Roof Insulation Makeup?

[ashthekid]

Hi,

 

I’m currently having a discussion with my builder about what makeup of insulation is best for our large open plan kitchen dining lounge area which has high vaulted ceilings with exposed oak trusses. The original plan was to put 100mm PIR in between the rafters, then 100mm underneath the rafters before battening and plasterboarding. 
He has suggested we not bother with any insulation in between the rafters and just up the mm to 150mm underneath which obviously makes his job a lot easier and quicker but will I suffer any issues with such a large gap between the rafters being left just open? It would be approx 100mm open gap. 
surely I need something in there or would it actually be better with that large gap. 
 

It’s worth noting I am concerned about heating this large open room so want to insulate as much as possible to retain the heat etc. Just wasn’t sure what difference this change may make.

 

Thoughts?

[ProDave]

What stage is the build at?

 

If not too far, making it a warm roof with insulation above the rafters would be my choice.

 

If it is a cold roof you cannot usually completely fill the gap between rafters as you have to leave a ventilation gap, so definitely as much as you can fit allowing that gap and then more underneath.

[PeterW]

You’ll have an issue finding decent fixings to install 150mm insulation under the rafters and then affix plasterboard to it. It will need a 2 layer approach and probably use of battens etc.

 

The other issue is that using 150mm he is reducing your insulation by a fair bit - what depth are your rafters though as 100mm full fill PIR needs an air gap unless you’ve got breather membrane and counter battens installed. 

[nod]

I can see how this will help him 

Stick to plan A He must have priced  to do the insulation twice 

[Mr Punter]

How deep are the rafters?  The insulation between them can be about 50mm less than the rafter depth.

[tonyshouse]

I worry about draughts and thermal bypass especially into the battened void. 
 

I would say insulate between let me inspect it then over the top and then a full vapour barrier welted and sealed to adjoining air tightness layers or plastered into the walls, fully sealed around openings and only then the battened void 

[ashthekid]

@ProDave already well under way in the project so cold roof is my only option. Roof has been felt, battened and slate tiled already.
 

They are quite substantial rafters(not sure if exact size off the top of my head - would need to check tomorrow) and there is 100mm in everywhere else leaving a 25mm air gap I believe. This is the last bit to do after my original contractor disappeared on me. In the Rory I could get away with 75mm in between to save money to put towards a thicker layer underneath?

The main reason for us wanting to increase the PIR underneath is to cover over the purlins so we have a nice flat ceiling. The purlins are 175mm so in my head I’m thinking 130mm solid PIR, then double batten (2x25mm) which would get me to 180mm and able to plaster over the lot.
 

So you think definitely put a layer of breathable membrane on again underneath the rafters before the next layer of insulation? Even though there is a layer under slate tiles?

 

 

[PeterW]

1 hour ago, ashthekid said:

So you think definitely put a layer of breathable membrane on again underneath the rafters before the next layer of insulation? Even though there is a layer under slate tiles?

Not required and a waste of money 

1 hour ago, ashthekid said:

The main reason for us wanting to increase the PIR underneath is to cover over the purlins so we have a nice flat ceiling. The purlins are 175mm so in my head I’m thinking 130mm solid PIR

OK so this is your real reason…!!  Time to blend the materials here to get to your target and reduce costs, but I would think carefully about PIR here as you will need 230mm fixings through both battens and into the joist behind. That will take a lot of skill, and a fair chunk of cash to do properly. 
 

I would be getting creative here and creating a ceiling off the main rafters using 3x2 and OSB gussets so that the 3x2 is level with the bottom of the purlins. I’m guessing this would give 300mm of airspace below the roof, so it would then be full fill with Frametherm 35, and then a stapled VCL to hold all that in. Then add a 62.5mm plasterboard / insulation layer over the lot and you have a very good thermal and acoustic performing roof for probably 2/3rd of the price of the thick PIR and fixings etc.