This week on stuff I mentioned that our frames (the external framing not the internal partitions) are slightly thicker than standard: 140mm rather than 90mm. It’s only a marginally different cost for a big impact to our overall plan: it solves both a structural and insulation issue – they can be cantilevered over the edge of the concrete base, and it means we can pack in more insulation to ensure we keep our thermal envelope intact.
Because we have 50mm of XPS on the concrete block walls below the top timber framing walls, we wanted an overhang to make top and bottom sit together like a nice neat box. If we'd had standard 90mm frames, we'd have had no ability for the frames to cantilever out - they wouldn't be structurally strong enough. Having that neat box, where the external skin of both the top (timber) and the bottom (concrete block) parts are flush, allows us to keep our thermal envelope in one clean, easy line.
140mm also allows us to pack more Earthwool/Glasswool insulation
The surprising thing is that it's cheaper to fill this 140mm frame with R3.2 Earthwool/Glasswool than it would be to fill a 90mm frame with R2.8 or even the minimum R2.6. I know, even my eyes are crossed at this point. But it's a win win for us. We can offset the cost of the cheaper insulation against the slightly more expensive cost of the timber and at the same time get a better R value all round. It doesnt quite come out even as the timber is slightly more expensive than the saving on the insulation, but still, it's pretty close and we get a much better insulation solution, especially as we needed the external insulation to dove tail in top to bottom anyway.
For those of you completely lost and not sure how to tell your Rvalue from your XFactor, its simply a matter of numbers. The higher the Rvalue of your insulation (and the same goes for windows if you're looking at double glazing etc), the better the effect. So the 3.2 means we get higher insulation properties from our walls. The reason, Mr Building Boxes tells me, for this seeming discrepancy (usually better is more expensive, right?) is that the lower R value in the 2.6 and 2.8 also comes with a corresponding increase in density for the product, so it can be squeezed into those standard 90mm walls. There's more product, packed into a smaller space. So it's more expensive.
Got it? Good...If you have any questions, feel free to let me know...and I'll get Mr Building Boxes to do his technical arm waving explanation dance for you.
A family of four: writer, scientist, lego engineer and destruction specialist. Our previous home is featured in New Zealand Interior Style and this new project promises to provide plenty of great, green, smart and maybe madcap solutions to new building in New Zealand.