What type of boarding above rafters

[Ed Davies]

3 hours ago, scottishjohn said:

but you have to have some faith that a breathable membrane actually works --they are a one way device -they breath out wards,not inwards 

 

As @JSHarris says, they're equally “breathable” both ways; the point is that they let vapour through while blocking liquid water.

 

There are a few “intelligent” membranes which are supposed to be in some sense one way. AIUI, they vary in their permeability depending on the vapour pressures with the idea that they'll let water in the roof/wall out more easily than they'll let it in. I don't understand the details beyond that but they aren't relevant to this discussion anyway as they're used on the inside except that their existence does support the notion that normal breathable membranes are two way.

[scottishjohn]

the point is a mute one in the real world., 

talking  in relative terms

if the air outside membrane  is cold  and on other side of membrane  it is hotter 

then vapour is not going to move inwards as hot air expands and cold contracts ,so the natural flow will always be from hot to cold ,so vapour will always  want move outwards

and  any condensation dripping from roof slates  etc that forms  on outside of membrane  is not a vapour --so it will not pass though anyway .

I would be first to agree if you have lots of vapour already on inside of membrane,which condenses ,then it will not flow through until it is vapour   .

so we go back to what i said in the beginning .

when you clad roof with OSB -then you must fit membrane same day if possible -before you wet the osb and potentially trap  vapour.

It is highly unlikley that vcl will have been fitted anyway at this stage , so maybe a couple of days before membrane is fitted  will not be the end of the world

it can dry out inwards untill the insulation  and  the vcl is fitted anyway .

this is why kit houuse suppliers who make panels have then pre membraned before arrival on site -to keep them dry 

I really don,t see anyone fitting osb in freezing temps anyway,so no worry on ice formation   

your VCL is what you must get right  and then there can be no big amounts of vapour coming from house on inside to condense

 give it a few weeks after building to settle and it will just get drier all the time on inside of membrane as what ever vapour is trapped there moves outwards 

 

[Jeremy Harris]

The movement of water vapour has nothing at all to do with hot air expanding, or cool air contracting.

[scottishjohn]

1 hour ago, JSHarris said:

The movement of water vapour has nothing at all to do with hot air expanding, or cool air contracting.

 

temp has an effect on how much vapour air can hold before it becomes saturated and becomes liquid  and therefore cannot pass the membrane .

 

so if roof is cold above membrane and hotter below there can still be passage of the hotter vapour from inside to outside ,where it will condense  or evporate and cannot go back inwards  and runs down roof as water

so yes hot air and cold air does a have an effect 

 for a very clever fella you seem to be one dimensional with your thinking when it suits  .

tin roof/slates heat up --that heats the membrane and boarding ,any damp  on inside will turn into vapour and then exit through the membrane  where it is evaporated by the air flow up the gap which is caused by the convection of  hot air  as it expands and rises

thats how all these membranes work ,

 

Edited February 2, 2019 by scottishjohn

[PeterW]

19 minutes ago, scottishjohn said:

thats how all these membranes work ,

 

Errr it’s not ..... they’ve been developed using a number of materials - they are usually multiple layers - that have different characteristics under varying vapour pressures and rely on in some instances the negative pressure created by the outside air temperature ... they also work when it’s cold, and the thermal gradient is inverted to how you have stated it. 

 

Simplifying it to a hot / cold thermal transfer is like saying it never rains in the summer as it’s warm ... 

[scottishjohn]

I did not for  one second say that its only to do with heat ,

you can look at my posts and take things in isolation if you like . as the last few have done 

 and make a post filled with non specifics  -fine with me 

i know how they work and what will make a good roof --  END OF

 

[Jeremy Harris]

25 minutes ago, scottishjohn said:

I did not for  one second say that its only to do with heat ,

you can look at my posts and take things in isolation if you like . as the last few have done 

 and make a post filled with non specifics  -fine with me 

i know how they work and what will make a good roof --  END OF

 

 

 

FWIW, the movement of any vapour through any solid (including membranes) is solely dependent on the permeance of the solid and the relative vapour pressure either side of it.  Vapour will always want to move from a higher vapour pressure region to a lower vapour pressure region, and vapour-permeable membranes try to allow this to happen, in both directions equally, by being fairly vapour permeable whilst retaining a high resistance to the movement of moisture through them.

 

 

[Miek]

If OSB can vary in vapor permeability then why not use something like Panelvent? 

 

"Panelvent DHF is a 13mm square edged sheathing board designed to replace OSB and plywood as an external lining board. With market leading water vapour resistance values, this board allows the designer the freedom to design breathing structures, free of the risks of interstitial condensation."

 

 

[Ed Davies]

AFAIK, the only objection to Panelvent is cost. Long time since I looked but, IIRC, it was about twice the price of OSB.

[Miek]

7 hours ago, Ed Davies said:

AFAIK, the only objection to Panelvent is cost. Long time since I looked but, IIRC, it was about twice the price of OSB.

Yes, at least twice I think. OSB is very cheap if you buy by the pallet. 

