Air tightness barrier/VCL on SIP walls (with EWI) and on SIP roof (with internal insulation)

[BrackensBuild]

We’re building with SIPs and will have 100mm EWI on the walls and 80mm insulation on the inside of the roof. For the walls, we will use Glidevale Thermo Extreme breather membrane outside, then Glidevale Protect BarrAir membrane on the inside. Currently trying to work out if we use the same membranes for the pitched roof and would welcome any thoughts or suggestions.

[Russell griffiths]

I’m baffled as to why you are building with sips and then having to add extra insulation. 
either get a better sips product 

or find a different method of building 

happy to be educated as to the reason. 

[saveasteading]

  On 18/03/2024 at 07:29, BrackensBuild said:

welcome any thoughts or suggestions.

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Also puzzled. Don't you have a formal design in place?

Why wouldn't you want the the same membrane?

 

[Iceverge]

Can you not just tape the sips to each other internally for airtighess? 

 

What are you planning for your final rain screen? A block external leaf or cladding hung from the SIPs?

 

 

 

[Square Feet]

  On 18/03/2024 at 08:12, Russell griffiths said:

I’m baffled as to why you are building with sips and then having to add extra insulation. 
either get a better sips product 

or find a different method of building 

happy to be educated as to the reason. 

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External insulation on top of SIPS is the standard I have found when reading up on this.

[Post and beam]

  On 19/03/2024 at 10:55, Square Feet said:

External insulation on top of SIPS is the standard I have found when reading up on this.

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Really.

We are hoping to start our Potton home soon and that is SIP's with a brick skin ground floor and Cedral cladding on the upper. No other insulation anticipated.

[markocosic]

  On 18/03/2024 at 10:26, Iceverge said:

Can you not just tape the sips to each other internally for airtighess? 

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Having tried taped OSB myself I would say no.

 

It's too rough and ready to take tape nicely and with a sip if you get any movement (usually at ridge beam) it turns into soggy Weetabix.

 

Better to go belt and braces with membrane.

[markocosic]

  On 19/03/2024 at 12:11, Post and beam said:

Really.

We are hoping to start our Potton home soon and that is SIP's with a brick skin ground floor and Cedral cladding on the upper. No other insulation anticipated.

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In countries that have winters you won't get a sup thick enough to meet building regulations.

 

400 mm walls and 600 mm roofs not practical. Build with smaller panels then add insulation externally to also keep the outer OSB layer toasty warm and dry.

[Square Feet]

  On 19/03/2024 at 12:11, Post and beam said:

Really.

We are hoping to start our Potton home soon and that is SIP's with a brick skin ground floor and Cedral cladding on the upper. No other insulation anticipated.

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According to this, yes.

[BrackensBuild]

  On 18/03/2024 at 08:12, Russell griffiths said:

I’m baffled as to why you are building with sips and then having to add extra insulation. 
either get a better sips product 

or find a different method of building 

happy to be educated as to the reason. 

Expand  

 

142mm SIPs + 100mm EWI = better u-value for the same wall depth versus just a 250mm SIP. In addition, we can reduce cold bridging where we have timber posts within the SIPs instead of the splines (where structurally required).

[markocosic]

Air barrier - stops bulk movement of air through a structure (important)

Vapour retarder/barrier - slows/stops movement of moisture through a structure (less important)

 

Science:

https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

 

SIPs on their own are problematic because:

- You have a symmetrical wall/roof (vapour retarders in the form of OSB on BOTH sides)

- That vapour retarder is made of weetabix (can't do moisture)

- You have leaks in the wall/roof (making joints airtight is difficult/impossible)

 

They don't fail so much at ground level (where cold air is sucked through the wall, heated as it goes through the insulation layer, and dries out in the process)

They do fail at the ridgeline (where warm moist air exits and dumps the excess moisture into the SIP assembly)

They really fail if the cladding/roof covering isn't well ventilated (and that excess moisture stays in the outer layer of weetabix)

 

 

I would suggest this variation: stick an airtight vapour barrier (can be polythene) on the OUTSIDE of a skinny SIP then go over the top of that with insulation. The air/vapour barrier on the OUTSIDE is easier to apply well (less likely to be compromise than one inside the house with service penetrations etc) and keeps the SIP dry until it's insulated, roofed, and cladded.

 

 

 

 

 

[MoDo]

Perhaps for this reason (moisture getting trapped in outer osb layer) Kingspan TEK systems are built with PIR installed internally. The supplier we used has the u-value data for different thicknesses of PIR on this page https://www.clays.com/sips-technical-data

[markocosic]

Still needs an airtight layer. That internal insulation layer is just to sell you more kingspan.ftom insulation perspective is better outside but practically they can't do that without turning the house into Weetabix due to non breathable nature of the insulation so are forced to put it inside and tape the living daylights out of it

[MoDo]

  On 07/05/2024 at 00:26, markocosic said:

Still needs an airtight layer. That internal insulation layer is just to sell you more kingspan.ftom insulation perspective is better outside but practically they can't do that without turning the house into Weetabix due to non breathable nature of the insulation so are forced to put it inside and tape the living daylights out of it

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No airtightness tape detailing that I can see on their Standard Details PDF. The system is designed to have plasterboard fixed directly to the OSB. Airtightness is taken care of by overlapping and sealed giant T&G joints of the SIPS panels themselves. PIR is just for higher u-values. A breather membrane is fitted to the outside. 

[markocosic]

Weetabix waiting to happen in that case.