Warm roof or Cold roof

[Chanmenie]

Hi Guy's

Am in the process of getting designs and quotes for my roof / attic trusses, assuming i don't have any restrictions with ridge height,

should I go for warm roof with insulation over and between rafters or a cold roof with between and under.

From the research i have done it appears warm roof is the most desirable as it removes the cold bridge of the rafters.

But I would be very interested in your experiences 

[ProDave]

For me, definitely warm roof. 

 

But as you are having room in roof (attic trusses) I would go one further, don't use attic trusses but use a cut roof suspended from a ridge beam.  That is so much easier to detail air tightness ans means any eaves storage space or service space is inside the insulated and air tight envelope of the building.  I still maintain that was our best design choice.

[Iceverge]

Cold roof here but following tyveks Sealed roof guidance. Thermal bypass is a problem otherwise. 

 

 

 

Trussed roof with raided heals aka Denby Dale. 400mm of blown cellulose over the top.

 

 

Pros: Minimal thermal bridging and smaller surface area for heat loss. Very very simple to run a membrane and service cavity under the trusses. Cheaper. 

 

Cons:  you can hear the roofing membrane in the wind as it billows. Only works really for a non habitable roof space so perhaps not for you.

 

 

 

 

Edited December 15, 2021 by Iceverge

[joe90]

Like @ProDave I think warm roof like I have is easier to make airtight, and I also didn’t use trusses but a cut roof from i joists full filled with insulation then lined with OSB bubbly glued to create a sealed surface, this insulation extends seamlessly with the same insulation within the cavity ?.

[Chanmenie]

Thanks for the replies guy’s, so we are in agreement that warm roof is the way to go,

but I don’t get how a cut roof is easier to detail for air tightness than trusses.

@prodave is it any more difficult to insulate all the the way to the eves with trusses ? 
I was certainly planning to fully insulate so the voids are within the insulated airtight envelope as.my MVHR  manifolds and pipes will be in the voids 

[ProDave]

With trusses, you have various bits of wood, typically with a loft truss forming the wall of the dwarf wall, and then a diagonal between.

 

IF you are trying to make a proper air tight house, then you will be trying to sheet the whole of the inside with an air tight membrane with all joints taped. So what do you do?  Take the air tight membrane following the roof line right down to the eaves and cut and tape around every single timber that sticks out from the roof line on each truss?  A lot oif hard work to get it right.

 

A cut roof hung from a ridge beam has single rafters from ridge to eaves with nothing else.  One nice flat surface to tape and seal, and then you build up the dwarf wall structure inside the sealed envelope.

[Dave Jones]

trussed roof is no more difficult than a cut roof to seal. Make sure whichever VCL barrier you use (Alutrix is best) is brought down onto the walls and sealed. 

 

You need to think about lights as you want zero holes made into the VCL ideally.

[joe90]

4 hours ago, Dave Jones said:

trussed roof is no more difficult than a cut roof to seal.

Depends where the membrane is.

[Chanmenie]

with a warm roof you can wrap it round the outside under the battens and tiles, nothing in the way then 

[Temp]

+1 If building again I would go for a warm roof with most of the insulation above the rafters. 

 

Downside - long fixings needed. Builders don't have experience.

Upside - much easier to make the insulation layer airtight (no gaps)

 

Will need counter battens under the tile battens. 

 

 

[G and J]

On 15/12/2021 at 14:00, Iceverge said:

Cold roof here but following tyveks Sealed roof guidance. Thermal bypass is a problem otherwise. 

 

 

 

Trussed roof with raided heals aka Denby Dale. 400mm of blown cellulose over the top.

 

 

Pros: Minimal thermal bridging and smaller surface area for heat loss. Very very simple to run a membrane and service cavity under the trusses. Cheaper. 

 

Cons:  you can hear the roofing membrane in the wind as it billows. Only works really for a non habitable roof space so perhaps not for you.

 

 

 

 

Interesting discussion with our architect yesterday.   I expressed my intention to have a cold but sealed roof à la @Iceverge.

 

Reasoning:

Loft to be non-habitable, cheapest fink truss based, very limited use for storage.

Cold loft with insulation above plasterboard is cheapest and quickest solution.

Hate insecty lofts, with a passion (as I’m now sitting underneath one, humph).

Improved thermal performance.

Fits nicely with airtight strategy as I’ll put a VCL directly above plasterboard ceilings.  Yes I’ll need to faff around sealing round wires and MVHR ducts but I’m happy with that.

 

So, the architect then mentioned a drawback: it transpires that he’s heard of cases of condensation forming under specific circumstances on the inside of the ‘roofing felt’ (I’m a wrinkly, I know it’s technically breathable tile underlay but everyone knows where roofing felt sits!).   The circumstances are that it’s freezing conditions, roofing felt freezes and becomes vapour impenetrable, condensation forms.   Hmmmmmm.  Don’t like the sound of that.

 

Now one of my less positive attributes is that I think very slowly, and hours later I realised that this may be a symptom of the architect not being as used to working with airtight house designs as they could be.  My thinking is that if I achieve an airtight ceiling then little water vapour will reach the loft and thus little or no condensation will form.

 

So, is that a safe assumption?  Will a sealed cold loft above an airtight house have running water?

 

[G and J]

Would it have been more appropriate if I had started a new thread?