Housing - Renovation: Fixing OSB airtight layer

1 to 30 of 56

- CommentAuthorpmagowan
- CommentTimeJan 26th 2012
OK, I'm finalising the spec for the roof and am looking at the airtight layer. Because I already have old T&G planks as sarking I had been originally planning on just putting down a layer of 500g polythene. I am worried that this will be easily damaged by the roofers and so now I am specing T&G OSB 15mm on top of this sandwiching it to the sarking. The Polythene will be overlapped by 150mm and sealed with duplex tape. The OSB will be joined with silicone sealant and then taped with Tescon No1. The polythene will be joined to the wall parge coat at eaves and chimney using Contega PV tape. The OSB will also abut the parge coat at the eaves and verge where the overhang will be cut away as previously described in another thread (Heavy duty counterbattons supporting the fascia).

A few questions;

Am I over-specing this?
What is the best way to fix the OSB, screws, nailgun?
Should the OSB be fixed through the 15mm T&G sarking and into the rafter or is it ok to fix it to the sarking alone?
Should I tape over every nail/screw in the osb or put a dab of silicon on?

For those that cant remember the roof structure from inside to out will now be;

100mm rafters
15mm T&G sarking
500g polythene sheet
15mm T&G OSB
150mm EPS Plat 70
50mm counterbatten
Roofing felt
Slate batton
Slates (Bangor Blue style)

I will be using Thor super-7 helical fixings

Thanks
- CommentAuthortony
- CommentTimeJan 26th 2012
Only 150mm of insulation?
- CommentAuthorpmagowan
- CommentTimeJan 26th 2012
Yes Tony, Sorry
I had planned 200 mm but it was just a nightmare to find fixings that were for the job. It would be a nightmare to insert them and difficult to spec size and numbers.
What I will do is put another 100mm of plastic bottle insulation between the rafters. I appreciate that this may require monitoring for condensation at the timbers.

Another question;
How much silicon do you need?
- CommentAuthorPeter_S
- CommentTimeJan 26th 2012
not sure what the osb gets you in this case (assuming the T+G boards are to be left in situ?) the insulation will cover the polythene sheet and protect it as long as the roofers take care with their feet.
- CommentAuthorpmagowan
- CommentTimeJan 26th 2012
This is what i am afraid of Peter. I can't see roofers managing to put on the insulation and the counterbatton all without tearing the polythene. All it takes is a clumsy foot, a thrown hammer or even a nail left sticking out of the sarking. I worry that the risk of failure is too great and I will only know when I get a blower door test and smoke. It would be awful if, after all this trouble, the roof looked like an upturned steam iron during testing!
- CommentAuthordavidfreeborough
- CommentTimeJan 27th 2012 edited
I'm not sure the polyethylene is earning it's keep & it's probably not a good idea if you're thinking of putting another 100mm of insulation between the rafters. Why not focus on making the OSB your primary air barrier & move the polyethylene to below the rafters to manage the condensation risk? I realise that this will be more difficult to seal, but it's not good having polyethylene part way through the build-up.

I would check the existing sarking boards are well fixed, fix 18mm T&G OSB3 to the rafters with 75mm screws & then you won't need to play "hunt the rafter" when fixing your counter battens.

David
- CommentAuthorpmagowan
- CommentTimeJan 27th 2012
Thanks David and Tony (I think you answered me in the other roof detail thread).

David, do you think that with the 15T&G sarking planks and the 15mm T&G osb that this provides a ridgid base whereby I dont need to worry about attaching the counterbattons directly to the rafters? That would be brilliant as I was worried about aiming through all the insulation. What do others think?

I was hoping the polythene would do where it is as most of my spec has been to reduce the difficult internal detailing. I just cant understand how you can seal polythene intenally round all the rafter ends etc. I have seen others do it with very carefull tape-work but it worries me. I suspect the tape would eventually come loose but perhaps that is because I am ignorant of these tapes. I realise that there is a risk with having it in the middle of the insulation but I was hoping that with an MVHR the relative humidity would be low and that 100mm of fluffy insulation would not put the polythene behind the dew point. I have to admit I am winging it with this calculation. It also concerns me that the OSB can remain airtight for long periods and thus my belt and braces approach. i may just be making work for myself!

