Saturday, September 1, 2012

Extension and SIPS construction

1 September 2012:  [Extension] More innovation! 
As I work on this, I have just realised that this is a new form of construction, adapting what is already a very innovative technology, that of SIPs (Structural Insulated Panel System), but improving its thermal design. I am using the construction method of the Sunbox to build this, essentially.
   Yes, it is a SIPS construction, I am using that for the structural concept of the wall, but it is as different from normal SIPS construction as my Sunbox is from a framed conservatory. I've been so used to teaching Passivhaus principles, that I am hunting down every risk of thermal bridging in the construction.

The differences is in the fabrication and in the corner jointing.

Fabrication: SIPs are sandwich panels of 18mm OSB enclosing a core of foam insulation. The thickness is variable, but in my case, I want a U-Value better than 0.15, so mine will be 150mm of foam, making it 186mm in total. Normally with SIPS, the company will take my architectural drawing, will do a shop drawing. The panels of OSB (oriented strand board) would be cut out on CNC machine, and the sandwich panels of OSB and polystyrene would be glued and put in an industrial press to set. Edge blocks would be included where bolting will be needed.
   I am building these panels myself in the back garden, and my industrial press is a set of clamps at the corners, plus 10 metres of high tensile Nylon threaded rods that will go through the panel at intervals and ensure that there is no risk of delamination. I am not using expanded polystyrene. In my case, I am using the best insulation, PIR closed cell foam, with higher U-Value, water proof and vapour resistant, the vapour barrier included. The U-Value will be better than Passivhaus.

Corner jointing: SIPs are normally framed. Even if SIPS is used in a monocoque fashion (i.e. no frame, the panels self structural), the SIPS are normally used like large slabs. They arrive from the factory on a low-loader, ready made. They are bolted together at junctions, because it is easiest that way, and very strong.
    On this extension, at each corner, the way that the timber and the foam meet are carefully considered, and any possibility of thermal bridging is removed by considering how they fold and butt to each other. The outer and inner corners are then secured with aluminium angles, 50x50 on outer corners and 40x40 on inner corners. This is how the Sunbox is built, with the triple skin polycarbonate carefully butted, and with external and internal metal angles, ensuring no thermal bridging.
    The Extension is a strong monocoque construction like the Sunbox. It could have been cantilevered from the wall if the house had been a little bit more elevated relative to the ground and if I thought that Rushcliffe Borough council building inspectors would understand how it worked. But for safety, there is a light concrete raft on which it will sit.
   At this moment, I have not quite decided on the eaves detail where the roof and wall meet, because I do want to maintain this unbroken line of insulation. But it will be decided in the next few days.

Another aspect I should add is that of demountability. I will make it so that in some future time, it could be carefully un-screwed, un-bolted and taken away, leaving only the floor slab. Just in case a future owner wants something different. Mind you, visitors to the house will be body searched to see if they are carrying screwdrivers or spanners!

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