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Retroanaconda

New workshop project

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The tyvek on top of your ply has nothing to do with condensation build up or air moving over the top side of the plywood.

 

If a room is warm and the roof is cold, the meeting of both elements have the ability to produce condensation at some point.  As you are constructing a cold roof scenario, an air gap is needed in the way you are doing this with a 50mm gap between the insulation ( the warm side ) and the underside of the ply ( the cold side )

You will need vents at the ridge unless you tie both sides with a horizontal ceiling which allows air movement to flow from one side to another. With onduline sheets it is probably a better solution for you.

 

Even with a warm roof construction, a physical barrier ( usually dpc sheeting ) between both materials is needed due to condensation build up in the tiny gaps left when laying the insualtion.

 

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I see. Well the Tyvek technical manual that I’ve been using seems to think that an air gap between the insulation and the boarding/membrane is not required, and indeed promotes it for a sealed roof system. It states that due to the breathability the air gap is unnecessary.

Like this, except I’m using Onduline rather than slates/tiles. 

B83E8C90-E6AE-44A6-8D19-03B4986B802F.jpeg.2290263829697d4076ec12d5c5ce6fac.jpeg

 

I will have an air gap in any case, and it’s a garage not a house so the level of moisture accumulation in the roof structure will be comparatively low. The internal wall covering will be OSB as the increased resistance to moisture penetration is desirable on the inner/warm edge. 

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Perhaps we’re at cross purposes here 😉

Your picture shows a standard roof with loft insulation between the joists and no insulation in the rafters.  In this scenario, you really should still have some form of vent within the roof to allow air to flow through the loft, no roof should be sealed without proper air management.  The temperature fluctuations during the year will promote unstable air within a sealed system.
 

I was under the impression you were insulating the rafters ( the pitch timbers ) which requires an air gap behind the insulation.  

a cold roof system is this ( this shows a flat roof system, but it’s the same principal for any cold roof system )

 

DFC78E0D-88CB-4561-894E-DC22434EC8E1.jpeg.61796dea039d0e15836438ee74a4c97d.jpeg

 

so to clarify, if you’re insulating the joists ( the horizontal timbers ) then either vents or allowing the air to flow through the roof will suffice.

 

if you’re insulating the rafters ( the pitch timbers ) then an gap is needed behind the insulation.

 

 


 

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This interests me - when I got my garage extended I knew I'd be insulating the (flat) roof so got the builders to put vents in the soffits.

However, after having a panel down from the ceiling recently I notice there's some discolouration of the roof board above, despite (theoretically) plenty of air flow above the insulation panels.

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5 hours ago, FridgeFreezer said:

This interests me - when I got my garage extended I knew I'd be insulating the (flat) roof so got the builders to put vents in the soffits.

However, after having a panel down from the ceiling recently I notice there's some discolouration of the roof board above, despite (theoretically) plenty of air flow above the insulation panels.

 

Is the discolouration due to moisture?

 

Airlfow isn't a 100% answer to zero condensation, it can still happen in some circumstances. 

 

Airflow will always assist in drying out the cavity should moisture gather.

 

 

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I am insulating the rafters yes, and the picture does show the insulation going across the joists but the point being made there was that it shows said insulation pushed right up against the underside of the boarding and so there is no ability for air to flow from the soffit and up the rafter line under the boards/membrane. This image is maybe better - again it has slates rather than corrugated sheets but same principle and it shows insulation filling the full rafter depth with the moisture being allowed to pass through the boards and Tyvek to be taken away by air movement under the roof covering itself:

Capture2.JPG.b6e75dad9c0638f814c614f72311931e.JPG

Fully sealed and air-tight roofs are quite common these days, fuelled by the desire for ever-more energy efficient buildings, and the Tyvek system is designed to be able to work in this way. Quoting from the technical manual:

Quote

Tyvek® is a vapour permeable material which, as a Type LR roofing underlay (BS5250), will offer a low resistance to the passage of vapour. During the winter, when a building is heated and the internal vapour pressure is high, a Tyvek® underlay will, by diffusion, allow water vapour within the roof space to permeate through to the batten space. Natural air movement through the joints of the roof covering will subsequently allow any moisture laden air to escape to atmosphere.

The ability of a Tyvek® underlay to provide this function of condensation control eliminates the need to ventilate any roof voids between the underlay and the insulation.

This is designed to have a VCL on the inside which limits the passage of vapour into the roof structure, and my internal OSB sheathing will not achieve 100% impermeability in this regard. However this is based on an occupied dwelling, with much higher levels of temperature and humidity than a garage will experience and so I don't anticipate this being an issue.

It's all a bit academic really as I will have an airspace above my ventilation anyway - belt and braces.

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What you have quoted is for a standard roof set up where tyvek is used to replace older bitumen style underlay and is an accepted way of doing it by building control.

The picture  is a snapshot of a possible set up that Tyvek show as a way of doing it, but i've never come across this set up as i can see so much wrong with it, especially the notion that moisture can travel through the timbers. You'd be looking at securing your tiles to a system which will break down and rot over time if exposed to moisture. 

