Thursday, September 6, 2012

Nylon Rods and SIPS explained a bit more

September 6, 2012: Matt F wrote a comment and question as a follow up to a posting below, and as this is a new topic because it goes into more detail about the rods.

Question: Matt F: Interesting that you are fabricating your own SIPs. 

  • Is this primarily a cost saving exercise? 
  • I note that you actually penetrate the SIP with the nylon rod - doesn't that create a thermal bridge in itself? 
  • I ask because we are quoting for a PV system on a SIP roof and are concerned about how to best mount the system. 

Reply: David Nicholson-Cole: Hi. we originally ordered only 10 metres of nylon rods, but have finished using 34 metres, they are that good!! Now that we have a technique, we use them more than first intended. We cut them to 200mm units with a handsaw. Theres no measurable thermal conduction through them, and they are so strong, no matter how much we tighten them up, not a single one has slipped its thread. As we only need 186mm each, so we cut them to length after the rods are tight and the panel glue has set. You need a tube spanner, ideally. We keep some over-long ones (225mm) for first-fixing at corners when the panel is first assembled.

Nylon rods don't stretch, shrink, creep, change thermally, rot, or vibrate undone. We like them so much that the entire extension (all 950 kilos of it) will sit on 8 steel feet, secured only by 8 M12 nylon rods, plus some wall bolts to the existing house. We use M8 (8mm) rods for the panels and M12 (12mm) for fixing it down.

If you have to work with a SIPS construction, you could use this method for fixing solar panel rails firmly. We countersink the thread so that the nut head is level with the face of the timber. The nuts have their own spreader profile, like a washer. If you are in the UK, they come from one supplier in the UK, called BLUEMAY 01380-821821. If you need only a small number, I might have plenty left over by the end of the week.

Another Reply:  David NC: We are hand crafting our own SIPS panels for several reasons.

  • One because at 1m x 3.6 m its probably too small for a commercial supplier to do. 
  • Two because its a complicated shape sectionally that they might not be willing to do. 
  • Three because its Fun and Research - I teach construction so this is a good technique to pass on.
  • Four because we want to hand craft each junction to avoid thermal bridging. 
  • Five, because all the materials, OSB board, PIR insulation, PVA glue, nylon rods and lots of screws are readily available. 
  • Six, we have had to learn as we go how much along how heavy each part would be, and the site is inaccessible to a crane. If we had it delivered in factory made panels off a truck, they would have been impossible to lift into position. With hand crafting, we can decide how to section the SIP cassettes and dry build it in the garden entirely before moving it to the final location, in chunks that can be lifted by at most, four people. The roof will be in 3 interlocking cassettes.
The downside is also in several parts,

  • One, we have to do this without the large workshops, CNC cutting machines and industrial glues and presses that the commercial boys have. 
  • Two, I still have to pay the labour charge for a joiner for skilled cutting, so its not free, 
  • Three, we have to apply massive levels of concentration to cutting out the parts to ONE millimetre accuracy, as the whole thing will have to be airtight and structurally fitting. 
  • Four, the boards have varying levels of slight curvature, so without an industrial press, we are reliant on Nylon rods, and we keep running out!
Question from Dr Chris Wood: I wonder why you went to the expense of buying foil backed insulation, when you have no cavity. The foil is only of benefit when inside an air void, as it provides low emmisivity and high reflectance. When sandwiched the heat flow is purely conductive.

Reply: David NC: Chris, that is how it comes, we don't have a choice. It makes it a lot easier to glue to as the foil is industrially glued to the insulation. It's easier to spread glue on the foil than on the matt face..... but I notice that the glue spreads into droplets somewhere, so there is a risk of poor adhesion. Thankfully, our nylon bolts will be 'lost' in the construction and ensure that the panels stay together. The foil also acts as a Vapour Barrier, although I don't think this type of insulation has a dewpoint in it.

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