Fixing PIR - any issues with this?

[Benpointer]

Hi All,

 

Our roof build-up is a bit complicated but below 254mm easi-joists which are filled with blown cellulose, we have:

• Tyvek VCL
• 25x50mm cross battens, which were required to stop the VCL from blowing off the easi-joists when the cellulose was blown in
• 100mm PIR
• 38x50mm vertical service cavity battens
• 15mm PB and skim

 

Here's a picture (albeit the easi-joists are shown as solid joists due to the limitations of my ubakus skills).

 

 

The plan is to fix the PIR on lightly with half a dozen 120mm screws and washers, then pin it properly in place with the service cavity battens using 200mm screws through the cavity battens, the PIR and (ideally) the cross-battens, then into the bottom web of the easi-joists, by 37mm min.

 

Ideally, the 200mm screws will hit the 25mm cross battens on their way but it's likely that some may miss the cross battens but of course they will still penetrate the easi-joist bottom web.  

 

If you can visualise all that, my question is: will the PIR deform if the screws are crossing a void rather than going through those 25mm cross battens?  (Bearing in mind the force of the screws will be spread on the underside by the 38mm vertical service cavity battens.)

 

If you can make sense of that and have a view, all responses would be appreciated.

 

 

[Nickfromwales]

The cross battens will be visible, and after placing the 100mm PIR you'd just run a marker to show where to put each fixing. I think this is an overcomplicated question for a simple install, and will become apparent how simple this is once these start going on. Basically, AFAIC, they can't miss?

 

If the chippys are installing this then I'd suspect a bit of micro-managing is creeping in lol. 

[torre]

Why not fit 25mm insulation between the cross battens?

 

Avoids any deformation and improves the insulation level.

[Benpointer]

8 hours ago, Nickfromwales said:

The cross battens will be visible, and after placing the 100mm PIR you'd just run a marker to show where to put each fixing. I think this is an overcomplicated question for a simple install, and will become apparent how simple this is once these start going on. Basically, AFAIC, they can't miss?

 

If the chippys are installing this then I'd suspect a bit of micro-managing is creeping in lol. 

 

Thanks Nick, take you point about micromanagement, if the chippies were putting this in I'd have absolutety no worries but its the insualtion team from Cornwall and tbh they don't inspire 100% confidence.   At one point they were suggesting pre-drilling the service cavity battens at 600mm centres to speed things up - but the cross battens are at 400mm centres 😬

 

I guess I was just wondering how much of an issue it would be if they did miss the cross battens.  (Obvisoutl they are not going to miss the 122mm wide rafters else the screws won't bite.)

[Benpointer]

1 minute ago, torre said:

Why not fit 25mm insulation between the cross battens?

 

Avoids any deformation and improves the insulation level.

Three reasons:

1. It's more work.
2. The cross battens are probably 22-24mm in reality (I'd need to measure).
3. In places the cellulose has blown pushed the membrane away from the rafters a bit (5-10mm max) between the cross-battens - the must pump it in with some force!

Another alternative would be to put short sections of 25mm batten vertically below each rafter (where the cellulose hasn't pushed it away from the rafter) but again it's a lot of extra work.

 

A pictura of the membrane and the cross-battens so you can see the current set-up:

 

[Thorfun]

Like Nick said I think you’re over complicating things just put a board up and chalk line where the cross battens are and you can’t miss! (Unless they’re screwing at a strange angle)

 

and we pre drilled and countersunk all our service cavity battens. Got a cheap table drill and eventually got it down to a short overhead of time. But we were doing it ourselves so weren’t paying someone the extra time it took. 

[Benpointer]

8 hours ago, Thorfun said:

Like Nick said I think you’re over complicating things just put a board up and chalk line where the cross battens are and you can’t miss! (Unless they’re screwing at a strange angle)

 

and we pre drilled and countersunk all our service cavity battens. Got a cheap table drill and eventually got it down to a short overhead of time. But we were doing it ourselves so weren’t paying someone the extra time it took. 

Haha! Snap!  Two long evenings with Mrs. P passing the battens and me drilling on a pillar drill with a pre-measured fence.    (Although we haven't countersunk the holes - I figured the screws will pull in to softwood battens just fine.)

 

Cheap labour! Plus my right arm is ready for the one arm bandits in LA now!

Edited August 6 by Benpointer

[Thorfun]

6 minutes ago, Benpointer said:

Haha! Snap!  Two long evenings with Mrs. P passing the battens and me drilling on a pillar drill with a pre-measured fence.    (Although we haven't countersunk the holes - I figured the screws will pull in to softwood battens just fine.)