You could argue that timber batens have the same possible outcome, however timber batens are designed to sit beneath a roof which, if installed correctly, shouldn't allow mositure to penetrate. In the real world this isn't the case in many respects, but i've replaced bitumen style underlay on 60 year houses and the timber batens are still doing their job.

 

Like you said, it's academical as you're placing an airspace above the insulation anyway.

 

Look forward to your progress, although if it's anything like the 65mph winds and rain we have at the moment, i can't see you up the roof much today!

 

 

 

 

 

 

 

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There’s no issue with headroom.

A sliding door would need to go sideways to clear the fence, but I’m sure it would cost more than me making a couple of timber-frames hung doors. The only issue with that is sealing them at the bottom, but I’ll see what I can come up with.

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55 minutes ago, Retroanaconda said:

There’s no issue with headroom.

A sliding door would need to go sideways to clear the fence, but I’m sure it would cost more than me making a couple of timber-frames hung doors. The only issue with that is sealing them at the bottom, but I’ll see what I can come up with.

For the door seal at the bottom... What about a brush on the inside and outside faces? Or a removable seal for it to shut against at the bottom?

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Something like THESE  ??

I found them very good for draught proofing the top of a roller door as well.

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I put a hardwood threshold across the front - the door closes against it and it stopped rain getting blown in under the door.

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4 hours ago, Paddy said:

Something like THESE  ??

I found them very good for draught proofing the top of a roller door as well.

I used a long one on the top of my roller door too. Made a huge difference. 

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A brush would work but I’m not sure how it would fair against wind driven rain. Ideally I’d like to not have anything on the floor, so things can roll in/out easily, but I may have to sacrifice this for a good seal.

You can get shaped rubber profiles that glue down and then a rubber piece on the door engages with it when shut, which may be an option. Something like this. Still a 15mm ‘bump’ though. 

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9 minutes ago, Retroanaconda said:

A brush would work but I’m not sure how it would fair against wind driven rain. Ideally I’d like to not have anything on the floor, so things can roll in/out easily, but I may have to sacrifice this for a good seal.

You can get shaped rubber profiles that glue down and then a rubber piece on the door engages with it when shut, which may be an option. Something like this. Still a 15mm ‘bump’ though. 

That’s why I was thinking something removable. I’m going to do something for mine that stops the middle of the roller door being pushed in wards. 

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I've  used thin rubber belt at work to keep dust and water out if you attach so it causes the rubber to deform slightly thus putting some pressure to the ground to act as a seal it should be ok the other method would be to cut out a narrow drainage channel and cover with some heavy duty mesh or punched plate  so water runs away and you can still roll over drain channel could also be scraped out occasionally regards Stephen 

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6 minutes ago, landroversforever said:

That’s why I was thinking something removable. I’m going to do something for mine that stops the middle of the roller door being pushed in wards. 

On my shutter door there is a hollow round section of rubber seal that compresses when door is at the bottom works well regards Stephen 

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I’ve thought about a rubber trim on the door that would seal against the slab, if it was positioned well it might work. I’ll have to see once I get the doors made.

I’ve not quite finalised plans for the area in front of the workshop yet - there will be a concrete slab there but I’m not sure yet how it will be laid out. When I do the drainage around the workshop I will include capacity for a linear drain across the front of the building which will no doubt help. As will the eventual planned car port.

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Having your concrete slab sloping slightly away from the garage will also help apologies if I'm telling you how to suck eggs regards Stephen 

Edited by Stellaghost

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Something like this in front of the doors stops water being on the surface, and therefore stops it being blown in:

https://www.aco.co.uk/products/raindrain

If you've already poured the approach slab, apologies, but this totally soled the blown rain problems on my parents garage, which just has normal up and over garage doors, and very old ones at that (80s?).

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Aco drains are the norm for the drainage side in front of the doors, however if you can slope the slab away from the doors and fit the aco further away, then you are removing the notion of any standing water near the doors ( if they're wood )

I think the key is to be able to seal the bottom of the door to stop draughts etc entering the workshop as well being waterproof to some extent.

Having a flat entrance will pose difficulties in sealing and water resistant as over time, things move.

Our up and over garage door has a flat piece of rubber at the bottom to reduce draughts and water ingress, but we also have a small step ( 15 mm ) that the door sits against which helps hugely in water getting in

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Yeah that’s what I meant by linear drain, although I’d get a version rated for heavier loads. Slab out front not poured yet.

Having the drain means that this slab can be flat, or even slope very slightly towards the garage if necessary. This helps as the ground slopes that way and will drain water away from the house, which is arguably more important. 

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Thinking outside the box slightly. What about tapering the slab downhill and then making the door have the opposite taper so that you've got a downhill slope running out of the garage. You can then put a bit of foam / rubber etc., because in normal operation it's not going to be dragging across the floor.

Can knock up a diagram if that didn't make sense.

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Personally, having to drag a car uphill into a garage is a pain. 

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Wouldn't have thought you've got much else of an option in Wales or Scotland. If it's downhill or flat it's going to flood :ph34r:

  • Haha 1

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