Any issues with mixing Warm and Cold roof features in an in-rafter insulated roof

Carlow52

Builder has just mis-read the spec and used 125mm deep rafters instead of 175mm.

The spec called for a warm roof insulation design with 50mm 'overlay' across the outside and 175 between the rafters.

Adding extra insulation as 'overlay' is not an option due to height issues so am looking at either
1: deepening the rafters inside with a 50 by 50 and use the 175 insulation as planned.
2: add a 50mm layer of insulation/ possibly as a 62.5mm composite board across the underside.

This latter construction detail is more like cold roof detailing so wonder are there any issues with mixing the two types?
Thanks

ps any other suggestions welcome also

sydthebeat

Carlow52 wrote: »

Builder has just mis-read the spec and used 125mm deep rafters instead of 175mm.

The spec called for a warm roof insulation design with 50mm 'overlay' across the outside and 175 between the rafters.

Adding extra insulation as 'overlay' is not an option due to height issues so am looking at either
1: deepening the rafters inside with a 50 by 50 and use the 175 insulation as planned.
2: add a 50mm layer of insulation/ possibly as a 62.5mm composite board across the underside.

This latter construction detail is more like cold roof detailing so wonder are there any issues with mixing the two types?
Thanks

ps any other suggestions welcome also

flat roof or pitched?
deepening the rafters internally wont result in a warm roof construction so you would need to keep a 50mm air gap over the top of the insulation + felt.
In our climate i always think its a good idea to retain this air gap, because the construction has to be pin point perfect in warm roof construction to avoid rafters decaying....

Carlow52

Thanks: It is a pitched roof

I may not have made it clear what the issue is

the detail I wanted was
Slates
Battens
Breathable membrane
Counter battens
50mm overlay insulation across top of rafters
175 rafters c/w 175 insulation in-between
vapour barrier
plasterboard

This I believe to be a warm roof construction: if not then....:)

The detail, due to wrong rafter size, I am looking at is

Slates
Battens
Breathable membrane
Counter battens
50mm overlay insulation across top of rafters
125 rafters c/w 125 insulation in-between
50mm insulation across rafters inside
vapour barrier
plasterboard

or

Slates
Battens
Breathable membrane
Counter battens
50mm overlay insulation across top of rafters
125 rafters with 50mm added on underneath c/w 175 insulation in-between
vapour barrier
plasterboard

Does this second one not still maintain the warm roof construction, whereas the other one is a mix or warm and cold or am I way off?

No6

Carlow what insulations are you intending to use, both over the rafters and between. Because the between rafters insulation is bridged by the timber you may get away with perhaps adding 25mm on the overlay, (I know you think his isn't possible) but you don't say why? a few mm's (25 -50) will not be a major issue with compliance.

Slates

Carlow52 wrote: »

Thanks: It is a pitched roof

I may not have made it clear what the issue is

the detail I wanted was
Slates
Battens
Breathable membrane
Counter battens
50mm overlay insulation across top of rafters
175 rafters c/w 175 insulation in-between
vapour barrier
plasterboard

This I believe to be a warm roof construction: if not then....:)

The detail, due to wrong rafter size, I am looking at is

Slates
Battens
Breathable membrane
Counter battens
50mm overlay insulation across top of rafters
125 rafters c/w 125 insulation in-between
50mm insulation across rafters inside
vapour barrier
plasterboard

Your roof build up should read
Slates
Battens
Counterbattens (fixed with structural fixings ,Helifix, through the rigid insulation into the rafters)
Breather membrane (IAB approved)
Rigid insulation over rafters
Insulation between rafters
Vapour barrier
Plasterboard
internal finish

Bit of a pain in the behind to construct

Easier construction (robust detail)

Slates
Battens
Counterbattens
Breather Membrane
Rafters infilled with insulation
Composit insulation/plasterboard with integrated vapour control to underside of rafters
Internal finish

In both details the BM goes between the counterbatten and insulation
and not the batten and counterbatten

Carlow52

Thanks quys:
re the extra on the outside, there are window sills just above the top of the A's which limit the scope as the slope is tight enough as is.

Re the position of the breather membrane, the installation instructions for the product show it between the two battens. will PM the details
I appreciate the HB manual shows it otherwise.:)

Slates

Carlow52 wrote: »

Thanks quys:
re the extra on the outside, there are window sills just above the top of the A's which limit the scope as the slope is tight enough as is.