 

Cheap labour! Plus my right arm is ready for the one arm bandits in LA now!

i had a countersink bit with the drill bit in it already so did both in one go. 😉 

[Nickfromwales]

14 hours ago, Benpointer said:

 

Thanks Nick, take you point about micromanagement, if the chippies were putting this in I'd have absolutety no worries but its the insualtion team from Cornwall and tbh they don't inspire 100% confidence.   At one point they were suggesting pre-drilling the service cavity battens at 600mm centres to speed things up - but the cross battens are at 400mm centres 😬

 

I guess I was just wondering how much of an issue it would be if they did miss the cross battens.  (Obvisoutl they are not going to miss the 122mm wide rafters else the screws won't bite.)

.....evening boss, sorry I'm late!

 

Ok, so the real elephant in the room is the AVCL. If they miss, then they'll withdraw the fixing, and go again, making lovely holes in your airtightness layer; a crime punishable only by a slow and painful death afaic.

 

This is the real crux here imo.

[Benpointer]

1 minute ago, Nickfromwales said:

.....evening boss, sorry I'm late!

 

Ok, so the real elephant in the room is the AVCL. If they miss, then they'll withdraw the fixing, and go again, making lovely holes in your airtightness layer; a crime punishable only by a slow and painful death afaic.

 

This is the real crux here imo.

Yes, I appreciate that is a Buildhub crime of the first order.  I was thinking of several mitigations we have in place which may or may not be effective:

1. On the other side of the VCL is 254mm of blown cellulose. I have been told that blown cellulose improves air-tightness by 50% but I am not sure what that would mean in practice.
2. We can tape the PIR boards before the service cavity battens are fixed, effectively making the membrane holding the cellulose redundant.
3. We are engaging Aerobarrier to do their magic after first fix.

But if we can avoid missing the cross-battens that would clearly be best.

[Nickfromwales]

2 minutes ago, Benpointer said:

• 1) On the other side of the VCL is 254mm of blown cellulose. I have been told that blown cellulose improves air-tightness by 50% but I am not sure what that would mean in practice.
• 2) We can tape the PIR boards before the service cavity battens are fixed, effectively making the membrane holding the cellulose redundant.
• 3) We are engaging Aerobarrier to do their magic after first fix.

1) In practice, these people would be bound by a jacket that ties up at the rear....

 

2) If I hadn't already had 3 beers, I'd drive there right now and apply the back of my hand to you, sir. Absolutely NOT on your NELLY! All of our work has clearly been in vain.....close BH, turn off the lights....the message just isn't hitting home. It's your AVCL!!

 

3) Why? And be honest. Lack of confidence, or other?

[Benpointer]

8 hours ago, Nickfromwales said:

1) In practice, these people would be bound by a jacket that ties up at the rear....

 

2) If I hadn't already had 3 beers, I'd drive there right now and apply the back of my hand to you, sir. Absolutely NOT on your NELLY! All of our work has clearly been in vain.....close BH, turn off the lights....the message just isn't hitting home. It's your AVCL!!

 

3) Why? And be honest. Lack of confidence, or other?

Lol - I'll take that as a no then 😂

 

To answer point 3: yes absolutely I lack confidence in our overall airtightness outcome.  Why?: I'm a novice at this; most trades don't seem to understand airtightness very well; as mentioned on my blog, we have a complex roof build up (maybe a poor choice but the bed is made now).  

 

Anyway, I thought you were a great advocate of Aerobarrier?:

 

 

 

[Thorfun]

10 hours ago, Nickfromwales said:

.....evening boss, sorry I'm late!

 

Ok, so the real elephant in the room is the AVCL. If they miss, then they'll withdraw the fixing, and go again, making lovely holes in your airtightness layer; a crime punishable only by a slow and painful death afaic.

 

This is the real crux here imo.

We missed with screws through battens and 80mm PIR in to 38mm wide studs often. Just slapped some airtight tape over the hole and move on!

 

I also told every trade who came on site about our airtightness requirements and said if you make a hole to let me know and we’ll seal it. In the end had to do that pretty infrequently as they all seemed to get what we were trying to do. 

[Nickfromwales]

The AVCL leaks behind the layers though, so the taping isn't really robust, but where do you stop if there's trades blasting through jobs.

 

The best thing I find is speaking to the trades before they get their tools out.

[Nickfromwales]

9 hours ago, Benpointer said:

Anyway, I thought you were a great advocate of Aerobarrier?:

I most definitely am!

 

I was just asking, because if there's a quality frame such as yours, and most AT methodology is known from day dot, you could avoid that cost, perhaps, that's all  Out of curiosity, when had you panned getting them in, and were you going to attempt a DIY blower (pre) test of any sort?

[Benpointer]

1 hour ago, Nickfromwales said:

I most definitely am!

 

I was just asking, because if there's a quality frame such as yours, and most AT methodology is known from day dot, you could avoid that cost, perhaps, that's all  Out of curiosity, when had you panned getting them in, and were you going to attempt a DIY blower (pre) test of any sort?

We're hoping to get them in w/c 23rd or 30th Sept.  That's after 1st fix, before plastering.  Just waiting for them to confirm they can do a day one of those weeks.