Warm pitched roof vs cold pitched roof

GraceKav

Just wondering what people's feelings on warm vs cold pitched roofs is? We are getting conflicting advice with one professional advising a warm pitched roof and another a cold pitched roof. We are trying to build to 1m3/hr/m2 so airtightness is a key consideration. Thanks a mil.

BryanF

Can you give the exact material build up of what you mean by the two options? It’s not really as simple as cold v warm - especially with a pitch roof

GraceKav

For warm roof, two spec options we have are:
1.
plasterboard
125mm spray insulation between rafters
150mm rafters
100mm kingspan over rafters
counterbatten
membrane
slate batten
slates

2.
220mm rafter
175mm xtratherm xt/pr rafterloc between rafters with 82.5mm
(70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab

For cold roof:
400mm mineral wool insulation
82.5mm (70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab

I am expecting that all of these options would be sufficient but given that we have conflicting advice on it, curious what people preferences are?

We think the below is true but would welcome advice please.
- Mineral wool has a better flammability rating than sprayfoam insulation
- Spray foam insulation likely cheaper
- Benefit of warm roof means less cutting into the envelope for mvhr pipes etc and mvhr pipes would be inside thermal envelope

BryanF

My Preference would be warm roof. But you can use cellulose with this type of build up

Re fire: Mineral wool better than domestic foam products : yes
Foam cheaper: not really
Mvhr pipe runs will need circa 100/150mm dia, and should be insulated : yes better within thermal envelope.

Can you outline who is advising you of these options?

sydthebeat

GraceKav wrote: »

For warm roof, two spec options we have are:
1.
plasterboard
125mm spray insulation between rafters
150mm rafters
100mm kingspan over rafters
counterbatten
membrane
slate batten
slates

2.
220mm rafter
175mm xtratherm xt/pr rafterloc between rafters with 82.5mm
(70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab

For cold roof:
400mm mineral wool insulation
82.5mm (70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab

I am expecting that all of these options would be sufficient but given that we have conflicting advice on it, curious what people preferences are?

We think the below is true but would welcome advice please.
- Mineral wool has a better flammability rating than sprayfoam insulation
- Spray foam insulation likely cheaper
- Benefit of warm roof means less cutting into the envelope for mvhr pipes etc and mvhr pipes would be inside thermal envelope

there are some obvious questions and issue with the above specifications


1. is a warm roof. How do you spray 125mm between 150mm rafters??? i assume the other 25mm not mentioned is the vent card. if so, why do you need it in a warm roof construction? Have you costed up the fixings for the 100mm PIR over ?? you might have a heart attack when you do.

2. is not a warm roof construction.

The cold roof specification.... are you talking here about insulating on the flat? if so thats not whats considered in a 'warm' or 'cold' roof... in these, we are referring to insulated on the slope.
If your talking about insulating on the slope... how do you install 400mm quilt?

GraceKav

BryanF wrote: »

My Preference would be warm roof. But you can use cellulose with this type of build up

Re fire: Mineral wool better than domestic foam products : yes
Foam cheaper: not really
Mvhr pipe runs will need circa 100/150mm dia, and should be insulated : yes better within thermal envelope.

Can you outline who is advising you of these options?

Thanks for the advice. These are from our engineer and our energy consultant. Trust both of them but just confusing when we’re getting such conflicting advice. Re the options outlined, yes personal preference comes into it but I suppose is there really much of a variance performance wise at the end of the day and cost wise? Might not figure out the latter until we get the tenders back. At the moment gone out to tender based on the flat roof insulation and the number two spec for our vaulted roof in one of the rooms.

GraceKav

sydthebeat wrote: »

there are some obvious questions and issue with the above specifications


1. is a warm roof. How do you spray 125mm between 150mm rafters??? i assume the other 25mm not mentioned is the vent card. if so, why do you need it in a warm roof construction? Have you costed up the fixings for the 100mm PIR over ?? you might have a heart attack when you do.

2. is not a warm roof construction.

The cold roof specification.... are you talking here about insulating on the flat? if so thats not whats considered in a 'warm' or 'cold' roof... in these, we are referring to insulated on the slope.
If your talking about insulating on the slope... how do you install 400mm quilt?

I’m probably really confusing things with my terminology here. Find this stuff so confusing. My understanding was that a warm roof insulation was when the thermal envelope was sealed at roof level and cold roof where it was sealed on the flat leaving the roof pitch outside the thermal envelope. This isn’t correct?

1 and 2 are the two proposed options for insulating on the slope. We have a vaulted ceiling.

1 was proposed to be the insulation method used for the whole house including the vault by one professional.