Tony, I am in agreement with your insulation obsession and you are one of the people who has changed this project from a straightforward reroofing to an insulation challange. Thanks!
- CommentAuthordavidfreeborough
- CommentTimeJan 27th 2012 edited
Posted By: pmagowanDavid, do you think that with the 15T&G sarking planks and the 15mm T&G osb that this provides a ridgid base whereby I dont need to worry about attaching the counterbattons directly to the rafters? That would be brilliant as I was worried about aiming through all the insulation.
I was suggesting 18mm OSB3 because that would give 33mm thickness in total & Ancon recommend 35mm minimum embedment, but yes that is the idea. It may be that when trying to fix helical nails midway between rafters that you get too much bounce, so its a good idea to try & align the counter battens with the rafters, but the alignment won't be critical.

I agree that its difficult to seal an internal vapour control layer around rafter/joist junctions, but that's the price you pay for putting some of the insulation between the rafters. If your polyethylene was inboard of all the timber then I would say give it a go. However, as it is, if there is a problem it will be hidden for a long time.

Is it an option to take off the existing sarking boards fit the polyethylene & replace them? That way, at least you'll be able to monitor the polyethylene for signs of condensation. There are some reinforced grades of polyethylene which can resist a boot.

David
- CommentAuthorpmagowan
- CommentTimeJan 27th 2012
I wonder if I put a thermometer on the inside of the existing sarking would this allow me to monitor for risk of condensation. The sarking would be at the same temperature as the vcl and I would have a hunidity monitor anyway. I am intending to set up a monitoring system which is linked to the mvhr etc. This way I could put an extract duct in the attic and if the relative humidity at the sarking would be close to the dew point then the mvhr could go on boost. I know I am skirting around the well known fix but I really hate the idea of trying to attach a vcl inside the rafters. Maybe this is stupid and internal vcl would not be as difficult as I thought.

I am really pleased with your idea re the OSB as I was almost losing sleep over the risk of missing rafters.
- CommentAuthorPaul in Montreal
- CommentTimeJan 27th 2012
Wouldn't it just be easier to use closed-cell sprayfoam on the inside? Maybe more expensive, but at lot simpler in the end.

Paul in Montreal.
- CommentAuthorpmagowan
- CommentTimeJan 27th 2012
Thanks Paul. I have heard a lot of bad stories about spray insulation and so i hadn't considered it. The problems being trapping moisture and also forming a permanent joint with the roof thus making future alteration and repair difficult. I want this roof to last 100years or more and it may need some maintainance. Does the closed cell squirty foam get around the problem of moisture? Does it form a vcl? Is it a good idea?
- CommentAuthorPaul in Montreal
- CommentTimeJan 27th 2012
Spray foam is used a lot over here as it's a good way to get airtightness. If you're really concerned about trapped moisture, Icynene is somewhat vapour-open, though is more expensive (as far as I understand) than regular closed-cell foams. We have an as-yet unfinished "bonus" room over a garage with a steel roof over trusses. There is no roof deck at all - the steel is installed directly on battens across the trusses. The only feasible way to insulate this is to sprayfoam the whole thing - there are lots of contractors over here so it should be straightforward. The rest of the house, which is not room-in-roof, just has blown cellulose above the ceiling (which has a layer of foiled PUR board between the trusses and the furring strips the drywall is attached to. Then about 2 feet of cellulose on top of that. The junction between the roof trusses and the timber frame is also spray foamed for air tightness (as it is between the upstairs and downstairs (where there are engineered floor trusses) and similarly in the basement. In retrospect I would have had sprayfoam everywhere but budget did not permit so we focused on the areas where the airtightness aspect was important.

Paul in Montreal.
- CommentAuthorTimSmall
- CommentTimeJan 28th 2012 edited
I'm doing something similar at the moment. I'm depth-constrained, and I have to line up with next door (it's a semi with a continuous roof - not interrupted by a firewall.

Have you considered:

plasterboard
38mm counter batten (service void - optional)
500g polythene, or Intelligent vapour membrane (for belt and braces)
100mm rafters (insulation of choice full-fill)
15mm T&G sarking
9mm OSB (since you're going over sarking, this shouldn't be difficult to air-tight by using sufficient silicone between and below butt-joins).
150mm EPS Plat 70
50mm counterbatten (is 50 really required? 38mm?)
Roofing membrane (breathable!, felt isn't).
Slate batton
Slates (Bangor Blue style)

Alternatively, you could go for Viking House's build-up with two layers of 100mm EPS, with battens inlayed for fixing. There's a thread somewhere about using a hot knife for cutting out the channels. Alternatively, you could use a router I suppose (it'll look like a blizzard, but hey!).

