Alterative option to 150mm celotex + 50mm celotex?

[flanagaj]

Can anyone advise here. Our architect has specified 150mm celotex between the rafters and then 50mm between the rafters, finished off with VPL and a plasterboard skim finish.  The interesting thing is that the roof space has a maximum height of 1.2m to the underside of the ridge and it's not a vaulted ceiling.  So not sure why there is plasterboard and a skim finish.

Any ideas as to what I can say to this.  Yes, I am getting a U value of 0.12 W/m2K, but that is not actually achievable.   

Edited yesterday at 15:07 by flanagaj

[BotusBuild]

16 minutes ago, flanagaj said:

Our architect has specified 150mm celotex between the rafters and then 50mm between the rafters

?? Seems strange wording

Care to clarify or change this? 🙂

[JohnMo]

16 minutes ago, flanagaj said:

150mm celotex

Good luck cutting that straight and square.

 

I would fill with cellulose or mineral wool. 

 

A drawings may assist visualisation.

 

Why not make it vaulted?

 

 

[flanagaj]

17 minutes ago, BotusBuild said:

?? Seems strange wording

Care to clarify or change this? 🙂

My mistake.  150mm celotex between the rafters and then 150mm below the rafters.

eg

[flanagaj]

16 minutes ago, JohnMo said:

Good luck cutting that straight and square.

 

I would fill with cellulose or mineral wool. 

 

A drawings may assist visualisation.

 

Why not make it vaulted?

MVHR pipes and I don't like vaulted ceilings.

What is the U Value difference between mineral wool and PIR?

[-rick-]

Given you want roof integrated solar. Make sure your roof make up is compatible. Specifically about fire resistance. I'm pretty sure I've seen that requirements have got tougher recently. Sorry can't remember more or point you to anything specific.

[JohnMo]

11 minutes ago, flanagaj said:

What is the U Value difference between mineral wool and PIR?

Depth for depth, done perfectly PIR on paper out performs mineral or natural wools and cellulose. But getting PIR to fit perfectly isn't the easiest job. So realistically natural or mineral wool may out perform PIR by a country mile. Plus there is decrement delay - how quickly the insulation allows heat to penetrate in to the room. The denser the better PIR is pretty rubbish.

[JohnMo]

Looking at the drawing, not sure how you will be able to assemble without loads of joints. The material between the rafters will be fine, just a pain cutting. The under drawn sheets you will never get them in big sheets in place there isn't the space.

 

Needs a bit of a constructability review 

[BotusBuild]

20 minutes ago, flanagaj said:

My mistake.  150mm celotex between the rafters and then 150mm below the rafters.

eg

If that's not a vaulted ceiling, as you say you dont want one, then it doesn't make sense to me.

 

I would "move" the insulation/PB/skim part to the ceiling of the room (the red horizontal line).

This would probably also address the fire risk mentioned above by @-rick-.

Might be worth a chat with the architect if you can

[-rick-]

You had another thread recently about trusses vs hand cut roof. If you aren't trying to vault the ceiling or preserve the option to build additional rooms in the roof then going for bog standard trusses is more attractive than otherwise. If planning a future expansion into the loft then I understand the placement of the insulation. If not, is it possible that the architect is working of old or incorrect assumptions?

 

If so could be building in extra cost/complexity in other areas too.

 

Edit: Also note your plans say zinc roof whereas another thread suggested youd choosen something else.

Edited yesterday at 16:22 by -rick-

[flanagaj]

30 minutes ago, -rick- said:

You had another thread recently about trusses vs hand cut roof. If you aren't trying to vault the ceiling or preserve the option to build additional rooms in the roof then going for bog standard trusses is more attractive than otherwise. If planning a future expansion into the loft then I understand the placement of the insulation. If not, is it possible that the architect is working of old or incorrect assumptions?

 

If so could be building in extra cost/complexity in other areas too.

 

Edit: Also note your plans say zinc roof whereas another thread suggested youd choosen something else.

It's all turned into a bit of a farce.  Roof has always been planned as a standing seam, but now we want to add solar.  I was hoping to fit the solar panels using the GSE mounting panels, so this sit nicely into the roof, but getting the flashing detail correct, does not look easy.

