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Sheathing Board Options


bissoejosh

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Currently our design is based on using 15mm Egger DHF for our external sheathing but having received some prices it seems an expensive option. Any huge issues (assuming the engineer OK's it) switching to either 11mm or 15mm OSB? I'm sure others have used OSB and it comes in at roughly half the price so a substantial saving on 300m2. My only thought was weatherproofing during construction as I know the Egger is pretty water resistant.

 

For reference the rest of the make up as follows:

 

PB & Skim

Service Cavity

12mm VapourBlock OSB (again quite an expensive option)

245mm i-Stud

15mm Egger

38mm Cavity

20mm Cladding

 

Many thanks

 

Edited by bissoejosh
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Are you planning on the (T&G?) Egger DHF being your Air tight layer? If so perhaps its cost is not so high, What would be your alternative airtight layer if you didn't use DHF

 

Do you need a Service cavity? You could use foil-backed PB as VCL rather than OSB and the service cavity. Perhaps just put the Service cavity where you need it. Electrics can run through insulated cavity, only plumbing (and MVHR) needs to be kept out of the insulated cavity.

 

38mm Is a small vent gap to external cladding. 

Edited by IanR
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Hoping to use the VapourBlock OSB as our airtight layer and VCL for durability and ease of construction.

 

Service cavity is currently 25mm directly behind PB. I thought minimum external vent was 25mm - BCO upped it to 38mm. I could up it to 50mm if necessary?

 

 

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1 minute ago, A_L said:

15mm OSB instead of the Vapourblock and Panelvent DHF instead of the Egger ?

 

https://www.ecomerchant.co.uk/panelvent-structural-vapour-permeable-sheathing-board.html

Another option, although the frame is drawn to allow for the 12mm internal board and changing now would be a bit of a pain. Panelvent instead of the Egger looks interesting - same sort of product just a little thinner and hopefully cheaper?

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6 minutes ago, Dudda said:

Internally the 15mm OSB wouldn't be as airtight as the 12mm VapourBlock OSB. I wouldn't go changing that considering this is your airtight layer.

 

Agreed, I think this stays as it is.

 

It may be the case that the Vapourblock is also sufficient for racking strength which makes the external sheathing rather redundant except for holding insulation in place and providing protection during construction.

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10 minutes ago, bissoejosh said:

Hoping to use the VapourBlock OSB as our airtight layer and VCL for durability and ease of construction.

 

Service cavity is currently 25mm directly behind PB. I thought minimum external vent was 25mm - BCO upped it to 38mm. I could up it to 50mm if necessary?

 

I hadn't seen OSB used as an Air tight layer, but have just checked and MBC have updated their build-up and appear to use the OSB inner layer for air tightness and VCL, When I considered using them (a few years ago) they used an additional membrane for this.

 

If that is a robust solution then both it and T&G Egger DHF would not be necessary. But, if you go airtight layer on the inside skin you do need a service cavity to avoid penetrations. If you went airtight on outer skin you could avoid the extra cost of service cavity - other than where you can't avoid plumbing routing along/down an external wall.

 

With regards external vent gap I was encouraged to go 50mm minimum. 

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21 minutes ago, bissoejosh said:

Hoping to use the VapourBlock OSB as our airtight layer

 

...another thought. Are you going with blown cellulose? If so that's a lot of holes in you airtight layer to re-seal.

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The designer of my house specified OSB3 as both the racking layer and the air tight layer (with taped joints) on the inside of the frame.  I later decided to add a proper air tight membrane on the inside over the OSB.  Then later the structural engineer upgraded it to 2 layers of OSB to give the required racking strength.

 

Definitely stick with a service void.

 

If changing the external layer, check with your SE re racking strength, particularly if moving from a T&G product to a square edge product.

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1 minute ago, IanR said:

 

Why's that Dave? it's a lot of extra cost.

It's the cost of some battens.

 

You will have the OSB anyway (or whatever you change it for) so omitting the service void just mean you put the plasterboard straight onto the OSB.  It then makes it a pain to run cables through the frame (drilling the web of the I beams) and then cutting through the OSB and plasterboard for back boxes.  Then there is the issue of making it all air tight.

 

I have omitted the service void in a couple of places where I know there are no services (stairwell head wall and bathroom outside wall) just to gain an extra 25mm in the rooms, but all other rooms have the service void.

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9 minutes ago, ProDave said:

The designer of my house specified OSB3 as both the racking layer and the air tight layer (with taped joints) on the inside of the frame.  I later decided to add a proper air tight membrane on the inside over the OSB.  Then later the structural engineer upgraded it to 2 layers of OSB to give the required racking strength.

 

Definitely stick with a service void.

 

If changing the external layer, check with your SE re racking strength, particularly if moving from a T&G product to a square edge product.

I've asked the question re racking. Did you have any external sheet materials other than the fiberboard? It seems to me that the Egger DHF is redundant for the most part but excluding it would mean I have to insulate from the outside as I would need the internal board up right away for strength - I can't decide if this is an issue.

