Architect drawings- VCL Airtightness layer in right place? Any input on general detailing.

[dbyter]

Hi, new to the forum and this is my first post. Looks like there is a wealth of information here and in time I hope to be able to contribute something useful back once we are further on with our new build in Shetland. Our architect has just come with the preliminary drawings in preperation for the build warrant submission and I'm wondering if the VCL is in the best place within the makeup for fitting and achieving good airtightness? 

 

Thoughts or input on other aspects of the detailing welcomed.  

 

Thanks, David.

 

[JohnMo]

I would push back on 170mm kingspan between rafters and use a more flexible and very easy to correct mineral wool, something like Rockwool Flexi. Do it thicker to to U values. Same for walls, but you may need to increase the 25mm inside to something a little thicker.

 

Back to your question, it looks fine, well protected. On your 1st floor joists you may need to be wrapped to join the 1st and 2nd floor wall VCL to join.

[ETC]

How are you venting the roof?

[JohnMo]

1 hour ago, ETC said:

How are you venting the roof?

The sarking boards do that, with the breather membrane. When they fully dry there is a gap between each board. Not sure why the architect has shown battens on the sarking, with real slate they are not needed.

[dbyter]

Thnaks for input. I did ask about roof venting and my architect told me that because its natural slate on sarking boards - nailed with gaps betwen boards for movement - there would be enough airflow through the sarking boards and the small undulations which occur with natural slate. Hoping this to be the case.

Good point re vcl between 1st floor joists. We are going for factory injected PIR panels for walls but will ask him re pros/cons of mineral wool for between rafters. 

Thanks. 

[dbyter]

We are going for integrated solar and slate hook fixings-maybe we could lose the battens and counter battens?

Edited 15 hours ago by dbyter

[Gus Potter]

2 hours ago, dbyter said:

Hi, new to the forum and this is my first post. Looks like there is a wealth of information here and in time I hope to be able to contribute something useful back once we are further on with our new build in Shetland. Our architect has just come with the preliminary drawings in preperation for the build warrant submission and I'm wondering if the VCL is in the best place within the makeup for fitting and achieving good airtightness? 

 

Thoughts or input on other aspects of the detailing welcomed.  

 

Thanks, David.

Hello David.

 

Welcome to buildhub. You'll get lots of different ideas here.. the best thing is to mull them over and adopt the ones that suit you best. Lastly, excuse my spelling and grammer!

 

You asked for thought! The following is to give you some power to your elbow, not intended to be any form of critisism, after all you are still at the preliminary design stage. Shetland eh!. My sister lives in Tiree both can be increadibly windy with driving rain. As such you really have to almost go above and beyond the normal call of duty when considering the design details, which are crutial. 

 

I'll start from the top down.. the slates. You have two choices in terms of nailing. Traditional or modern.

 

Traditional method: In the islands for smallish slates we often go for one nail in the head of each slate then every third course head nailed and also cheek nailed. The cheek nails restrain the slates below as they have effectively 3 nails in a triangle. Now at the verges and eaves, around Velux or chimneys.. anywhere we are going to get horrible wind vortices we do one nail in the head and fully cheek nail. Now the great advantage of this is that it works! but also lets you maintain the roof for the next 60 years.

 

Modern method.. as per most manufacture's recommendations. Two nails in the head..but the roof will cost more to maintain in the long run, likely to leak more before you spot it. There is a tendency these days to use larger slates. each slate gets more wind load and the nailing points get more stressed.. more failures. Consider the benefits of a smaller sized slate.. even though they cost a bit more to buy and fix. The NHBC, slate suppliers only guarentee for say 10 years. If a slate cracks you don't see it as it does not often fall out as it has two nails in the head! In other words.. I often suspect that the suppliers and warranty providers are aiming to reduce their liability over the warranty period and don't care what happens after that. 

 

Suggestion: Have a chat with your Architect and if possible traditional local slaters.

 

This part of your drawing has potential conflicting information and design approaches.

 

 

The slates are shown on battens and counter battens on a breathable membrane on sarking. If you go traditional then the slates get nailed directly to the sarking, no battens or counter battens. At the eaves there is no undercloak slate shown, the overhang into the gutter is short. In Scotland we often use timber sarking. Us SE's make good use of the roof stiffening effect it provides. Normally we specify that the timber sarking has a 5.0 mm gap between the boards and is no wider than 100mm. This provides suficient repeating gaps to allow the breathable membrane to function. 

 

Now you don't have any eaves ventilation shown..that is ok as you have the sarking gap. Sometimes you see the permeable insulation between the rafters going right up to the underside of the sarking. Your drawings show a 50 mm gap. This indicates that there is possible a lack of understanding. A 50 mm gap is usually mandatory if you don't have sarking gaps etc and rely on eaves and ridge ventilation. Normally what we do, and I do a lot of this as do many Architects I work with, is to leave a 20 mm gap under the sarking.. just to disperse the water gas so it reaches the sarking gaps and gets out. Now that means you can make the insulation between the rafters 30mm thicker which takes it up to 200mm. Now often we are looking for buildability and at material procurement. It just so happens that TP10 etc is commonly available in 100mm thick sheets. And what you try and do is use same sheets where ever you can in the rest of the build. 