The cut roof was to simply save money as I was going to cut it myself.  My time is free as I get a good amount of annual leave.  But that said, if cutting the roof adds complexities elsewhere, then I may have to just go with trusses.

Only reason I opted to insulate between the rafters was so that we could have MVHR in the loft and I could board some of the ceiling floor for storage.  It's too low for anything of use. 

[-rick-]

3 minutes ago, flanagaj said:

Roof has always been planned as a standing seam, but now we want to add solar.

 

Just saying that the detail around a zinc standing seam is likely to be somewhat different to the version you posted about before. Zinc can be soldered/custom formed in a way that steel can't.

 

Quote

The cut roof was to simply save money as I was going to cut it myself.  My time is free as I get a good amount of annual leave.  But that said, if cutting the roof adds complexities elsewhere, then I may have to just go with trusses.

 

Seems to me that you might be underestimating how much time you need to get this (plus everything else done).

 

Quote

Only reason I opted to insulate between the rafters was so that we could have MVHR in the loft and I could board some of the ceiling floor for storage.  It's too low for anything of use. 

 

That is a good idea if it can be done without too much extra effort (or if it saves a lot of effort that would otherwise be spent dealing with the MHVR). If you aren't really targetting a specific size for the roof space - just enough for MHVR and maybe some other plant then there maybe options that avoid the tricky bits. Simplist that comes to mind is insulate under rather than between the rafters (losing space but its easier).

 

Need to think about airtightness carefully though.

[flanagaj]

2 minutes ago, -rick- said:

Seems to me that you might be underestimating how much time you need to get this (plus everything else done).

 

That is a good idea if it can be done without too much extra effort (or if it saves a lot of effort that would otherwise be spent dealing with the MHVR). If you aren't really targetting a specific size for the roof space - just enough for MHVR and maybe some other plant then there maybe options that avoid the tricky bits. Simplist that comes to mind is insulate under rather than between the rafters (losing space but its easier).

 

Need to think about airtightness carefully though.

Fair point regarding the amount of time I actually have.  Trusses will be much quicker and make it easier to install the insulation.  It's the air tightness part I am struggling with.  If you use wool, then that is obviously not going to perform as well as PIR which is taped along the rafters.

[ADLIan]

If the roof space is not being used and there is a horizontal ceiling use mineral wool at horizontal ceiling level. Will save you £1000s!!!

[Iceverge]

A 25 deg roof will be near impossible to insulate correctly to the eaves and airtighess will be a mare too. 

 

As @ADLIan  says you could do a cold attic.

 

You could still store stuff there. I would put an airtighess membrane and then a 20*70mm battened service cavity below the joists before a double layer of plasterboard. 

 

The MVHR could penetrate the membrane but you'd need to be diligent with taping the penetrations. Mineral wool or preferably cellulose over the top. 

 

 

 

[Iceverge]

If you do want a warm attic I would move the airtighess layer to above the rafters as a taped sheet of OSB that wrapped around the rafter ends and through the wall plate to the inner wall.

 

Have you a plan view or your roof and a bigger cross section? 

[flanagaj]

36 minutes ago, Iceverge said:

A 25 deg roof will be near impossible to insulate correctly to the eaves and airtighess will be a mare too. 

 

As @ADLIan  says you could do a cold attic.

 

You could still store stuff there. I would put an airtighess membrane and then a 20*70mm battened service cavity below the joists before a double layer of plasterboard. 

 

The MVHR could penetrate the membrane but you'd need to be diligent with taping the penetrations. Mineral wool or preferably cellulose over the top. 

That's not possible. I only have 2350mm of ceiling height as it is.

[Mr Blobby]

3 hours ago, flanagaj said:

Fair point regarding the amount of time I actually have.  Trusses will be much quicker and make it easier to install the insulation.  It's the air tightness part I am struggling with.  If you use wool, then that is obviously not going to perform as well as PIR which is taped along the rafters.

 

We have a similar roof construction to your design.  Cold roof, warm loft, standing seam on top.  It works well giving us a warm space in the loft and plenty of space for the MVHR in a warm space, but is not the cheapest option.

