Warm v's Ventilated Roof - Which Way to go ?

[bob the builder 2]

Hi All

 

It decision time on roofing at our build and i wonder if anyone had experience of specify the best approach (Warm v's Ventilated) 

 

The key constraint is roof depth as we are working to a tight height restriction.

 

Ideally we would like to go with a zinc or Greencoat PLX standing seam finish.

 

Reading around there seems to be various approaches to deliver the roof structure above timber rafters the TF company will leave us with.

 

Any experience in achieving the best mix of cost and depth would be greatly appreciated.

 

Thanks

Bob

[Iceverge]

Can you supply a cross section of your roof for clarity please? A sketch will do. 

[TerryE]

An estate agent gave me a piece of advice that proved invaluable: if you think that you might ever convert your loft into living space or warm storage, then by far the cheapest time to do it is when you build the house; you can always defer second-fit until when you need the space.

 

So we went for warm-loft and now my adult son who lives with us uses the loft floor as his bed-sit.

Edited March 12, 2022 by TerryE

[ProDave]

Room in roof or just an empty loft?

 

At the simplest level, warm roof would give a lower profile as no need to leave an air gap for ventilation.  I much prefer warm roof, so much easier to detail in many ways.

[Temp]

Assuming there is a room in the roof or might be in future..

 

Warm roof has the insulation above the rafters (eg rafters are on the warm side).

Cold roof has the insulation between or below rafters (eg rafters are on, or partly on, the cold side of insulation).

 

By that definition a warm roof is usually deeper overall.

 

Cold roof has options...

 

a) Ventilated void - if the membrane is NOT vapour permeable there must be a 50mm ventilated void below it. Typically the membrane drapes/sags 25mm into this void to prevent water being trapped above tile battens.

 

b) No void and counter battens -  if the membrane is vapour permeable you can reduce or eliminate the void. If you eliminate the void by fully filling between rafters you should add counter battens on the outside to raise the tile battens off the flat membrane and allow water to run down.

 

If the membrane is in contact with insulation (eg Cold roof with no void/drape) then you should use a membrane approved for that as not all are.

 

A room in the roof design (aka warm loft) can have any of the roof types above.

 

I would go with cold roof b) as the counter battens add 25mm but you avoid the 50mm void. So overall this saves you 25mm for a given thickness of insulation.

 

If height isn't a problem  I would build a warm roof with as much of the insulation above the rafters as possible. If you want to build a hybrid (some above and some between) then you must get an Interstitial condensation risk analysis done. I believe the recommended ratio 2/3rds above and 1/3 between. If you put more between the rafters that "moves" the top of the rafters nearer the cold side increasing condensation risk.

 

 

 

[Temp]

If building a house with a low pitch roof (so no prospect of a room up there) then you could consider a cold loft/cold roof design. This has the insulation at joist level not rafter level. 

[Temp]

https://www.kingspan.com/gb/en-gb/products/insulation-boards/insulation-technical-hub/articles-and-advice/what-is-the-difference-between-a-warm-pitched-roof

 

 

Quote

 

Warm pitched roof

A warm roof building will make the entire structure of the building warm in an attempt to avoid any cold bridging. It has s insulation layer above the rafters, and immediately below its weatherproof membrane. A warm roof construction has many benefits over a traditional ‘cold roof’, essentially it is a ‘breathable roof construction’, which allows moisture to escape which in turn prevents damp and any associated decay problems. A warm roof construction allows heat to be conserved within a property – without the need for a ventilation system.

A warm roof is recognized as being the most suited form of roofing to the UK climate, providing both a cost and thermal efficient solution. Kingspan Kooltherm K107, a premium performance phenolic insulation and Thermapitch TP10, a high performance PIR insulation are both fibre free, rigid thermoset insulation which can be used to insulate warm roof spaces in ventilated or unventilated pitched roofs, under tiles or slates.

Cold pitched roof

A cold pitched roof is where the insulation is placed either ‘between’ or ‘between and under’ the rafters or at ceiling joist level. This type of insulation system can be ventilated or not ventilated. However, if the roof is not ventilated then it will require a breathable membrane between the insulation and structure. If you live in an old building you may find that your roofing underlay is sarking felt, if so, you will need to allow for a 50 mm area of ventilation between the insulation and the top of the rafters.

