Hi all,

We are planning to use SIPs for our self build and I'm currently trying to develop strategies to improve the U value and decrease the thermal bridging. The manufacturer has offered a couple of suggestions - using a thicker panel and/or using a reflective breather membrane and a foil VCL.

What are people's opinions on adding another 75-100mm of celotex/kingspan to the internal face of the SIP. I have read a lot of discussion about adding extra insulation to the outer face and the condensation risk that it poses, but not much about adding it internally. If we were to do that, how would we go about it? I don't really want to batten out twice, and where would we put the VCL to avoid it being punctured by services and plasterboard fixings?

My theory is that adding the extra insulation would bring down the U value to approx 0.1 and combat some of the thermal bridging, although the manufacturer is already using insulated splines so hopefully the bridging at panel joints shouldn't be too bad. Unfortunately, due to the plot size and planning restrictions, our house is a non-optimal shape, so I'm keen to make the most of reducing the u values where I can.

Thanks for any input, this forum is so flipping helpful!

divorcingjack

I put 50mm PIR on the inside of my SIP Panels. The SIPs themselves are the Kingspan ones and I got a confirmation from Kingspan this was an acceptable method plus a U-Value calculation.

Not sure if 100mm is risking condensation as the inner face of the SIP may actually be getting a bit cold.

HTH.

I would add insulation to the outside, probably preferring eps and have a vapour barrier on the inside face behind the presumed plasterboard.

Adding any insulation to the OUTSIDE of the SIP will move the point of possible interstitial condensation (dew-point) OUTWARDS. So, it 'may' be in the middle of the PIR.

However, is you use foam-jointed, foil-faced, taped-seamed PIR, there is almost no risk of water vapour migrating through 142mm SIP and the foil face of the PIR, especially with a taped VCL inside the dwelling.

Insulation on the inside of the SIP will bring the dew-point inwards, inside the SIP possibly?

If you want to email the proposed build-up, and I am happy to do a condensation-risk analysis for you.

Cheers

FYI....

The Kingspan Tek SIP Lighthouse in the BRE Innovation Park (the carpark!) used 2 no. layers of 142mm SIP to get the required U-values for CfSH L6.
Then Kingspan did not have a problem with insulation on the o/s of SIP construction.....

Our SIP panels are 142mm. Kingspan (and the co we are using) definitely advise additional insulation to be internal but I can't remember why.

I was advised that a thicker panel offered no advantages and an additional layer (whether in or out) would have fewer thermal bridges than a thicker panel

http://www.tek.kingspan.com/uk/energy_efficiency.htm

Simon, that was my thinking. When you get to that level of U value, it seems that the thermal bridging losses become more significant.

If you don't mind me asking, what foundation design are you going with? I have been advised that the insulated raft foundation is not cold-bridge free for SIP as they have to be supported underneath both OSB panels.

dj,

As far as I remember, the Lighthouse was 2 no. 142mm SIPs back-to-back.
The tech guy was there was giving us the spin, when I trained with BRE.

Insulated raft founds can reduce cold bridging, but it does not matter whether it is SIPs, T/F or blockwork sitting on the raft? The cold-bridging or otherwise is not specific to SIPs.

Cheers

david,

Perhaps I am missing something here, but a SIPs construction sits on a sole plate on the raft, yes?
If the raft is insulated where does the bridge occur?

Cheers

Sorry, I misunderstood 'insulated raft'.
When I spec an insulated raft, it would have 300mm XPS underneath and also 200mm 'upstand' around the edge. So the only bridge would be 18mm OSB outerleaf of the SIP....?
But yes, to insulate the o/s of the SIP is the best way IMHO.
Cheers

.. yes.
You're right, I was still thinking that the OP was putting insulation on the o/s of the SIPs, that would meet up with the upstand of the insulated raft....

I would speak to Kingspan technical. You also need to be sure that whoever is signing off/providing a warranty *plus* the BCO is content with the build-up. Kingspan recommended to me insulation inside and other parties were happy with that.

Oh and the choice of external finish makes a difference as well.

I would say ventilated void between any finish

One of the only structural decay problems that ever happened to SIPs construction was a result of air-leakage between panel-joints in a roof of a building in Alaska. Water Vapor diffusion through SIPs panels has never caused structural decay, so worry about the air-tightness and don't get too worked up about the Condensation Risk Analyses result, if there's no moist air at the Dew Point moisture can't condense. If you do a CRA on External Insulation it usually shows that you'll get condensation 10mm into the material measuring from the outside but it never happens in reality because there's not sufficient moisture present at that point because there's no moist air movement.

For condensation to happen on structural timbers the temperature usually needs to be -10 outside, for mushrooms to grow it needs to be +12 degrees, by the time the temperature increases by 22 degrees the condensation has already dried! If you put just 10mm of insulation over your rafters you'll reduce the amount of condensation on them by 90% so externally insulating the SIPs is a safer way to build, but it doesn't really matter if you build air-tight.
Even Kingspan's Timber Frame details keep the insulation on the inside so I'm not surprised, their theory is to allow moisture to condense on the timber and allow air movement in the cavity to dry it. If you insulate on the cold side you prevent condensation forming on the SIPs which is better in my view!

As well as being Passive House builders we also supply Passive Slab foundations to people building Block, Timber Frame, ICF and SIPs houses, we've constantly tried and failed to produce a perfect cold bridge free detail for SIPs construction, if you sit a SIPs panel on the edge of the slab there's thermal leakage at the edge of the slab, if you sit the SIPs on an insulated ringbeam there's thermal leakage at the top inner and outer corners of the ring-beam.

I've attached the cold bridge free detail we use for our Twin-Wall timber frames.

While the Kingspan guy is here I wish to question him on the claimed Lambda values of SIPs panels that have no foil to prevent Pentane gas leakage, I've seen Canadian University results showing the Lambda of PUR dropping to 0.025W/m2.K after just 100 days when the foil face was cut. The PUR insustry claims it takes 7 years for the Lambda to fall to this level.