 

As others have said, use wool between the rafters and PIR under.  No way you will cut the PIR to fit snugly.  Fix and tape membrane under the PIR with battens.  If you want to plasterbard then fix the plasterboard to the battens, thus keeping the membrane intact.  We attached our cieling joists to a timber fastened to the wall under the joists so that we could run the airtight membrane behind it without being penetrated by joists.  Like this:

 

 

Frget about GSE inset panels on a standing seam roof.  Use S5 clamps instread.  And Standing seam is too nice for panels so install them at the back only.  You will understand as soon as the roof is on.

 

I'm assuming you have a timber on top of a steel ridge beam.  Its not clear from your drawings.  You need to keep the steel on the warm side, not in the middle of the insulation.

 

Assuming you have gable ends then build the inner leaf lower than the outer so that you can pack some insulation between the top of the wall and the timber directly under the standing seam.

 

We used sarking instead of ply or osb.  More time and cost but the roofers loved it.

 

In retrospect I would have rn the airtight membrane vertically instead of horizaontally.  I think it would have been easier to install crease free.  And better if the roofers had cut battens to install the membrane in sections but, for whtever reason, my roofers hate cutting battens.  

 

 

Edited yesterday at 20:41 by Mr Blobby

[JohnMo]

By far the easiest is to put the insulation on top of the rafters, is that possible?

[Mr Blobby]

4 minutes ago, JohnMo said:

By far the easiest is to put the insulation on top of the rafters, is that possible?

 

Our original design was to put the insulation on top and have a warm roof but it proved problematic.  I forget the detials but I think the SE didnt like it.  Something to do with screw length to fix PIR and sarking.

[flanagaj]

9 hours ago, Mr Blobby said:

 

We have a similar roof construction to your design.  Cold roof, warm loft, standing seam on top.  It works well giving us a warm space in the loft and plenty of space for the MVHR in a warm space, but is not the cheapest option.

 

As others have said, use wool between the rafters and PIR under.  No way you will cut the PIR to fit snugly.  Fix and tape membrane under the PIR with battens.  If you want to plasterbard then fix the plasterboard to the battens, thus keeping the membrane intact.  We attached our cieling joists to a timber fastened to the wall under the joists so that we could run the airtight membrane behind it without being penetrated by joists.  Like this:

 

 

Frget about GSE inset panels on a standing seam roof.  Use S5 clamps instread.  And Standing seam is too nice for panels so install them at the back only.  You will understand as soon as the roof is on.

 

I'm assuming you have a timber on top of a steel ridge beam.  Its not clear from your drawings.  You need to keep the steel on the warm side, not in the middle of the insulation.

 

Assuming you have gable ends then build the inner leaf lower than the outer so that you can pack some insulation between the top of the wall and the timber directly under the standing seam.

 

We used sarking instead of ply or osb.  More time and cost but the roofers loved it.

 

In retrospect I would have rn the airtight membrane vertically instead of horizaontally.  I think it would have been easier to install crease free.  And better if the roofers had cut battens to install the membrane in sections but, for whtever reason, my roofers hate cutting battens.  

 

 

Thanks for explaining the build up detail.  as I am most likely going to go with trusses, I don't think I can use the same build up as you have.

Point taken regarding the solar, but sadly, we have either a north facing roof or a south facing roof, so I think the decision is made on that one.  I am sure it will grow on us using the clamps.

I was hoping that we won't require a steel ridge beam.  The technical architect hasn't made mention of it and I am still trying to find an SE who responds to my request for a quote.



 

[flanagaj]

9 hours ago, JohnMo said:

By far the easiest is to put the insulation on top of the rafters, is that possible?

Not without increasing the ridge height and I am reluctant to put in a change for that at this late stage.

[Nickfromwales]

52 minutes ago, flanagaj said:

Not without increasing the ridge height and I am reluctant to put in a change for that at this late stage.

If you’re overlooked that is a possible issue, if you’re not then a ‘tweak’ of the ridge height often goes unnoticed. If it’s < +200mm then I doubt anyone will care tbf. 

[crispy_wafer]

Just be mindful, I wasnt...  With trusses you may get cross bracing included! makes insulating a pain in bottom as the bracing can get in the way, it can be done though, just takes a little more time.

[JohnMo]

Limited head room already, I really would explore the big room feeling of a vaulted ceiling, will make your life so much easier, the additional headroom will make room feels way less cramped. Make a central flat section for running MVHR ducts.