Roof Buildup

[Mr Blobby]

Sorry if this is answered elsewhere but here goes...

 

Following the warnings on here about  warm/hybrid roofs we are going for a cold roof over a warm loft construction.  Well I thnk we are, I've not done this before, hence the need for some guidance on here.

 

The proposal from the architect is like this:

 

1. standing seam

2. standing seam breather

3. ply

4. batten + ventilation gap

5. breather membrane (replacing old roofing felt type)

6. batten + ventilation gap

7. PIR between rafters

8. Airtight membrane

9. PIR under rafters

 

Cut roof hanging off two steel beams AFAICS

 

Architect diagram like this:

 

 

 

So my questions are:

 

1. Cutting PIR between rafters seems like a challenge that will end up with lots of air gaps.  What about mineral wool between rafters and PIR below rafters?

2. Builder says we can do mineral wool between rafters, full fill up to the breather membrane.  The 50mm gap between breather membrane and PIR is not needed for mineral wool.  Is that right?

3.  Architect says mineral wool is no good above rafters because cold air will blow through it like a wooly jumper on a windy day.

4. Is this cold roof / warm loft thing a typical design?  Any general way to improve this?

 

I like the idea of mineral wool for the sound insulation too. Any advice on buildup here very welcome.

 

Edited July 30, 2023 by Mr Blobby
terrible spelling

[Gus Potter]

Can you post the drawing again as the notes are hidden a bit odd?

 

Also can you post a ridge detail and what you are doing up there?

 

Do you have dormers? what is happening there at that interface?

 

Are you putting an ensuite in the attic?

 

It does look like you have some kind of hybrid roof, I have nothing against them but you need to understand how the tiles are fixed, vapour control.. what you have posted looks far off what you need to certify the design is ok both in terms of insulation and tile fixing.

 

Oh and lastly has anyone thought how all that is going to work structurally? Can you tell us a bit more about that?

 

 

 

 

[Chanmenie]

Mineral wool is not as efficient as PIR. And if the rafters are plumb and square cutting the PIR should not be over challenging plus you can seal with gapotape or airtight foam. 
Have a look at Robin Clevit videos he shows a jig he made for cutting PIR that will keep the blade straight and true.
Im cutting PIR for my roof using two layers, 100mm and 75mm as thinner sheets are much easier to cut than one thick one, plus I’ll have a better chance of achieving the airtightness.

[Mr Blobby]

12 hours ago, Gus Potter said:

Can you post the drawing again as the notes are hidden a bit odd?

 

Here you go...

 

 

12 hours ago, Gus Potter said:

Also can you post a ridge detail and what you are doing up there?

 

Good question. No ridge detail available yet. I assume it is vented.

 

 

12 hours ago, Gus Potter said:

Do you have dormers? what is happening there at that interface?

 

No Dormers

 

 

12 hours ago, Gus Potter said:

Are you putting an ensuite in the attic?

 

No plans to do that.

 

 

12 hours ago, Gus Potter said:

It does look like you have some kind of hybrid roof, I have nothing against them but you need to understand how the tiles are fixed, vapour control..

what you have posted looks far off what you need to certify the design is ok both in terms of insulation and tile fixing.

 

Roof will be covered in standing seam aluminium, so no tiles.  Is there anything missing here?

 

12 hours ago, Gus Potter said:

Oh and lastly has anyone thought how all that is going to work structurally? Can you tell us a bit more about that?

 

Well I hope the SE and architect have figured this out right!

 

AFAICS there are two steel purlins supporting the roof.  Otherwise roof is standard 35 degree cut pitched roof with gable ends, not a hipped roof.  No dormers.

 

 

 

 

 

[Mr Punter]

17 hours ago, Mr Blobby said:

1. Cutting PIR between rafters seems like a challenge that will end up with lots of air gaps.  What about mineral wool between rafters and PIR below rafters?

Yes you can do this but lower insulation from the mineral wool.  Use Rafter Roll 32 for good performance.

2. Builder says we can do mineral wool between rafters, full fill up to the breather membrane.  The 50mm gap between breather membrane and PIR is not needed for mineral wool.  Is that right?

Yes.

3.  Architect says mineral wool is no good above rafters because cold air will blow through it like a wooly jumper on a windy day.

The mineral wool would be between the rafters with a breather membrane above. 

4. Is this cold roof / warm loft thing a typical design?  Any general way to improve this?

Yes it is fairly typical.  Make sure your final spec is approved by the standing seam manufacturers.

 

 

[Mr Blobby]

Thanks @Mr Punter

 

2 minutes ago, Mr Punter said:

3.  Architect says mineral wool is no good above rafters because cold air will blow through it like a wooly jumper on a windy day.

The mineral wool would be between the rafters with a breather membrane above. 

 

 

My mistake sorry, I meant mineral wool between rafters of course, not above. 🙄

So no ventilation gap is required between breather membrane and mineral wool between rafters.  That's good.

Our architect seems to be keen on PIR between rafters.  Not sure why.  We can hit our target u value with 150 mineral wool between rafters and PIR under rafters ok. 

 

Detail diagram has airtigtht membrane placed between layers of insulation.  I would have thought easier to install the airtight membrane under the insulation. 

At the purlins the membrane is just taped to either side of the steel I reckon.  Under, over, does it matter?

[Mr Blobby]

I'm going round in circles with roof buildup 😫 and decision time is here. 

We started out with a warm buildup but with spray foam between the rafters (as an airtightness strategy that should be difficult for a main contractor to get wrong) but I was not keen on. 

I have taken responsibility on site for the aritghtness so the foam will be replaced with an airtight membrane in the loft.

 

The architect then proposed the cold-roof, warm loft buildup above but I've been told by the insulation sales guy that it will present a condensation risk if we put metac between the rafters and then 100mm PIR underneath the rafters.  He's not done the condensation anlaysis on this yet though.

 

So, starting afresh, our requirements are:

 

1. We have no ridge height restriction to limit roof height so insulation on top of rafters is no problem

2. Standing seam aluminium covering

3. Some vaulted ceilings but about 90% ceiling josts and loft space

4. Cut roof supported by 2 purlins

 

What roof buildup can I install to get a low u-value pitched roof without condensation risk?  It can't be this difficult, surely 🤨

 

What about this....

 

1. prefa aluminium standing seam

2. seperation layer

3. ply

4. battened +50mm air gap

5. breather membrane

6. PIR 

7. Do I need another layer of ply in here between the rafter and the PIR ?

8. 150mm rafter

9. 150 mm Metac or similar between rafters, or does this hybrid roof present a condensation risk?

10. Airtight membrane like intello plus or Isover Vario

11. plasterboard

 

How much PIR can I put on top of the rafters to reduce the overall u-value?  Is there a sensible limit?  is this done in two layers and taped?

 

Roof construction is starting in about two weeks so I need to decide this now 😬.  Please help!

 

Edited August 13, 2023 by Mr Blobby