Re the position of the breather membrane, the installation instructions for the product show it between the two battens. will PM the details
I appreciate the HB manual shows it otherwise.:)

Get a look at the BM Iab cert, no counterbattened BM on the Irish market works (I stand to be corrected) as a non-vented roof with the BM between the batten and Cbatten.

RKQ

Slates is correct about the counter batten, the breather felt is under the counter batten.
(Its called a "counter batten" because it is positioned counter or perpendicular to the slate batten) The counter batten maintains air flow to the underside of the slate etc.

What exactly is the problem you are refering too ? - window sills just above the top of the A's.

All BM manufatureres usually show counter batten in there literature. I would avoid mixing types - use one contruction or the other.

Carlow52

Thanks for replies, much appreciated

http://www.nsai.ie/modules/certificates/uploads/pdf/IAB040157.pdf

shows, on page 3, in figure 3 and 4 the setup for sealed and vented roofs.

Slates is right for unvented/sealed roofs. Batten/counter-batten/ taut BM

for vented: batten/ draped BM/counter-batten


In relation to the window sills: the roof in question is an extension to an existing dwelling where the second story windowsills limit the height of the A roof and to achieve a decent slope the A is already at the limit vis-avis the clear height to underside of sill

topcatcbr

Carlow52 wrote: »

Thanks for replies, much appreciated

http://www.nsai.ie/modules/certificates/uploads/pdf/IAB040157.pdf

shows, on page 3, in figure 3 and 4 the setup for sealed and vented roofs.

Slates is right for unvented/sealed roofs. Batten/counter-batten/ taut BM

for vented: batten/ draped BM/counter-batten


In relation to the window sills: the roof in question is an extension to an existing dwelling where the second story windowsills limit the height of the A roof and to achieve a decent slope the A is already at the limit vis-avis the clear height to underside of sill

Would you not consider moving the sill and replacing window.

If the insulation is outside timber no need to ventilate
if the insulation is inside timber you require min 50mm ventilation (interstatial condensation forms within the timber as warm air passes through the rafter from the warm inside face to the cold outside face) no matter where the breather membrane is placed. the ventilation removes this

Carlow52

Thanks for all the answers; much appreciated.

Following on, the spec for the job as priced by the builder states that all current regs to be met.

The sloped ceiling finish is 18/20mm TGV timber board.

Builder of opinion that this will meet fire regs and that no need for plasterboard over the vapour barrier. The HB manual does not specifically cater for this design.

Would welcome any insights: my instinct is to put on the plaster board.
Thanks

Slates

I have a non-vented warmroof with the following build-up

Slates
Battens
Counterbattens
Breathermembrane
222x47mm rafters at 400 centres

I am going to full fill the rafters with a combination of glassfibre and quinntherm

Which of the following options would be better

1, glass fibre against the Breather Membrane with 100mm quinntherm underneath the glass fibre flush with the underside of the rafters.
then a vapour check followed by 12.5x38mm plaster/laminate board

2, 100mm quinntherm against the BM with the glass fibre flush with the underside of the rafters,then a vapour check followed by 12.5x38mm plaster/laminate board

From a breathability stance does it matter what way it is done? Will the glass fibre and quinntherm give the same u-value what ever the arrangement?

sydthebeat

Slates wrote: »

I have a non-vented warmroof with the following build-up

Slates
Battens
Counterbattens
Breathermembrane
222x47mm rafters at 400 centres

I am going to full fill the rafters with a combination of glassfibre and quinntherm

Which of the following options would be better

1, glass fibre against the Breather Membrane with 100mm quinntherm underneath the glass fibre flush with the underside of the rafters.
then a vapour check followed by 12.5x38mm plaster/laminate board

2, 100mm quinntherm against the BM with the glass fibre flush with the underside of the rafters,then a vapour check followed by 12.5x38mm plaster/laminate board

From a breathability stance does it matter what way it is done?

Will the glass fibre and quinntherm give the same u-value what ever the arrangement?

If you use quinntherm PU insulation then your construction isnt breathable. Therefore you need to ensure that your vapour check layer is absolutely perfectly sealed at every junction. Worst case senario would be weretheres a leak in the VC allowing moisture / vapour into the roof construction. This moisture is drawn to the external due to pressure differentials..... It cannot pass through the PU board so its only outlet is through the timber joists... This leads to very serious problems of decay /rot and loss of structural integrity..

Therefore my opinion would be either to make a construction completely breathable, or completely non-breathable. You get problems when you have a mixture of both.