 

 

 

 

[bob the builder 2]

Thanks to everyone for the input so far.

 

We are going with a well known TF provider who will leave us with a vaulted roof as per this picture (hope you can all see it)

 

So the question is if we want to go standing seam (zinc or PLX) what is the best in terms of depth and performance (we are just a bit tight on headroom hence the desire to make the depth as narrow as possible)

 

Thanks again for your thoughts

 

Bob 

[Patrick]

Always always ventilated. No question about that .... It s not even close.

It s so much more secure and less option for mistakes ...

On the paper , both might have pros and cons.... In reality I haven't seen much evidence for warm roof being a great idea.

[Radian]

11 minutes ago, Patrick said:

Always always ventilated. No question about that .... It s not even close.

It s so much more secure and less option for mistakes ...

On the paper , both might have pros and cons.... In reality I haven't seen much evidence for warm roof being a great idea.

Would you say that for flat roofs as well?

[TerryE]

2 hours ago, Patrick said:

Always always ventilated.

 Ours is -- by the MVHR -- because the top floor in the roof is part of our living space.  One a more serious point, having a ventilated cold roof is one approach, but not the only one from a design and construction PoV -- Yes, you've got to get the thermal design, RH gradients vapour barriers etc correctly placed, but this is all entirely doable and we've talked all of the issues to death on previous threads on the forum.   The main advantage of a cold roof is that poor builders are less likely to screw it up. 

[Patrick]

1 minute ago, TerryE said:

The main advantage of a cold roof is that poor builders are less likely to screw it up. 

Thats exactly the reason why I made that statement. Level of skill nowadays is shocking ... So better safe then sorry

[TerryE]

1 minute ago, Patrick said:

So better safe then sorry

 

I understand where you are coming from and you clearly have to guard against potential poor workmanship, but IMO that's a matter quality control and not a cause to accept that you've lost control of your subs before you begin. 

[PeterW]

@bob the builder 2 you’ve asked the wrong question and everyone is answering it ..!! If your TF provider is insulating using blown cellulose then you have a warm roof to start with. You can’t change that unless you change the insulation and a whole load of other things. 
 

What I think you are asking is whether to ventilate behind the standing seam zinc and the answer there depends on what the supplier requires as some will be happy to go onto your current counterbattens and others may require OSB or fleece backed underlay prior to laying. 
 

If you want to ventilate the rafter level space then you have a lot of work to do to join membranes and sort partial fill insulation which will need to be a change to your TF spec. 

[Patrick]

46 minutes ago, PeterW said:

@bob the builder 2 you’ve asked the wrong question and everyone is answering it ..!! If your TF provider is insulating using blown cellulose then you have a warm roof to start with. You can’t change that unless you change the insulation and a whole load of other things. 
 

What I think you are asking is whether to ventilate behind the standing seam zinc and the answer there depends on what the supplier requires as some will be happy to go onto your current counterbattens and others may require OSB or fleece backed underlay prior to laying. 
 

If you want to ventilate the rafter level space then you have a lot of work to do to join membranes and sort partial fill insulation which will need to be a change to your TF spec. 

To answer how to do that best (imho) . As i got the same setup in my house. Blown cellulose , Standing Seam on top :

 

 

 

Sarking boards on counterbattens above Timber frame filled with Cellulose.

 

[bob the builder 2]

All

 

Thank you as ever some great advice.

 

So i think this is what i must do then given my timber frame is specified to a 'warm roof' build-up.

 

Am i right to call this a 'Ventilated warm roof' ?

 

Many thanks 

[Patrick]

3 minutes ago, bob the builder 2 said:

All

 

Thank you as ever some great advice.

 

So i think this is what i must do then given my timber frame is specified to a 'warm roof' build-up.

 

Am i right to call this a 'Ventilated warm roof' ?

