We are building above an existing Garage, the Architect has specified eco-joists for the ceiling of the Garage/ floor of the new build.
For insulation he has specified rock wall between the joists, but as far as I can see, by the nature of eco-joists it is going to be pretty near impossible to get a complete thermal barrier this way. Is any one aware of a standard or recommended approach for insulating eco-joist floors? Or is the rock wool approach OK?

Thanks for any help

How deep will the joists be? (How far is the new floor above the garage ceiling?)

Unless the joists are quite deep, there won't be space for enough wool to keep your extension insulated from the cold air in the garage, so you might want an extra layer of insulation boards under the joists, or a higher spec insulation between them.

The architect will also be thinking about fire barrier between the garage and the extension.

No experience of these joists, but looking at them per this :

https://www.shmdirect.com/eco-joists/

there will still be a risk of thermal bridging along the metal webs, because they come nearer to the boundary of the insulation : the web plates look like they come within an inch of the surface of the flange... Taken collectively, the webs will provide a likely route for heat-loss (presumably) downwards into the unheated garage... As DJH says above, some other (additional) form of insulation might be a worthwhile consideration.

gg

Jerseyman,

if you use a wool insulation with a lambda value of 0.032W/mK (I personally like knauf frame therm 32, other makes are available), you would achieve a Uvalue of 0.138 W/m2K.

That's a pretty good value, and into a garage (unheated space which will not have wind chill or as low temps as externally), I'm not convinced of the cost/benefit of driving that lower.

You could go for a cheap loft roll wool insulation (0.044) with a 50mm skin of PIR or PUR underdrawing the floor, and that would give you a Uvalue of 0.126W/m2K. That may be more costly, and certainly more work.

Actually, you might get a little better than above, as I've used timber "I" joists, so a slither of timber rather than open metal straps, but same ball park.

If the rest of the build is as good as roughly 0.13W/m2K, you'll have a really good build, but focusing on the "easy" part of Uvalues in the chunky bits can blind you to the tricky areas of corners and junctions full of timber, that lead to high psi values for cold bridging for long strips of structure. As you drive down the general Uvalues, so these parts become increasingly significant. Ask your architect to show you details for all those bits (probably 6 to 8 specific details) and make him earn his money.

Including through the joists I hope

Wao. I can't believe easy joists are the same (fractionally worse actually), for thermal bridging as solid timber. I've read they Wolf Easy-joist tech guide, and that's what they say. So must be true.

If at all possible, go for timber I joists, as they perform better thermally than solid timber (ditto easy-joist), and less timber being used.

So taking the example of wool 32 as the insulation, for 250mm thickness;

- solid timber / easy-joist = Uval 0.147 W/m2K
- timber I joist = Uval 0.136 W/m2K (rechecked this)

getting towards 10% less heat loss.

PS...in case anyone happens to suggest filling with glass beads....