Sun, 06 Sep 2026 10:40:36 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277323#Comment_277323 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277323#Comment_277323 Mon, 09 Dec 2019 10:35:49 +0000 jerseyman 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]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277327#Comment_277327 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277327#Comment_277327 Mon, 09 Dec 2019 11:36:33 +0000 djh
Rock wool will be a bit messy, it can be very dusty; be sure to wear protective clothing. One alternative would be warmcel. It should fill the space better though it will cost more.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277329#Comment_277329 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277329#Comment_277329 Mon, 09 Dec 2019 12:03:29 +0000 WillInAberdeen
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277335#Comment_277335 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277335#Comment_277335 Mon, 09 Dec 2019 17:43:35 +0000 jerseyman https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277337#Comment_277337 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277337#Comment_277337 Mon, 09 Dec 2019 21:56:10 +0000 gyrogear
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]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277338#Comment_277338 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277338#Comment_277338 Mon, 09 Dec 2019 23:37:02 +0000 tony https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277339#Comment_277339 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277339#Comment_277339 Tue, 10 Dec 2019 08:23:50 +0000 GreenPaddy
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277342#Comment_277342 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277342#Comment_277342 Tue, 10 Dec 2019 09:03:17 +0000 jerseyman https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277352#Comment_277352 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277352#Comment_277352 Tue, 10 Dec 2019 15:26:03 +0000 tony https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277354#Comment_277354 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277354#Comment_277354 Tue, 10 Dec 2019 16:33:08 +0000 djh https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277356#Comment_277356 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277356#Comment_277356 Tue, 10 Dec 2019 17:37:09 +0000 GreenPaddy
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....]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277445#Comment_277445 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16357&Focus=277445#Comment_277445 Tue, 17 Dec 2019 20:53:38 +0000 bogal2