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vivienz

How well sealed are SIPs in real life?

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OH and I mentioned SIPs to one of the architects we saw recently, whilst discussing different construction methods and kit house manufacturers.  He sucked his teeth quite enthusiastically and said that he'd seen a few constructions of this type and more than a bit of expanding foam used to seal gaps that shouldn't have been present in high-precision, pre-fabricated builds.  We were surprised as we've heard a lot of good things about SIPs for an airtight house, but the seeds of doubt have been sown.

 

Any thoughts, anyone?

Edited by vivienz

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Good!  Let me sow some more.... is it sensible to combine insulation with structure? 

 

Thermal bridging is massive with SIPs especially at the sole plate, round windows, up corners, big solid timbers are built in, often double ones.

 

air tightness is about careful site work, I have seen 50mm gaps under windows, odd gaps where roof joins walls, service penetrations done with a club hammer.

 

it is good to think about air tightness before you start, you are on the right track.

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Thanks, Tony, that's very helpful and rather the opposite of what the all-in kit house manufacturers say.

 

I'm going to stay with friends for a few days tomorrow and I'm going to drop in at Scandia Hus in East Grinstead on my way, so I'll play devil's advocate and see what they have to say from their side of things.  I'm always cynical when it comes to someone singing the praises of something they want to sell me, especially when it's very expensive.

 

 

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Many thanks for the information, chaps, very helpful.  The links to the previous threads were a real eye-opener, I have to say.  I'll do a summary of them in a few days but the main thrust of the various threads seems to be the issue of the thermal bridge between the walls and sole plate which have the potential to condensation and, in time, rotting.  I really can see the sense of the points being made and the difficulty for the buyer when you have a groundworks supplier and separate house supplier.  The poor old consumer will fall between the two, quite literally if the base of the house collapses, with no satisfactory recourse to either.

 

I've looked at the MBC system that has been frequently mentioned on this forum and it certainly has its advantages.

 

Off to do some more research.

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Hi we built a SIP house 4 years ago, and as stated its the site work that dictates how well sealed the structure is. The weak points are the first floor as the joists sit on the sip panels and any roof valleys.

We spent a good week going over the roof junction as the foreman was off when the roof went on and there were a few gaps that were quickly sealed and taped.

It is no different to any other house in that attention to detail is the key.

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I looked at sips in detail and contributed to the ebuild thread. That thread well and truly put me off at the time.

 

They do have some benefits though and I've been thinking about them again recently. 

 

Timberframe quotes seem to have increased significantly in the last few years, so the price gap has narrowed. My house will also have a open roof space and open plan layout on the upper floor. Which makes sips and attractive option. 

 

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Do we have any air tightness test results for real ones? I like to see second test after six months.

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Just thought I'd update this thread with some real life observations now our own SIPS house is in place. Really surprised at the comment above re 1st floor being supported *on* top or inserted into  the panels - they should really be using joist hangers so there's no penetrating the structure at all.This is kind of fundamental to take advantage of all that insulation really. Even where we had to have steels in because of some.large spans they've cleverly been terminated without going through the wall (big strong upstands on massive timbers) The only things that go actually through the construction of our SIPs (done by clays) inside to outside (that is to say exposed in the 55mm cavity between the sips and the stone outer wall or under the roof slates) are 2 beams where we have a very large roof window I believe called a cabrio?) which needed that extra support, and ditto on our really large dormers, which would not be a feature in a normal 2 storey. However, even then since we are applying a layer of insulation inside too, nothing in fact will be cold bridging dorectly into the interior of the house. We certainly don't have any of those massive timbers referred to acting as cold bridges. There are large timbers involved but have quite cleverly been designed to be *within* the envelope.

 

As for gaps,  so far the only air gaps we have been able to discover were some timy ones between the sole plate/DPC and the thermal blocks it is sat on - and I think they were down to slight irregularities in the blocks/block laying actually.  But these were sealed and we've gone over it all ourselves again and silicone the edges above and below the DPC even though it wasn't really necessary. Similarly since we'd bought boxes full of sealant, we sealed all the joins in the panels ( which was pointless to be honest as I saw how they were sealed together but attention to detail seems to be key, and I've tried to provide jobs that everyone in the family can feel part of available for them to do) Where you may see expanding foam used is I watched when they fastened the panels together - talk about thorough... the panels were spline joined, so no big timber cold bridges (the splines also being SIPS themselves), and they "glued" the insides first with low expansion sticky type foam, then the panels were pulled together under a lot of pressure with a device that reminds me of a fence tensioner, so a lot of the foam squeezed out at the joins of course then literally about 100 nails were fired in each edge( I stopped counting at 100. I reckon those house must weight an extra ton from all the nails ?) I was quite genuinely amazed how thorough the guys were - particularly when we got torrential rain and they were literally soaked to the skin through their waterproofs even.  Needless to say, there's not  been a lot of gap finding to do despite us going over quite literally every mm of join - but I do have to say that the guys were unbelievably thorough, I mean to a degree *way* beyond my expectations and possibly not all firms will be quite so exacting.  I also think bridging from big timbers had been thoroughly thought about, because you *could* build it in a simpler faster way for the builder, but have those timbers bridging between the cavity and the interior.  I think our checking out of the various firms for a few months paid off.   One thing I have noted is that the breather membrane touted as waterproof for up to a few weeks has not stood up to some of the heavy rain we've had amd has definitely let water through... but the roofers started today and being a belt-and-braces type is actually felting over the lats too! So even if we lost a slate at some point in the future  there would be 2 layers in fact between the outside and the osb face.

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