No problem with the spray polyurethane, the horror stories are all about it's miss-application. It's expensive in the UK tho (I'm going to be using some for some hard-to-access detailing).

Can you do a DIY air test at the point that the OSB goes on? That's what I'm planning.

BTW, the above is similar to the construction which I'll be overseeing in the next few weeks at my house, but I'll be using 100mm PIR over the OSB, with another 100mm + 25mm phenolic between 125mm rafters (non-foil-faced PIR - so vapour open - I got a load of SIPS factory rejects very cheap) instead of EPS. I got the 25mm phenolic cheap too !
- CommentAuthorpmagowan
- CommentTimeJan 28th 2012
So the general consensus seems to be to bring the polythene internally and attempt to make an airtight seal from it. I am not overly adverse to this except that the polythene would be really useful externally for all the detailing around eaves and chimney etc. It would make it quite easy to make a good seal. I suppose I could use some here anyway to bridge between the OSB and the external wall parge coat. The other problem is how to seal the polythene internally, it looks like a very fiddly job since the air tight layer on the walls is external. Technically it shouldn't be required as an airtightness thing internally but just as a vapour barrier but the problem is still there. How to seal moisture out.
- CommentAuthordavidfreeborough
- CommentTimeJan 29th 2012
Butyl rubber or EPDM is probably a better choice for bridging between OSB & masonry.

Have you thought about using a variable resistance vapour control layer internally, such as Pro Clima Intello? If moisture does get behind it during the winter it will dry out during the summer preventing long term build-up.

David
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
I don't understand these high tech fabrics. If vapour can get through then surely it doesn't work as a vapour barrier? If the internal RH remains at 60% for instance then it will diffuse across a membrane (if any permiability) and be 60% on the other side. Then if the OSB gets below a the dewpoint I will get condensation and problems. Am I missing something.

Paul inM, suggested closed cell icynene which would appear to work as an airtight layer as well as vapour control layer. Does anyone have an opinion? It's permanence worries me but it would add to the insulation while fixing all the other problems. Does anyone know if you can DIY it?
- CommentAuthorPaul in Montreal
- CommentTimeJan 29th 2012
Posted By: pmagowanPaul inM, suggested closed cell icynen

Just to note that Icynene is open cell and is somewhat vapour permeable. I know it's being using in Scotland on some heritage buildings - there was mention of that in a thread somewhere here. It is definitely airtight.

Paul in Montreal.
- CommentAuthorborpin
- CommentTimeJan 29th 2012
I have been fiddling with Builddesk this morning while watching the tennis, using Icynene;

Buildup in to out, 600mm rafter centers;
Icynene 150mm between rafter,
9mm OSB
100mm Cleotex GA3000
Breather
Vent
Slates

U Value 0.12 no condensation risk.

Condensation risk is sensitive to the Icynene depth and this could be an issue if the attic trusses are 200mm deep although it is only for one month in the year. Waiting for a price for this Icynene stuff as it works on the walls as well so thinking of using it for the whole of the inside of the frame (rather than warmcell).
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
They seem to do both an open and a closed cell version. Does putting it between rafters form a vapour barrier or would you have to cover the rafters completely? Most applications I have seen on YouTube you can still see the rafters internal edge. Does anyone know a rough cost? I suppose it is much more expensive than a polythene sheet!
- CommentAuthorborpin
- CommentTimeJan 29th 2012
I am not intending to have a VCL. Just let it breathe. The air tight layer is the OSB.

The BBA certificate I have found (08/4598) has values for thermal conductivity of 0.039 and vapour permeability of between 2 and 5.
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
If you are not having a VCL then you will have to be very careful about where the dewpoint is, as far as I understand it. What you don't want is for the dewpoint to occur Inside the OSB otherwise the wood will become damp and rot. This is my concern with 100mm of internal insulation to the 150mm EPS plat external insulation. I have not done any clever calculations though
- CommentAuthorborpin
- CommentTimeJan 29th 2012
That was the point of the post; BuildDesk shows that with that build-up stated there is no risk of condensation. The general rule is that you need more insulation outside than inside the OSB layer. That is why the risk is sensitive to the internal thickness of Icynene.