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13 minutes ago, ProDave said:

It's the cost of some battens.

 

You will have the OSB anyway

 

Nope, it's the cost of battens, OSB and labour.

 

My build up from out to in is:

Cladding

vent gap

breather membrane

T&G DHF Egger (airtight layer & racking )

I-Joist

Foil-backed PB (VCL) & skim

 

So unless the loads on the structure require a structural layer inside and out the extra OSB is just closing off the cavity to hold the insulation.

 

Edited by IanR
Added external breather membrane layer
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6 hours ago, IanR said:

 

Nope, it's the cost of battens, OSB and labour.

 

My build up from out to in is:

Cladding

vent gap

breather membrane

T&G DHF Egger (airtight layer & racking )

I-Joist

Foil-backed PB (VCL) & skim

 

So unless the loads on the structure require a structural layer inside and out the extra OSB is just closing off the cavity to hold the insulation.

 

Lots of thinking after this post. Perhaps I'm missing a trick not detailing in a similar manner - Cullen design asked me a while ago if I wanted to go down the airtight on the outside route but I had already gone 'traditional' in my mind. Any downsides having done it this way or things you would do differently? I think I'd be tempted to still opt for a cheap VCL+ batten to create service void & help support the mineral wool in our frame.

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1 hour ago, bissoejosh said:

Lots of thinking after this post. Perhaps I'm missing a trick not detailing in a similar manner - Cullen design asked me a while ago if I wanted to go down the airtight on the outside route but I had already gone 'traditional' in my mind. Any downsides having done it this way or things you would do differently? I think I'd be tempted to still opt for a cheap VCL+ batten to create service void & help support the mineral wool in our frame.

 

I think the build-up is very cost effective and I would go the same route again.

 

A service void is nice to have, but is it worth the extra cost and wall thickness? That's for you to decide.

 

There are no downsides that are specific to this buildup. If you get your air tightness from a rigid layer then your detailing of corners and junctions needs to be good. I'm sure CTD will get the design correct, so you'll need to ensure the build quality is up to the job. I was told an external breather membrane was needed for the roof but not necessarily for the walls, but chose to cover both to protect from the weather while cladding and roof was going on.

 

Electrical 1st fix was very simple within the cavity (pre-insulating), but any penetration through the DHF (wires for outside lights, door bell, MVHR ducts to outside etc.) need an air tight gasket.

 

I'd question your decision for mineral wool. Blown cellulose will improve air tightness and I assume has a higher decrement delay (I've not checked mineral wool). You should also perhaps get some advice or a Wufi analysis on mineral wall to make sure it's suitable behind the DHF for clearing any moisture out. Maybe the insulation manufacturer will be able to support this or can confirm it's suitable for this application.

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On our wall build up I have 11mm OSB on the inside for racking, not sure why you would need the Egger and OSB.

My wall build up from outside

Larch cladding

25mm batten

25mm vertical batten

Breather Membrane

40mm Pavatherm

275mm Timber with Rock wool Flexi

Vapour Barrier

11mm OSB

25mm Service void

12mm Plasterboard

 

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14 hours ago, JamesP said:

On our wall build up I have 11mm OSB on the inside for racking, not sure why you would need the Egger and OSB.

My wall build up from outside

Larch cladding

25mm batten

25mm vertical batten

Breather Membrane

40mm Pavatherm

275mm Timber with Rock wool Flexi

Vapour Barrier

11mm OSB

25mm Service void

12mm Plasterboard

 

So your Pavatherm acts as a retaining layer for the RockWool. We're not having Pavatherm so the question is does the RW need sandwiching or will it stay put with only one (external) side covered by OSB?

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3 minutes ago, bissoejosh said:

So your Pavatherm acts as a retaining layer for the RockWool. We're not having Pavatherm so the question is does the RW need sandwiching or will it stay put with only one (external) side covered by OSB?

The Rockwool Flexi 140mm slabs are very stable and would stay put. Personally I would use a cheaper option to the Egger board on the outside. Some wall build ups

have OSB just on the inside or outside, the breather membrane would seem a bit vulnerable with out it. I used Pavatherm to increase the insulation and have a warm roof. My project below.

 

 

IMG_8515.BH.jpg

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  • 2 weeks later...

Bit of an update...

 

We've upgraded to 300mm jji beam walls and blown cellulose throughout. To manage this from a cost point of view we've opted to use OSB instead of the egger dhf and also to use 11mm osb and a membrane internally with just a membrane on the ceiling. 

 

My next question for those that have sheathed with OSB...Did you use tongue and grooved or square edged? Our egger dhf was t&g which I guess eliminated some cutting for joints etc but I hope square is fine as I believe most are done this way? 

Edited by bissoejosh
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25 minutes ago, jamiehamy said:

We used square edge for the upstairs part. Make sure you order the right size and don't get caught out with 2440x1220 boards (rather than 2400x1200)  which will not tie in with 600mm centres. 

 

Good advice, I'll make sure I specify and the supplier confirms!

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