Can a SIPs flat roof ever be called a warm roof???

[GrumpyBear]

Hi all,

 

Re-post from the roofing section of the forum.  Not had any traction there and hoping I might get some interest here. 

 

[Original Post follows]

 

We're currently building a 50m2 rear and side extension using SIPs.  The SIPs structure has been completed, and includes a flat roof structure also made with SIPs.  To make a long story short, I've been told by various sources that converting the flat roof from a vented cold roof design to a hybrid roof with tapered insulation on top is a feasible, though not officially approved solution.  I have had WUFI and DELPHI hygrothermal analyses of the proposed build-up sent to me by smart membrane manufacturers which show that this will work all the way down to 60mm of tapered PIR insulation.  The tapered insulation manufacturer has just come back, however, stating that the only AVCL they will permit is above the deck (above the SIPs), that they won't approve the use of their products if another Smart Membrane is used below the panels, and that the minimum thickness should be 120mm of PIR (most of which will end up being between 150mm and 255mm) The problem with this is that the structural panels will be exposed to humidity from below without protection from an AVCL, and will quite likely soak up moisture over time.  They are referring to our build-up as warm roof, despite the fact that the panels are insulated and sit between joists.  In addition, I'm pretty sure they're using the simpler glaser method dew-point analysis as opposed to the more sophisticated and accurate WUFI analysis.  I'm thinking of two options here:  

 

1.  Agree to get the tapered insulation with the impermeable membrane above the deck and below the tapered insulation. Seal all panel joins and joists with tape and/or sealant without using a smart membrane on the underside of the panels.

 

2.  As above, but use the smart membrane anyway, as the WUFI (from Partel) and DELPHI (from SIGA) analysis says that it will work.

 

Current build up as proposed is the following:

 

OUTSIDE

1.2mm EPDM

tissue backed, PIR tapered insulation (120mm minimum)

bituminous vapour check membrane

11mm OSB3

172 grey EPS

11mm OSB3

(optional smart membrane like SIGA Majrex, Intello Plus or Partel Vara Plus)

50mm service void

plasterboard

INSIDE

 

Happy to attach the analyses if helpful...

 

Many thanks in advance for the input and help.

 

GB

[Russell griffiths]

Why do you need 120mm pir on top of 172mm eps. 

Do you have solid timbers as stiffening in the sips. 

Isnt a sips roof a warm roof already??

looks like somebody is making a fairly simple build up complicated. 

Ive never done a sips roof, but understand the principles of vcl. 

[markc]

@Russell griffiths think the build up is sips between joists or rafters rather than on the top. We erected the Sips “Oyster House” at the ideal home ex many years ago and that was all done with sips infills in a glulam  grid structure

[GrumpyBear]

@Russell griffiths  yes, 195mm deep timbers 4 aside, as well as two RSJs perpendicular to each other.  Panels sit in-between and are insulated.  As for complicated buildup - that was the vented cold roof design by the architect which requires over fascia vents on the internal side of the parapet, along the existing house and even along some sides of the roof lights.  Those roofing details strike me as being prone to failure and a potential source of water ingress with driving rain.  Not to mention potentially ineffective at venting the roof.

 

SIPs are normally considered a cold roof construction.

 

As far as "needing" the 120mm of PIR - that's what the tapered insulation providers have said is the minimum to avoid interstitial condensation.  They are basically shifting the dew point as far away from the SIPs structure as possible.

[Temp]

My understanding is that to avoid an interstitial condensation risk the rule of thumb is that you need at least 2/3rds of the insulation above the structural members and no more than 1/3rd between or below. This ensures that the structural members are far enough towards the warm side that condensation does not occur on them. 

 

It's not the thickness that matters but the thermal resistance. So for example the layers above and between can be the same or similar thickness if the insulation above has twice the thermal resistance of the insulation between/below.

 

So if you have structural rafters/joists between SIP panels you will need a fair amount of insulation on top to ensure they are far enough on the warm side.

 

Putting a vapour barrier below the SIPs isn't usually considered sufficient to prevent a problem occurring. I'm not aware of any reason why you can't put a vapour barrier below the SIP if you also have enough insulation above them. 

Edited August 26, 2022 by Temp

[GrumpyBear]

@Temp My understanding is pretty much the same.

Quote

I'm not aware of any reason why you can't put a vapour barrier below the SIP if you also have enough insulation above them. 

Edited 8 minutes ago by Temp

This is exactly what I found puzzling.  The tapered insulation manufacturer probably has an internal policy to not touch hybrid roofs - so calling it a warm roof allows them to sell me 3k of the stuff!  My thinking is that the smart membrane on the internal face will actually protect the panels and structure during the wet trades/screed.  I've read time and time again that preventing air leaks into the structure is a much bigger concern than vapour diffusion, so I'm edging towards sealing all the internal panel joints and joist junctions with an airtight liquid membrane (soudatight lq) - then installing a smart membrane on the inside face.  

 

Cost wise, the cold roof buildup is similar to the hybrid buildup - only the hybrid gives me 0.08 u value, and tapered internal gutters - whereas the cold roof is a headache detailing wise, and that's where the cost ends up being swallowed. 

[ADLIan]

As above the tapered roof design Co are trying to get the 'balance' of insulation correct to avoid condensation issues. This can be modeled quite well using BS EN 13788 without resorting to WUFI. The are specific design issues with warm flat roofs that cover the install of the AVCL - most manufs will want this fully supported on the structural deck/below the insulation. See MIs and BS 6229.