VCL in warm roof

[LA3222]

Now this may be a daft question and the answers may already be out there on the forum - I'm speedily working through all the posts and could have missed the relevant info.

 

I've read all the roofing posts - currently on foundations at the minute then that sections done, but a thought keeps niggling at me about the warm roof VCL that I can't recall reading about.

 

After reading @ProDave and others numerous plugs about sarking and warm roofs etc. I am sold that it is absolutely the way forward in my build.  It seems a no brainer to me.

 

So, as I understand it the roof makeup more or less is tiles, tile batten, counterbatten, breathable membrane, sarking (I'm sold on wood fibreboard), rafters, insulation fill between & below, VCL, plasterboard.

 

Right as I understand it, the internal VCL stops warm moist air from the house permeating into the insulation where it could condense at dew point in the insulation.  The external breathable membrane is there to stop moisture from rain etc. coming back into the structure but let moisture that may be in the insulation out - a one way system like a diode?

 

If my thinking is correct about the above, my niggle is how do I continue the VCL down from under the rafters to the external wall of the rooms below to ensure a sealed box essentially for the whole house? Do I have to faff about bringing the VCL down between each joist at 600 centres and then  taping/sealing around each one?

 

A lot of the pictures I have seen show the VCL in the bedrooms running along the ceiling - but surely that creates two separate boxes - the roof and then the house.  This would also mean every penetration through bedroom ceilings would need careful attention to ensure an air tight Seal?

 

If I do then so be it?  Just after a bit of clarity from those who have been there and have the t-shirt so to speak!

 

TIA for any guidance.

 

Jamie

 

[ToughButterCup]

This might help. Hope so.....

[ProDave]

My roof make up outside to in is:

 

Tiles

Tile battens

Counter battens

Protect VP400 breathable membrane

100mm wood fibre board

195mm rafters full filled with Frametherm 35

11mm OSB

Protect Barriair air tight membrane

25mm battens

Plasterboard

 

The Barriar air tight membrane continues down all the walls completely sealing the inside of the building, all joints taped, taped to windows and doors and sealed to the floor downstairs.  And a "Tony Tray" as described seals around the joist ends at the ground floor - first floor joint.

 

Here is the most complicated room, the big bedroom with the vaulted ceiling and mezanine floor. Only some of the battens for the service void are fitted in this picture

 

[Temp]

2 hours ago, LA3222 said:

So, as I understand it the roof makeup more or less is tiles, tile batten, counterbatten, breathable membrane, sarking (I'm sold on wood fibreboard), rafters, insulation fill between & below, VCL, plasterboard.

 

Right as I understand it, the internal VCL stops warm moist air from the house permeating into the insulation where it could condense at dew point in the insulation.  The external breathable membrane is there to stop moisture from rain etc. coming back into the structure but let moisture that may be in the insulation out - a one way system like a diode?

 

Yes. It's possible for rain and snow to blow up under tiles. The idea is it then runs down the membrane which is draped into the gutter at the bottom. The counter batten lifts the tile batten so water can run down under it.

 

If you plan to fully fill the space between the rafters you should check that the membrane manufacturer allows it to be in contact with the insulation. If you were a boy scout you will know that you shouldn't touch the inside of a tent when it's raining as that can cause the water to come through. Its not quite that bad but not all manufacturers recommend putting insulation in contact with their membrane. Last time I looked Kingspan make one that is approved for contact.

 

For some reason manufacturers that don't allow it to be in contact with insulation, still allow it in contact with the rafters (otherwise it would be no use to anyone). Go figure.

 

I've also used the VP400. It's pretty strong stuff. They say you can leave it exposed for a long time and I did just that. Had some exposed for a year on an out building and it was fine.

 

[LA3222]

I may be wrong but from what I've read I believe there are two varieties of membrane - tenting/non renting.

 

I believe that for a warm roof construction a non tenting membrane can be laid directly onto the sarking - in my case tgis will be something like 100mm wood fibreboard.  This will then have counter battens attached running from ridge to eaves on the rafters.  There will then be another set of counterbatten running horizontal to give something for the tiles to attach to.  The first set of batten ensures any rain etc that gets under the tiles can freely flow out into the gutter.

 

I think I have that right and therefore am comfortable with the external make-up of a 'warm' roof.

 

Internally I believe I can fill the rafters with insulation, I will probably then batten horizontally along the interior of the rafters and fill this with eps or some such.  This can then be covered with a VCL and plastervoard completing the job.

 

I think...i may be wrong.....that this is the way to go.  

 

The bit which blows my mind is the continuation  of the VCL from the loft to the external walls of the rooms upstairs to ensure there are no gaps in my VCL.