 

Many thanks 

Thats the correct setup . You can get away with putting tanalised timber boards cloesely spaced underneath the Standing seam instead of OSB - saves a bit of wood and a bit of Hairy chest - but both methods are pretty rock solid .

[Patrick]

2 hours ago, TerryE said:

 

I understand where you are coming from and you clearly have to guard against potential poor workmanship, but IMO that's a matter quality control and not a cause to accept that you've lost control of your subs before you begin. 

100% true and this is down to the individuals- so my view is highly biased as i dont trust and always try to eliminate the possibilty of getting things wrong as far as possible .  If you can trust your team or you have a site manager (are the site manager yourself) that is 100% accurate in his apporach -  other aspects are more important .

 

So i think the answer is as so often - it depends....

[Temp]

11 hours ago, bob the builder 2 said:

Thanks to everyone for the input so far.

 

We are going with a well known TF provider who will leave us with a vaulted roof as per this picture (hope you can all see it)

 

So the question is if we want to go standing seam (zinc or PLX) what is the best in terms of depth and performance (we are just a bit tight on headroom hence the desire to make the depth as narrow as possible)

 

Thanks again for your thoughts

 

Bob 

 

 

OK so some people will call this a warm roof because the loft space is heated or at least on the warm side of the insulation. I prefer the term "warm loft, cold roof".

 

This is a cold roof because part of the structural roof (the rafters) extend through the insulation to the cold side.

 

The gap between the metal roof and the membrane needs to be well ventilated. This would involve using a ridge system that provides a full length vent with matching vent at the eaves. The vent at the eaves would be equivalent to a 25mm wide slot. The vent at the ridge would be equivalent to a 5mm wide gap each side of the ridge "tile". Insect mesh may reduce the effectiveness area of the vent so allow for that by making them bit wider. 

 

It shows counter battens and I would expect horizontal battens to be added on top of these to raise the steel roof further to form the gap. The membrane should be a vapour permeable type (Exoperm is breathable) and approved for contact with the insulation (I haven't checked).

 

 

Metal roofs have a history of attracting condensation on the underside of the metal so I'd be tempted to increase ventilation above what normally recommended. Bigger vents at the ridge? As the roof supplier.

 

[Iceverge]

4 hours ago, Patrick said:

You can get away with putting tanalised timber boards cloesely spaced underneath the Standing seam instead of OSB

 

Excellent, I didn't know this. I never liked the idea of a flat sheet of metal on a flat sheet of OSB. Far to much chance of moisture between the layers not being able to dry out quickly.

 

The closely spaced battens is a far most robust idea. 10/10. 

 

 

 

[JamesP]

I have a ventilated warm roof fitted with Tata Steel SSR system. Pay attention to the closely fitted battens otherwise it is very noisy when the wind blows.

[Patrick]

2 hours ago, JamesP said:

I have a ventilated warm roof fitted with Tata Steel SSR system. Pay attention to the closely fitted battens otherwise it is very noisy when the wind blows.

Hairy chest underneath is the answer ( tyvek metal or Dorken DeltaTrela ....)

[Russell griffiths]

10 hours ago, Iceverge said:

 

Excellent, I didn't know this. I never liked the idea of a flat sheet of metal on a flat sheet of OSB. Far to much chance of moisture between the layers not being able to dry out quickly.

 

The closely spaced battens is a far most robust idea. 10/10. 

 

 

 

this is how we did ours. 

[Mr Blobby]

On 12/03/2022 at 11:20, bob the builder 2 said:

So the question is if we want to go standing seam (zinc or PLX)

 

 

 

Or aluminium for standing seam?  Something like prefa?

[TerryE]

Much as with @Temp and @Russell griffiths we have a warm-roof, where the roof profile was sarked, partly for structural reasons (to give racking stiffness to the loft storey)  and partly just to provide a physical barrier between the roofing and the house interior.  This was then "felted" with roofing membrane and vertically battened. The slater later added the horizontal counter battens in line with his slating plan.  We had a continuous horizontal breather at the eaves and under the ridge tiles to facilitate airflow between the felt and the slate to keep the under-slate area ventilated and dry.   

Edited March 13, 2022 by TerryE