2 and 3 methods were to be used for the vault and the rest of the house respectively by the other professional. The 3rd method was for insulating on the flat when we don’t have a vaulted ceiling. Hope this clarifies.

sydthebeat

they are all pretty heafty specifications, a good bit above what i would consider the standard min u values in current building regs.
why are you trying to reach these u values, may i ask?

so, only the first specification is a warm roof.... (the best way to recognise this is where there is insulation specified 'outside' of the structural roof timbers.

the second one is a cold roof

both are for a vaulted sloped ceiling

and the third one is just insulating on the flat section of the roof.

GraceKav

sydthebeat wrote: »

they are all pretty heafty specifications, a good bit above what i would consider the standard min u values in current building regs.
why are you trying to reach these u values, may i ask?

so, only the first specification is a warm roof.... (the best way to recognise this is where there is insulation specified 'outside' of the structural roof timbers.

the second one is a cold roof

both are for a vaulted sloped ceiling

and the third one is just insulating on the flat section of the roof.

Thanks a mil. I was hoping that would be the case and that all 3 would be where we want them to be re performance. We are going for as low energy as we can afford and so are aiming for very low airtightness, <1 m3/hr/m2.

Thanks for the clarification re cold vs warm, that makes a lot more sense. I thought it was referring to the temperature of the attic space :rolleyes:

Thanks for the help.

Calahonda52

OP:
Just looking at this one first:

I appreciate that you may not be comfortable with the terminology but stick with this, you are doing fine thus far

1.
plasterboard
125mm spray insulation between rafters
150mm rafters
100mm kingspan over rafters
counterbatten
membrane
slate batten
slates

1: where is the AT layer in the above?
2: what is the spec on the membrane?
3: why is it between the two battens?

Assuming the pb and KS are both foil lined, then, even if the foam is breathable, the rest of the roof is not, so if not then why the counter batten.
My concern here is that the professionals here are mixing and matching the materials without fully understanding the way each one works, with a focus on cost and at the same time being able to tell the client they are using breathable materials etc.
These membranes are not cheap but I have seen many jobs where they are used like in the spec above,or similar, complete waste of money.

The iron clad test for where the AT layer is to following it around on a section drawing of the house, without lifting the pencil


Have a look here for ideas and buildups.
https://www.ecologicalbuildingsystems.com/Ireland/Products/Pro-Clima/Windtight-Systems

The other question is for the vaulted ceiling, is there a service cavity inside the AT layer?

ps, reading the post below: same question applies if using the IB under the ceiling rafters: how will lights etc be dealt with?
Finally 400mm in the attic will require good planning when laying power cables, can't bury in the insulation unless ducted to allow air cooling

sydthebeat

GraceKav wrote: »

Thanks a mil. I was hoping that would be the case and that all 3 would be where we want them to be re performance. We are going for as low energy as we can afford and so are aiming for very low airtightness, <1 m3/hr/m2.

Thanks for the clarification re cold vs warm, that makes a lot more sense. I thought it was referring to the temperature of the attic space :rolleyes:

Thanks for the help.

are you going passive certed, or have you a target BER rating?

there is a point at which diminishing marginal returns makes it uneconomical.
the flat roof u value must be something like 0.07 which is incredibly low.... and in my opinion a 32.5mm board would do just as much in reality to insulate against heat loss, as an 82.5mm board... though on paper it wouldnt.

you must have a very generous budget to play with

GraceKav

Calahonda52 wrote: »

OP:
Just looking at this one first:

I appreciate that you may not be comfortable with the terminology but stick with this, you are doing fine thus far

1.
plasterboard
125mm spray insulation between rafters
150mm rafters
100mm kingspan over rafters
counterbatten
membrane
slate batten
slates

1: where is the AT layer in the above?
2: what is the spec on the membrane?
3: why is it between the two battens?

Assuming the pb and KS are both foil lined, then, even if the foam is breathable, the rest of the roof is not, so if not then why the counter batten.
My concern here is that the professionals here are mixing and matching the materials without fully understanding the way each one works, with a focus on cost and at the same time being able to tell the client they are using breathable materials etc.
These membranes are not cheap but I have seen many jobs where they are used like in the spec above,or similar, complete waste of money.

The iron clad test for where the AT layer is to following it around on a section drawing of the house, without lifting the pencil


Have a look here for ideas and buildups.
https://www.ecologicalbuildingsystems.com/Ireland/Products/Pro-Clima/Windtight-Systems

The other question is for the vaulted ceiling, is there a service cavity inside the AT layer?

ps, reading the post below: same question applies if using the IB under the ceiling rafters: how will lights etc be dealt with?
Finally 400mm in the attic will require good planning when laying power cables, can't bury in the insulation unless ducted to allow air cooling

I'll be honest, you've lost me on some of the above, sorry. I'm guessing you're more of a fan of spec 2 and 3 so? Can I check what the AT layer is (prob silly question I know)...