 

If someone can clear the fog it would be great.?

 

Getting ahead of myself maybe - missed that Ian and Dave have replied - will read in a minute.  Stuck in the middle of a rave at a holiday camp and two feral kids to track down!!

Edited July 28, 2018 by LA3222
Missed some posts that may have solved my ignorance!

[ProDave]

Just one point not mentioned, racking strength.

 

The Sarking board is a Scottish building regs requirement, partly to give the roof extra racking strength (English regs just let you drape roof felt over the gap and don't call for any racking strength)  Wood fibre board is not considered strong enough for the racking layer, which is why I instead have OSB lining the inside..

[LA3222]

Apologies for the crap picture, I'm on my phone so not the best.

 

It's the continuation of the VCL (red) across the junction (blue) that is puzzling me.

 

I'm sure I am missing the obvious but until it's pointed out I don't have a clue what the obvious is?

[LA3222]

3 minutes ago, ProDave said:

Just one point not mentioned, racking strength.

 

The Sarking board is a Scottish building regs requirement, partly to give the roof extra racking strength (English regs just let you drape roof felt over the gap and don't call for any racking strength)  Wood fibre board is not considered strong enough for the racking layer, which is why I instead have OSB lining the inside..

I didn't consider this - noted and OSB is now on my list!

 

I'm in England so sparking not required - but for the small additional cost (in the grand scheme of things) I am keen to increase the racking strength of the roof.

[ProDave]

5 minutes ago, LA3222 said:

Apologies for the crap picture, I'm on my phone so not the best.

 

It's the continuation of the VCL (red) across the junction (blue) that is puzzling me.

 

I'm sure I am missing the obvious but until it's pointed out I don't have a clue what the obvious is?

Quite a lot of people (like me) making a warm roof, do so for a room in roof application, and the roof is supported on a ridge beam, so you don't have that cross piece.

 

If your roof is made of pre made trusses, then I am afraid you are in for a lot if fiddly cutting and taping of the air tight membrane around each individual truss end.  If it's cut and made on site then you can do the Tony Tray idea and take the membrane around the  end of the horizontal timber then folded back and build the roof above it. 

[LA3222]

33 minutes ago, ProDave said:

Quite a lot of people (like me) making a warm roof, do so for a room in roof application, and the roof is supported on a ridge beam, so you don't have that cross piece.

 

If your roof is made of pre made trusses, then I am afraid you are in for a lot if fiddly cutting and taping of the air tight membrane around each individual truss end.  If it's cut and made on site then you can do the Tony Tray idea and take the membrane around the  end of the horizontal timber then folded back and build the roof above it. 

Cool, this confirms then in my mind that attic trusses = a lot of fiddly detailing! 

 

So logic would say that a cut roof would allow you to wrap the VCL around ceiling joist/rafter tie and then go about fitting the rafters from the eave up to the ridge board completing the triangular section.  Hopefully my thought process is right.

 

Seems I have some thinking to do about my roof so I have my COA established when the time comes.

 

The plans for my plot show two steels running the full length.  Having read all the posts in the roofing section it is my understanding these are purlins and will support the weight of my roof.  It appears to me that my design is a raised tie truss design.  So I think that this may be done as a cut roof, which means I can get the VCL running up the wall, behind the steels and then through the joint where the roof tie joins the opposing rafters and up to the apex of the roof.

 

I'm sorting out my electric issue at the minute so haven't formally discussed this with an architect.  I am on a crash diet of buildhub to learn and understand how/why things are done so I am able to have a two way conversation  with my architect about what I want to achieve and how.

 

FWIW I do not plan on deviating much from current plans so the roof will likely remain as is in terms of purlins carrying the weight etc.  I am mulling over swapping the steels out for glulam and making a feature out of them, but at 14.5m long the depth required may be too much to be practical.  

[ProDave]

I would take your roof design 1 step further and swap the ridge board for a proper supporting ridge beam, then the roof is supported by that an no need for a cross peice, leaving you the option of a full height vaulted veiling if you wish.  If you have not yet got that high it is not too late to re design it.

[JamesP]

I would recommend what @ProDave suggests if possible.

We have supporting ridge beam for 1st floor bedrooms ( 2 x 6m glulam) and main living area (Steel, 10.5m spliced).

[PeterW]

There are a lot more reasons for using a collar tie than just aesthetics and a ridge beam is not the answer to all roof designs. 

 

Collar ties are required in some roof designs where the engineer is using them to stop splaying at the walls, and also where steels or dormers are introduced. 

 

I’d also suggest that in standard construction (ie masonry walls with cut or truss roof) that OSB as racking isn’t required and actually adds complexity.