In terms of your OP, I suggest the OSB itself is sufficient as an air tight layer as long as it is well laid, and sealed as necessary. I like Fostertom's use of galvanised strip rather than noggins http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=6890&page=3
- CommentAuthorViking House
- CommentTimeJan 29th 2012
I had a few issues with water getting into houses because the roofs weren't properly sealed over Christmas so I think we will start putting a layer of Polythene over the OSB before we put on the 200mm of EPS externally. It will be wrapped down over the sides of the houses to aid air-tightness. The roof can breathe in and out from the OSB/Polythene layer.
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
Well, Borphin, if your calculations are correct I have nothing to worry about. I am only going to have 100mm of plastic bottle wooly batts on the inside and 150mm of platinum EPS outside. I think VH's post has pushed the polythene back out, wrapped exactly as said. I can always monitor the sarking for temperature and humidity and put an internal VCL in if problems arise. If only there was a way to cover the whole house in polythene to make the EWI airtight. As it is I am going to have to rely on a parge coat of EWI adhesive but I am sure that is very expensive. I thought about polythene and just mechanical fixings but then there would be loads of perforations and I am worried with 200mm it would still be quite a load on fixings.

The other thing I still cant get round it external fixings such as security lights and cameras etc. I think the best way is to burry a ply square into the insulation at set points and put a couple of fixings through it then rendering over the top. If I mark it carefully then I will be able to screw on fittings with ordinary screws etc. Might do this with downpipes etc or use Jim's method with these lower loads of just squirty foaming in a plug.
- CommentAuthorPaul in Montreal
- CommentTimeJan 29th 2012
Posted By: Viking Houseso I think we will start putting a layer of Polythene over the OSB before we put on the 200mm of EPS externally.
Wouldn't something like Tyvek be better? It's water proof but vapour permeable.

Paul in Montreal.
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
Which one PiM? They seem to do loads. I presume Airguard or such-like. It is about 5 times the price of polythene! Do people think it is worth the piece of mind?
- CommentAuthorborpin
- CommentTimeJan 29th 2012
Posted By: pmagowanThe other thing I still cant get round it external fixings such as security lights and cameras etc.
Or Gutter downpipes! This was one the reasons I have gone back from EWI and render to a block outer skin - just not robust enough I feel.
- CommentAuthorpmagowan
- CommentTimeJan 29th 2012
I don't think there is a massive load on gutter downpipes and Jim showed his system worked with just a plug, glued in with foam. I am in two minds whether to use backing plates for the gutter downpipes or not. If I do they should be strong enough but there will be more thermal bridging from the fixings. Also if there is a very strong load it would cause greater damage to the EWI. If I don't and use the plug method the worst that could happen is the plug pulls out and a bit more squirty foam is needed. Since almost all EWIed houses must have this problem it is odd that there isn't a standard solution. In fact it is odd that there is not a standard solution to so many of our queries. I trawl the internet and find very little. I even use google translate to see if the germans have any ideas. I am, at the end of the day, a big fan of the idea of EWI and am now committed to using it but it could be a lot simpler.
- CommentAuthorViking House
- CommentTimeJan 29th 2012 edited
Posted By: Paul in Montreal Wouldn't something like Tyvek be better? It's water proof but vapour permeable.
Hi Paul, It doesn't need to be breathable does it? polythene is cheaper and more airtight. We will use Tyvec on the outside before slating!
- CommentAuthorPaul in Montreal
- CommentTimeJan 29th 2012
Posted By: Viking HouseThe roof can breathe in and out from the OSB/Polythene layer.

Posted By: Viking HouseHi Paul, It doesn't need to be breathable does it? polythene is cheaper and more airtight. We will use Tyvec on the outside before slating!

You stated it should be breathable! I don't think Tvyvek is any less airtight than polythene.

Paul in Montreal.

1 to 30 of 56



	Site Map \| Home \| View Cart \| Pressroom \| Business \| Links	Contact Us
Logout
© Green Building Press

Green Building Forum

Categories

Feeds

Housing - Renovation: Fixing OSB airtight layer