[MarkyP]

I have OSB 3 sarking, as designed the intent was to full fill with mineral wool between 170mm deep rafters and then 70mm PIR below. This was passed at bc subsmission stage but since on site BCO has questioned condensation risk on the inside of the OSB. A VCL would if perfectly fitted address this from a CRA point of view but there is a tricky detail at eaves (this section is of the building is retrofit) and around dwarf wall uprights in the trusses which will be a fiddle to detail the VCL perfectly so perhaps not unreasonable of BCO to question whether I will 100% prevent vapour (or even air) moving from warm side into the build up.  So my plan, while it will be a chore, is to drill two rows of 8mm holes @ 200mm centres through the OSB in each rafter bay and then run a strip of air tightness tape over each row to prevent wind washing of my insulation once fitted as the OSB was all carefully taped to be wind tight. Above the OSB is a ventilated counter batten void, breather, then batten/slate.  I need to double check the tape of choice to ensure sufficiently vapour permeable. Have yet to discuss with BC but think this will address concerns. 

 

[Cpd]

34 minutes ago, MarkyP said:

So my plan, while it will be a chore, is to drill two rows of 8mm holes @ 200mm centres through the OSB in each rafter bay and then run a strip of air tightness tape over each row to prevent wind washing of my insulation once fitted as the OSB was all carefully taped to be wind tight.

Sorry but I am a bit lost here, (happens quirte a lot...) if you drill a row of holes through the osb and then tape over them with air tight tape....... won’t that be a complete waste of time as you have just sealed it back up again........ sure I am just reading this wrong..... I am in a similar dilemma as I build my shed roof out of floor grade ply........ what a baboon I was to move away from traditional 22x150mm white wood sarking.... I was pretty ill at the time and was not in full control of this part of the project .... it’s still my fault but hay ho. I now have a surface that is not nearly breathable enough and am getting condensation build up and black mould.  My plan is to take the tin off and  the breather membrane that was damaged in the frost and cut some big old holes along the length of the roof top and bottom through the ply and then put new membrane on  and re tin........ it was a stupid mistake..... but I was only functioning at about 20% on a good day when it was going on......  

[MarkyP]

yes this why I said I will need to double check choice of tape. If the tape isn't sufficiently vapour open then this would indeed will be a waste of time. My aim is to improve the vapour permeability of the OSB with a series of air tight but vapour open holes.

 

[Carrerahill]

What you are looking for is standard practice in Scotland, I have never seen a roof in Scotland without boarding (sarking) on it, well once by a major house builder building cheaply.

 

This is just my garage and it's fully boarded!

[Russell griffiths]

The holes in the osb that scanda hause use are about the size of a 2p

why not fit your membrane directly onto the osb with holes then counter batten. 

No wind washing then. 

 

 

[Cpd]

2 hours ago, Russell griffiths said:

why not fit your membrane directly onto the osb with holes then counter batten. 

That’s what I intend to do, cut me a bunch of holes, new membrane over the lot and put the tin back on. 

[makie]

On 04/02/2019 at 14:15, Carrerahill said:

What you are looking for is standard practice in Scotland, I have never seen a roof in Scotland without boarding (sarking) on it, well once by a major house builder building cheaply.

 

This is just my garage and it's fully boarded!

Every house builder I have worked for in Scotland just does felt over the rafter now.

[ProDave]

8 minutes ago, makie said:

Every house builder I have worked for in Scotland just does felt over the rafter now.

Not in Scotland, a sarking board is a (sensible) building regs requirement.

[makie]

7 minutes ago, ProDave said:

Not in Scotland, a sarking board is a (sensible) building regs requirement.

I work in the central belt of Scotland and very rarely will house builders put any form of sarking or boards on roofs.

[Crofter]

8 hours ago, makie said:

I work in the central belt of Scotland and very rarely will house builders put any form of sarking or boards on roofs.

That's interesting. It's common knowledge that houses in Scotland use sarking.

Perhaps this is not a hard and fast rule, just common practice.

It sounds like it may be possible to comply with regs if you can demonstrate that the roof is strong enough without sarking. I think the onus will be on the designer/developer to prove it, though.

[Cpd]

52 minutes ago, Crofter said:

I think the onus will be on the designer/developer to prove it, though.

And on that note......... they seem very capable of proving anything on paper, building it and getting it passed.........but what you get In reality seems very different 

[jamieled]

59 minutes ago, Crofter said:

It sounds like it may be possible to comply with regs if you can demonstrate that the roof is strong enough without sarking. I think the onus will be on the designer/developer to prove it, though.

 

I've spoken to two structural engineers about this for our house and both stated sarking isn't required from a structural point of view. It's main function is additional weatherproofing. I had a quick look at the Scottish regs and couldn't see anything in the structural section about sarking being required.

[Carrerahill]

11 hours ago, makie said:

I work in the central belt of Scotland and very rarely will house builders put any form of sarking or boards on roofs.

But then again, builders in the central belt of Scotland can't even put walls up properly so that doesn't really say anything...

 

 

[makie]

10 hours ago, Crofter said:

That's interesting. It's common knowledge that houses in Scotland use sarking.

Perhaps this is not a hard and fast rule, just common practice.

It sounds like it may be possible to comply with regs if you can demonstrate that the roof is strong enough without sarking. I think the onus will be on the designer/developer to prove it, though.

 

Slated houses need sarking but if there is tiles on it the battens alone help hold the trusses together also. The only time I see it now on sites is when they are being built for the council.

 

8 hours ago, Carrerahill said:

But then again, builders in the central belt of Scotland can't even put walls up properly so that doesn't really say anything...

 

 

 

Aye persimmon, Cala and all the other major builders are poor these days.....

[ProDave]

All I can say is all houses I see being built up here have sarking on the roof.