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

[gaschick]

1 minute ago, gaschick said:

SO, the timber frame is being made off site between 7-10th April. The windows were ordered from the timber frame design to fit into the timber frame in December, and are being delivered 8th April. This is a very common thing to do with the specifications on the timber frame being so accurate. Otherwise, we would maybe be waiting 3+months for windows to be delivered.  The top slope of the window is 13 degrees, but not shown terribly well in this plan! As you can see, there still isn't much space for the flashing at the point where the roof joins the exterior wall of the house. I hope that this drawing makes more sense.

Forgot to add, there are structural steels going into the walls as well, which complicates matters internally!

 

[SimonD]

16 hours ago, gaschick said:

the timber frame is being made...The windows were ordered

 

As per my earlier question, what has already been finalised and what is still open for re-evaluation and re-design? Presumably the windows were ordered when the timber frame was signed off for production, which means that's essentially fixed...

 

16 hours ago, gaschick said:

This is a very common thing to do with the specifications on the timber frame being so accurate.

 

Thanks, I really needed educating on that one 😉

 

16 hours ago, gaschick said:

I hope that this drawing makes more sense.

 

Unfortunately a 1:100 scale drawing isn't much help when doing the details as even a bold line can hide a multitude of sins equal to as much a 100mm. But I don't think your problem is with the eaves, it's probably with the detailing around your qausi dormer window. Do you know the actual dimensions or do you have a detail drawing for that dormer to roof area?

 

Assuming that your tf is not open to any change, then your options, if you want to go for a natural insulation, are to build up the total thickness of your roof either to the outside or to the inside of the timber frame. Each of those options comes with its own compromises. To the inside you'll lose ceiling height, which wouldn't be great at the lowest point as it's already fairly low. To the outside, it is probably easiest but you have the detailing around the window and possibly the eaves (the eaves are probably not a great deal in the grand scheme of things). To the outside you're probably looking at a minimum of 100mm extra.

 

Also, if you did go this way, you'd probably also then be looking at modifying the wall buildup to replace your pir in there with woodfibre as there doesn't seem much point only doing it on the roof, IMHO. This means implications across all the build.

 

HTH

[Kevin J]

Underlying question for any warm roof. Is the Vapour Control Layer adequate for purpose, adequately installed and 100% sealed against leakage into the roof void?

if the answer is a resounding Yes, a warm unventilated roof is possible. 
if in doubt however, ventilation. By way of a minimum 50mm void directly under the substrate timber, plywood or OSB3.

note: a ventilated Softwood under zinc is how it’s been done in Europe. Plywood and OSB3 were not around. The standing seam technique is also relatively new, in the grand consideration of zinc.

if you use OSB3 or Plywood almost every zinc manufacturer will recommend a Structured Underlay such as ISO-Mat Metal. Even if the rear of the zinc has been coated!

by far going with ventilated softwood, Vac Vac treated. (Avoid Copper treatments)

 

GreenCoat PLX is not susceptible to underside corrosion so you have greater liberty on the support structure. 

Aluminium is another proposal. More traditional detailing at penetrations. 

Metal cost will undoubtedly have a bearing.

 

the ventilation void must be increased on lower or longer slope lengths to potentially evacuate moisture build up - a chimney effect. 
 

wet insulation doesn’t perform correctly either. 
finally, we live in a wet climate. Moisture retained during construction has to escape. Ventilation