Site-made SIP floor detail
Hi, I plan to make a ‘homemade SIP floor’ for insulating my tiny house’s suspended floor or maybe rather an IP floor since the panel will not be structural in the same sense as SIP but I don’t seem to find much info on the method I intend to use and am hoping you can advise.
The sandwich panel will consist of a bottom ‘skin’ of 16mm (5/8”) Magnesium Oxide (MgO) board (supported structurally by timber floor joists), which on its own would be sufficient structurally as a floor if no insulation was needed. On top of that will rest a 50mm (2 inch) higher density EPS (I was thinking of using 1.25 lb/ft3 density) and then on top of that will rest or float the top ‘skin’ of 9mm (3/8”) MgO. (The 9mm is thin but if supported by a solid substrate (the EPS) I don’t see that it will flex/fail if it does not span anything?)
1) Originally I thought to glue the EPS to the 2 MgO boards, but then reading about the volatiles in the adhesive I have been advised would work to bond EPS to MgO, I started wondering if I can do without the glue (also it is quite expensive)?
2) My original plan was to ‘join’ the ‘panels’ in the typical SIP way of timber splines, as it also gives the top board a place to be fixed to. But then I started wondering if this is even necessary? Can’t I just lay the EPS continuously and then fix the MgO boards to each other by using a ‘false box spline’ i.e. a strip of MgO below the joint (I would have to recess the EPS by 9mm so that the boards lay flush on the ‘strip’) – and then screw each side to the strip? Would save a lot of weight if I don’t do the timber splines.
3) If joining the panels to each other and to the wall edges is not enough to prevent the floor from trying to lift at one end when say a heavy load is applied on the other end, would it work to fix the top board with a long enough drywall screw through the EPS to the bottom board? (this assumes that the compression strength of the EPS is sufficient to not deflect much under typical loads). And in that case could I leave out the ‘false spline’ strip completely (since it will save work on making the recesses in the EPS)?
4) Regarding 3) is 1.25 lb/ft3 density ‘stiff enough’ for the job? I can also go to 1.5 or 1.9 lb/ft3 but the cost increases. (Here, 1.25 is used to insulate domestic concrete slabs, 1.5 for commercial and 1.9 for industrial but my application is different than a slab of concrete on top)
5) I live in sunny South Africa with a relatively mild climate – temperatures in winter only occasionally dip below freezing and in summer goes to 86F with occasional temps of 95F or higher. So the insulation will be mostly to minimize energy for heating/cooling to keep it at a comfy temp. Our building code requires minimal R-value for flooring of 1 only (but more for walls and roofs).
6) Am I missing anything? (I would fill up the gaps between the top boards with a filler that allows some movement, and the floor finish would likely be a vinyl covering the joins)
Looking forward to get advice if anybody has done something similar, thanks!
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Replies
Lii,
I built a floor for an insulated shed much the same way. Floor joists, plywood, 2" EPS, and a t&g plywood subfloor. Both layers of plywood get fastened to the floor joists. The bottom one with nails, the top screwed through the foam. No splines or grooving necessary. With 2" EPS the joists aren't hard to hit. Use deck screws, not drywall screws through.
Thanks Malcolm. What density EPS did you use and is it holding up under your normal loading? I thought of using drywall screws since that is the recommendation of the MgO board supplier (the black phosphate coated ones) but have not checked with them specifically if this is also for floor (but I think their application is typically directly to the joists not through EPS). Will look at deck screws, not sure what the difference is but someone else with building knowledge also recommended that instead.
Lii,
Drywall screws may a be a different animal in South Africa. here they are low-grade steel with a thin shank, that should not be used for anything else.
I used EPS from a big box store, which is low density, but has proven fine. The plywood does compress at the seams a bit when you screw it down, but I've had that happen with much higher density XPS too.
Lii -
I would back up whatever detail you use to air seal the floor panel at joints or to other elements of your tiny house with a high quality 6-inch minimum acrylic pressure sensitive adhesive tape (such as Huber ZIP tape, or a Pro Clima or Siga tape). Over time, all joints move and all materials contract and expand and the tapes are a good "relief valve" for that movement, since they can accommodate that movement, particularly if they are mechanically trapped by subsequent layers in your assemblies.
Peter
Thanks Malcolm, the MgO supplier in the meantime confirmed use of drywall screws (I think you are right here they are different in SA - it comes in lengths of 100mm (4'') and more so maybe not only for fixing 'drywall' type boards). Alternatively they indicated we can drill through the boards first and use screws and nut washers which I assume is what 'deck screws' are.
Peter, the tape backup at joints seems sensible, but I would need to find a local substitute. Before you mentioned tape, I was thinking of putting a bead of low density polyurethane (sprayed from a can) between the joints, do you think that will do a similar job? I also think because I am now not bound by the splines, that the assembly will be more airtight as I don't need to lay my EPS and MgO boards in the same direction so not overlapping the joints of the one at the joints of the other.
*Correction* I meant to write "low expansion polyurethane"