 

Your drawing shows a timber frame. The detail at the wall head looks off. The cavity barrier (fire stop) is not going to work as fire will bypass that. The fire stop batten needs to have a panel top rail or head binder behind it. You are building in Shetland.. it's also very wet at times (likely any wet timber delivered) will stay wet so you have to be much more aware of timber shrinkage. The external walls stay pretty much still but the kit shrinks thus you need to leave a gap between the top of the blockwork and anything connected to the timber frame. This is the time to get your head around some of this stuff in case it later causes havoc with the detailing of your windows and door height for example. On first glance this means you fascia boards need to be deeper as in Shetland wind driven rain goes upwards more often than you think!

 

Suggestion: Have a chat with your Architect about whether you are mixing and matching design approaches. It's ok to do that.. but you have to be clear about the design philosophy. 

 

Turning now to this top left part of your drawing:

 

 

 

In Scotland in the central belt BC 99% of the time accept render on block with no cavity tray. But where you are you get not just ferocious wind and rain but persistent rain and a bit of steady wind. This is also very onerous.. watch out! 

 

I would put cavity tray over the door and window openings and dress the breathable membrane over that. They don't need to always be expensive ones. A good brickie can form up a tray with a weep vents out of a standard roll of DPC.  The thermal break in the timber lintels should be further out. I would with my SE hat on omit this and if need be use deeper timbers for the lintel and lose possibly the timber frame (TF) top rail over the lintel. It's also got to do with timber shrinkage and the postition of the loading from the floor joists.

 

In terms of insulation. You show the opening reveals as having 25mm of insulation. If you are loking for a high performance house then aim for 50mm.. but then the windows don't fit! I often use 50mm reveal (called ingoes in Scotland) insulation but to make this work we need to ask the window / door supplier to include a thermally broken frame extension kit. 

 

Suggestion: Have a chat with your Architect, ideally make contact with BC and see if they will give you some advice, hands on experience of what works in Shetland. 

 

Turning now to this bottom left part of your drawing:

 

 

 

I'll not make any comment on the floor slab etc for now. But you'll need more of a drip on the window cill. NHBC require 40mm, the arrangemnt shown is not adequate. As it's TF you need a fire stop under the window cill and you'll need cassette vent to ventilate the timber frame cavity. 

 

Ok that is probably enough for now for you to digest. I would go back and review, learn a bit more before you spend too much time calculating in detail exact u values etc as once your SE get's their hands on this and Scottish BC you don't want to end up back at the beginning again.  

 

Hope this helps and all the best. 

 

 

 

 

 

[Gus Potter]

22 minutes ago, dbyter said:

I did ask about roof venting and my architect

Posts overlapped!

[JohnMo]

15 minutes ago, dbyter said:

We are going for integrated solar and slate hook fixings-maybe we could lose the battens and counter battens?

We did integrated solar and slates. The solar trays are screwed directly to sarking. The slates are copper nails direct to sarking. Breather membrane on sarking boards.

 

Slate hooks, do they comply with the Scottish wind loads?

[Gus Potter]

50 minutes ago, JohnMo said:

The solar trays are screwed directly to sarking.

Ok that works if you have an eaves and ventilated ridge with a 50mm air gap.

 

1 hour ago, dbyter said:

We are going for integrated solar and slate hook fixings-maybe we could lose the battens and counter battens?

Slate hooks are a nightmare, and dangerous for folk going onto the roof to maintain it. As part of HSE stuff you design to make sure, say in roofs, you don't have exposed sharp nail heads, slate hooks are nasty and are a puncture / trip hazzard. 

 

Why not just do the job properly? Slate hook are more associated with remedial works. 

[dbyter]

@Gus Potter Thanks for taking the time to do put your thoughts down. I will raise these with the architect. Point also taken re slate hooks. Lots to learn!

[dbyter]

@JohnMo Thanks also. I think they must comply as they are on a few roofs up here.

Edited 13 hours ago by dbyter

[Iceverge]

Kingspan salesman strikes again.........

[JohnMo]

9 hours ago, dbyter said:

@JohnMo Thanks also. I think they must comply as they are on a few roofs up here.

That really doesn't mean it complies. If you structural engineer says nails, that's what your structural design certificate covers. If you want hooks, make sure the structural engineer is happy.

 

10 hours ago, Gus Potter said:

Ok that works if you have an eaves and ventilated ridge with a 50mm air gap.

Pretty much don't comply with any of that. Full fill Icynene, open cell foam. Proctor Roofshield, so a very open membrane. Full double taped vapour control membrane and service cavity inside house.  GSE trays have plenty of air channels to allow ventilation. When generating plenty of thermosyphon driven ventilation also.

[torre]

43 minutes ago, JohnMo said:

Pretty much don't comply with any of that.

Warranty providers now want to see compliance with ventilation for solar. For example NHBC guidance changed last year to treat in roof solar as impermeable. If you do want to full fill below the membrane you'd need to batten above for ventilation.

 

Any single manufacturer could  certify their system doesn't need ventilation (not sure any do though) but I don't think you'll get that assurance for warranty fitting trays and separate panels.