membrane, 50mm gap, 75mm PIR in rafters, 75mm over rafters, service void with batten, board.

 

Might be the wrong word to use but thought the above made it obvious. I have my answer anyways, cheers.

2 minutes ago, Super_Paulie said:

 

Might be the wrong word to use but thought the above made it obvious. I have my answer anyways, cheers.

I've seen it defined under and over - maybe a regional thing, so had the vision of what you were doing, it's confusing, thankfully @Gus Potter has an eye for detail and asked the question.  I'm a bad dobbie and must ask more questions before giving answers!  You'll probably do this anyway (sorry), but don't forget foam if needed and foil tape for the joints...

9 minutes ago, crispy_wafer said:

I've seen it defined under and over - maybe a regional thing, so had the vision of what you were doing, it's confusing, thankfully @Gus Potter has an eye for detail and asked the question.  I'm a bad dobbie and must ask more questions before giving answers!  You'll probably do this anyway (sorry), but don't forget foam if needed and foil tape for the joints...

Will do. I've already done my between rafter work, foamed and taped. Last thing to do is the overboard/underboard and that's all my insulation finished. BC described it as overboard and suggested horizontal, however I guess it's underboard and vertical now...!

Dont forget to mark on the new boards where the timbers are hidden!  Don't ask me how I know why this is a good idea... 🤣

24 minutes ago, crispy_wafer said:

Dont forget to mark on the new boards where the timbers are hidden!  Don't ask me how I know why this is a good idea... 🤣

Yep, marked them all up on the wall so I'm good to go ✌️

Edited September 28, 20233 yr by Super_Paulie

1 hour ago, Super_Paulie said:

Will do. I've already done my between rafter work, foamed and taped. Last thing to do is the overboard/underboard and that's all my insulation finished. BC described it as overboard and suggested horizontal, however I guess it's underboard and vertical now...!

that's the beauty of our language isn't it? i'd say it's still overboarding but under the rafters and internal to the rafters. then you could also use 'on top of the rafters internally' to describe what you're doing but that could also cause confusion! i think the way you described the full make-up from external to internal gave the best description of what you're achieving.

 

8 hours ago, Super_Paulie said:

Tiles-batten-membrane-gap-pir-pir-batten-board. Pretty standard install.

 

and i understand the requirement for brevity as we're all busy people (apart from @Pocster as the only thing he's busy on is handing out virtual biscuits) but sometimes you have to put all the details so there's no ambiguity in the written word.

 

so you'll often see someone state 'build-up from external to internal is ........' and such.

13 hours ago, Super_Paulie said:

75mm over rafters

 

Above or Below may be better to avoid confusion.

13 hours ago, Super_Paulie said:

Cold roof, breathable membrane, 50mm gap, 75mm PIR in rafters, 75mm over rafters, service void with batten, board

i suppose i would have said "75mm PIR over rafters, 75mm in rafters" in that order if i was going for a hybrid/incorrect setup, thought the order made it clear. Anyways, lets move on, its a debate i didnt intend, i was just after opinions on PIR orientation as per the title.

Cheers guys

It makes no difference which way the below rafter PIR goes.  Do whatever is easiest.  You can tape the joins instead of full VCL.  You will need some fairly long screws to hold the service battens in place.  I guess the rafters are only 125mm deep.

Just now, Mr Punter said:

It makes no difference which way the below rafter PIR goes.  Do whatever is easiest.  You can tape the joins instead of full VCL.  You will need some fairly long screws to hold the service battens in place.  I guess the rafters are only 125mm deep.

correct.
Yeah i'll crack on tonight, insulation washers and 100mm screws. Its just to hold it until my battens are up, at which point i'll fix the battens in place with 125mm screws.

9 hours ago, Super_Paulie said:

correct.
Yeah i'll crack on tonight, insulation washers and 100mm screws. It’s just to hold it until my battens are up, at which point i'll fix the battens in place with 125mm screws.

Are 125mm screws enough? With 25mm battens that’s only 25mm into the rafters. Doesn’t seem deep enough. We put 100mm PIR under our rafters and with 25mm battens used 180mm screws which gave a 55mm bite. Happy for someone to tell me that was overkill but I didn’t want them coming out!!

I'll see how I feel about it, 125mm, 140mm something like that. No harm in going longer if I can get em.

6 hours ago, Super_Paulie said:

I'll see how I feel about it, 125mm, 140mm something like that. No harm in going longer if I can get em.

I bought a load of 150mm screws online. So much cheaper than Screwfix. 

On 28/09/2023 at 13:55, Super_Paulie said:

i suppose i would have said "75mm PIR over rafters, 75mm in rafters" in that order if i was going for a hybrid/incorrect setup, thought the order made it clear. Anyways, lets move on, its a debate i didnt intend, i was just after opinions on PIR orientation as per the title.

Cheers guys

I have done the same way. 70 mm pir recticel between rafters and 70 mm under rafters taped and plasterboard. BCO also approved. One reservation I have though is risk of condensation between PIR layers. Usuall they say pir thickness on colder side should be larger thsn on warmer. Like 100 and 40 or 50. Did you have sny issues with condesation? I guess it is not easy to check

I'm yet to board as I'm playing catch-up and doing the whole house boarding at the same time. Both my layers of PIR are foamed and taped so I'm not envisaging any problems between the layers.