For the vaulted ceiling, the MVHR will be either at the gable end or we have a false bulkhead over the windows which can have it. We're not doing spots so the only things running though the ceiling will be cabling for pendant lights. Hadn't thought to ask about the cabling under the insulation on the flat. Will get querying on this.

Thanks for the help. I'm so lost on all this stuff but the advice on here gives me a place to start in asking questions. Really appreciate it.

GraceKav

sydthebeat wrote: »

are you going passive certed, or have you a target BER rating?

there is a point at which diminishing marginal returns makes it uneconomical.
the flat roof u value must be something like 0.07 which is incredibly low.... and in my opinion a 32.5mm board would do just as much in reality to insulate against heat loss, as an 82.5mm board... though on paper it wouldnt.

you must have a very generous budget to play with

We're not going passive. Like you say above, diminishing returns. We are trying to get as highly insulated and airtight as we can though. Think the u value on the flat rood is 0.1.

Definitely wouldn't say we have a massive budget but we've cut back on size and number of rooms in places so that we can build to as good a spec as possible. Planning this to be our forever home so makes sense to us to put the money into the fabric.

Calahonda52

GK:
re this
I'll be honest, you've lost me on some of the above, sorry. I'm guessing you're more of a fan of spec 2 and 3 so?

No, just trying to figure out the build up

Can I check what the AT layer is (prob silly question I know)...
well if you are going for
We are trying to build to 1m3/hr/m2 so airtightness is a key consideration

You need to know
I would have expected the following: so if its not that, I ask

1. plasterboard
2. [optional batten for service cavity, can be insulated]
3. Air tight/breathable layer
4. insulation between rafters: breathable
5. rafters
6. breathable insulation over rafters
7. wind tight breathable membrane
8. counterbatten
9. slate batten
10. slates

Is it possible the order of the two battens was mistyped?

GraceKav

Thats entirely possible. I may have just gotten the order mixed up when reading from the plans.

Doing our best to educate ourselves on this but still a work in progress clearly! Thanks for the help.

Calahonda52

GraceKav wrote: »

Thats entirely possible. I may have just gotten the order mixed up when reading from the plans.

Doing our best to educate ourselves on this but still a work in progress clearly! Thanks for the help.

Glad to be of some assistance.
Just to explain the design concepts:

once moist air gets through the AT layer, as it will, the buildup allows it to pass through all the way to the air space created by the counter batten,if positioned correctly, which is a ventilated space.
this negates the need for the old style ventilated soffits in trad design houses.

the other thing that occurred to me is re the vaulted ceiling:
depending on the aspect, it may be subject to solar gain in the summer which might make it uncomfortable.
research decrement delay
http://www.greenspec.co.uk/building-design/decrement-delay/
essentially you need a denser insulation than the XP/EPS/wool.

Have fun and keep up the learning

GraceKav

Calahonda52 wrote: »

Glad to be of some assistance.
Just to explain the design concepts:

once moist air gets through the AT layer, as it will, the buildup allows it to pass through all the way to the air space created by the counter batten,if positioned correctly, which is a ventilated space.
this negates the need for the old style ventilated soffits in trad design houses.

the other thing that occurred to me is re the vaulted ceiling:
depending on the aspect, it may be subject to solar gain in the summer which might make it uncomfortable.
research decrement delay
http://www.greenspec.co.uk/building-design/decrement-delay/
essentially you need a denser insulation than the XP/EPS/wool.

Have fun and keep up the learning

Had to go do some research

Presuming when you say decrement delay, you're saying the likes of Wood fibre, cellulose, softboard would be better as roof insulation? Have heard that cellulose is very expensive?

Calahonda52

Can't comment on price.
my point is that many professionals spec the build up using all the right materials for full breathability/ solar gain etc and then the client calls them out on cost.
The first thing to get substituted is the insulation, mostly for "XPS"

They leave the expensive inner and outed membranes, which are now worthless.

GraceKav

We've decided to go to go with the below. Tender has come back in and its just within budget so going to trust the professionals we've hired and push the button on it. Based on your feedback above don't feel that we're a million miles away. Thanks for the advice and tips here guys. Will be asking about ducting of power cables for cooling and ensuring the MCHR ducts lie under the mineral wool insulation. Feel like it might be too late to change spec to something with higher decrement delay for the vaulted portion so just going to have to trust that the difference won't be huge. Appreciate the help on this.

2.
220mm rafter
175mm xtratherm xt/pr rafterloc between rafters with 82.5mm
(70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab

For cold roof:
400mm mineral wool insulation
82.5mm (70mm insulation and 12.5mm plasterboard) xtratherm liner insulated slab