Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    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!

    • CommentAuthorDarylP
    • CommentTimeFeb 4th 2014
    ... why would adding extra insulation to the o/s face of a SIP lead to condensation?
    If it was me, I would use a 142mm SIP with 100mm PIR glued/screwed to the o/s. Cuts down on thermal bridging at the joints.

    Decent U-values, the SIP stays at room temps, and gives you an decent skin to fix to internally....

    Good luck:smile:
    • CommentAuthorborpin
    • CommentTimeFeb 4th 2014
    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.

    We're planning on 70mm inside our Kingspan SIPS - just trying to confirm where we place the airtight point. I'd prefer to have it at the SIP level and put the additional insulation on top.
    • CommentAuthortony
    • CommentTimeFeb 4th 2014
    I would add insulation to the outside, probably preferring eps and have a vapour barrier on the inside face behind the presumed plasterboard.
    Thanks all for your input, very reassuring!

    DarylP, I read that adding insulation of the same type (PUR) to a SIP risks moving the dew point to the middle of the insulation/OSB. Perhaps I read it incorrectly, but it scared me at the time! Is it OK to screw the insulation tight to the OSB - would the VCL go on the inside of the additional insulation then, under the pasterboard?

    Simon Still, that's what my architect suggested, but I wasn't sure about the condensation. Has your SAP guy/kit supplier got anything to say about it? It would be great to have the airtight layer away from services and plasterboard/fermacell screws. What size are your SIP panels?

    Tony, can I ask why your would have the insulation on the outside and why EPS?

    I'm thinking of a 175mm SIP plus 75mm celotex/kingspan, but I have to work out what the potential U value would be.

    Cheers again,
    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014
    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.

    I thought it was 'definitely beyond debate' that, with a relatively clean sheet of paper, putting insulation on the outside is a no-brainer - probably every 10th thread mentions it!
    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014

    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.....:bigsmile:
    • CommentAuthortony
    • CommentTimeFeb 5th 2014
    I do not like the basic philosophy of combining insulation with structure but having done so it is vital to protect the structure, so insulate it on the outside

    EPS is cheap, breathable easy to work with
    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

    DarylP, that is really generous of you, thank you. Will look up the detail later and post on here. Did the lighthouse use two SIPs back to back, with no cavity then?

    Gotanewlife, I believe that it is a specific problem with SIPs because they are faced with the same material inside and out.

    Tony, thanks for the info. Is EPS more breathable than PUR then? Are there any brand-names to look for like kingspan or celotex, as they seem to be fairly easy to source. Would you add the insulation direct to the outside face of the SIP, or have a cavity?

    Thanks again,
    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.
    • CommentAuthortony
    • CommentTimeFeb 5th 2014
    add direct PUR is not breathable
    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014

    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.

    The problem with SIPs is that both skins of the SIP need to be supported. So either the concrete raft bypasses the SIPs insulation causing a thermal bridge or you have two independent leaves which is difficult with a raft. It is probably simplest to build a cavity wall below DPC & then use a suspended concrete slab instead of a raft. This will work in most ground conditions.

    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014

    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 :smile:
    • CommentTimeFeb 5th 2014
    Posted By: DarylPPerhaps 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?

    The outside of the SIP is cold; the inside of the SIP is warm. If you sit both sides on concrete then you're bridging the concrete to the outside temperature. Normally with an insulated raft you sit the warm structure on the slab, and insulate it externally.

    You either need thermally separated supports for each edge of the SIP, or a SIP design that doesn't need support at both edges. That could be some reinforcement at the base of the SIP so it can sit on the inside edge and overhang the slab, much as a timber frame with Larsen truss. Or it could be a separate ringbeam and a smaller amount of cantilevering. The last is what we're planning to do - there's a similar design issue with straw bales.

    Thinking about it, I suppose another possibility might be to put the SIP on the slab and then insulate the SIP externally. There'd still be a bit of a bridge, but hopefully not too much of one. THERM would tell.

    Google shows a few hits for 'passivhaus foundation sip' so there is some knowledge out there.
    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014
    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.
    • CommentTimeFeb 5th 2014
    Posted By: DarylPWhen 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....?

    Yes, the cold outer leaf of the SIP is what makes the bridge, together with whatever timber there is at its base.
    • CommentAuthorDarylP
    • CommentTimeFeb 5th 2014
    .. yes. :smile:
    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....:shamed:
    So, is there more condensation risk adding the insulation externally? Hopefully darylp will be able to test the build up :)

    Djh, we think our foundations will have to be dug down quite far because we have a near neighbour whose yard is about 4m below our plot level, so engineer concerned about their wall collapsing. Don't know if your foundation design (twin ringbeams?) would work with that.

    David freeborough, aren't there problems with a suspended slab in terms of making it airtight?

    I have to admit that I am very worried about the foundation thermal bridging :( thought I had it all sorted out.
    • CommentAuthorborpin
    • CommentTimeFeb 5th 2014 edited
    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.
    It's going to be a mix of render, timber cladding and steel roofing as an external finish.

    I will ask the SIP provider tomorrow to see what they say . Confusing!
    • CommentAuthortony
    • CommentTimeFeb 5th 2014
    I would say ventilated void between any finish
    Posted By: divorcingjackDavid freeborough, aren't there problems with a suspended slab in terms of making it airtight?
    No, a cast in-situ suspended concrete slab is just as airtight as a raft foundation because its built into the external wall in a similar way. The one that's difficult to make airtight is a ground bearing slab because you can get relative movement between the slab (which bears directly on the ground) & wall (which has its own concrete strip foundations).

    However, all of the above are better than a pre-cast concrete or timber suspended floor with ventilated void below.

    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.
    Hi Divorcingjack

    We would recommend the use of an insulated plasterboard such as our Kooltherm K18, which can be break bonded and mechanically fixed to the inside of our 142mm Kingspan TEK panel.

    Our Specification Manual which can be found at www.kingspantek.co.uk/Literature/Product-Literature. Page 23 may be of interest or feel free to contact our Technical Services Department on technical.uk@tek.kingspan.com or 01544 387482

    Do let me know if we can be of further assistance.

    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.
    Hi David thanks for the info. Could I possibly ask you for a drawing/detail? Would you have one? Visual is so much easier to get hold of. Would you use insulating blocks to build the cavity wall to minimise the thermal bridge?

    We are hoping to use UF heating in the slab and had been considering powerfloating the finish of the structural slab to add thermal mass and avoid having to pour an additional screed. Would this work with your suggested approach?

    Viking, thanks for the clarification - I really don't think our site conditions are going to be suitable for an insulated raft due to the next door yard below the level of our site. It would be the simplest solution but i reckon our engineer won't go for it. I have to confess that I am still really worried about condensation and the SIP because of the risk (albeit tiny) of structural failure.

The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press