Gooman Posted July 31, 2019 Share Posted July 31, 2019 The floor level of the garage that we're converting is around 100mm below the rest of the house. I know we can achieve the required u-levels with either 50mm Celotex and 50mm screed, or 100mm Celotex and chipboard floating floor. I can save money on the build by going for the thicker Celotex and floating floor, as I can fit that myself, so that's got a lot going for it. But are there any good reasons why I should go for the screed floor instead? Don't want to regret scrimping if it's a worse result. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 31, 2019 Share Posted July 31, 2019 Not sure how you can get the same U value with 100mm PIR or 50mm PIR and 50mm screed. Ignoring perimeter effects (just because I don't know the perimeter for your floor) I make the basic U value of 100mm of PIR about 0.22 W/m.K (not great, and will be worse when the perimeter losses are taken into account) and for 50mm PIR plus 50mm screed the basic U value increases to about 0.432 W/m.K, which is far too high to meet building regs. The true U values will be slightly different, depending on the concrete floor thickness, the floor build up above the insulation and the insulation level, if any, around the perimeter. Link to comment Share on other sites More sharing options...
Gooman Posted July 31, 2019 Author Share Posted July 31, 2019 I had calculated the floating floor u-value, but just assumed (damn, there's the problem) that other resources that suggested 50mm Celotex and 50mm screed were correct. Guess you've answered my question for me ? Perimeter is 20m. On a related note - Celotex's application note reckons GA4000 at 100mm gives a u-value of 0.18 ... unless I've misunderstood? https://www.celotex.co.uk/assets/garage-conversion_application-datasheet_nov18.pdf Kingspan's TF70 brochure reckons it will give a u-value of 0.17 for a timber floating floor: https://www.kingspan.com/gb/en-gb/products/insulation/resources/product-literature/thermafloor-tf70 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 31, 2019 Share Posted July 31, 2019 3 minutes ago, Gooman said: I had calculated the floating floor u-value, but just assumed (damn, there's the problem) that other resources that suggested 50mm Celotex and 50mm screed were correct. Guess you've answered my question for me ? Perimeter is 20m. On a related note - Celotex's application note reckons GA4000 at 100mm gives a u-value of 0.18 ... unless I've misunderstood? https://www.celotex.co.uk/assets/garage-conversion_application-datasheet_nov18.pdf Kingspan's TF70 brochure reckons it will give a u-value of 0.17 for a timber floating floor: https://www.kingspan.com/gb/en-gb/products/insulation/resources/product-literature/thermafloor-tf70 Do you know the area and the depth of the concrete garage floor? To work out the U value all the layers need to be taken into account, together with the surface resistance both sides and the perimeter/area ratio. The timber floating floor will improve things slightly, as will the surface resistance effects, and the thickness of the underlying concrete. The insulation manufacturers tend to be a bit optimistic in the general examples they quote, so the best way to get a reasonably accurate answer is to use the λ value for the insulation material, and the λ value for all the other layers that make up the floor, together with thicknesses of those layers, the perimeter/area ratio and the surface resistances. That way everything is accounted for and the result is likely to be fairly realistic. Link to comment Share on other sites More sharing options...
Gooman Posted July 31, 2019 Author Share Posted July 31, 2019 Area is 20.5 m2. No idea of the concrete depth I'm afraid ... it's an early 1970s build if that helps with any assumptions. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 31, 2019 Share Posted July 31, 2019 100mm of PIR laid over the concrete, with a floating timber floor on top, and a perimeter/area ratio of 1 gives a total U value, allowing for surface resistance, of about 0.18 W/m.K. 1 Link to comment Share on other sites More sharing options...
PeterW Posted July 31, 2019 Share Posted July 31, 2019 Check the floor is actually level too - most old garages were laid to a 1:150 slope which can catch you out so need levelling before the PIR goes down 1 Link to comment Share on other sites More sharing options...
Gooman Posted July 31, 2019 Author Share Posted July 31, 2019 Cool. Looks like a goer then! For wall insulation, on one side the wall is shared with the house (so heated on the other side) and the other side is a 100mm single skin brick party wall shared with next door's garage (unheated). Would I be right in assuming that the party wall will be treated as an external wall for Building Regs purposes, but that the house wall won't be? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 31, 2019 Share Posted July 31, 2019 Yes, the shared wall will be treated as being to an unheated space, as it probably will be most of the time. Link to comment Share on other sites More sharing options...
nod Posted July 31, 2019 Share Posted July 31, 2019 A screed will seem quite cold compared to insulation and CB But also depends on the final finish of the floor Link to comment Share on other sites More sharing options...
wozza Posted July 31, 2019 Share Posted July 31, 2019 (edited) 2 hours ago, PeterW said: Check the floor is actually level too - most old garages were laid to a 1:150 slope which can catch you out so need levelling before the PIR goes down Most of our double length garage floor has been removed because of this. 2 hours ago, nod said: A screed will seem quite cold compared to insulation and CB But also depends on the final finish of the floor Sorry to hijack the thread but it got me thinking Will screed on PIR feel cold? Our extension floor will be 100mm or so of concrete, 100mm of PIR and then a 75mm screed - it meets the old floor that is just 100mm concrete (and its cold in the winter) so we awaiting a price to see how much to replace the old floor to match the new so we don't have half insulated and half not insulated. Also we will be fitting 25mm perimeter insulation - should we also fit this against the internal walls (that were previously external walls) as they are built straight off the footings? Edited July 31, 2019 by wozza Link to comment Share on other sites More sharing options...
nod Posted July 31, 2019 Share Posted July 31, 2019 1 hour ago, wozza said: Most of our double length garage floor has been removed because of this. Sorry to hijack the thread but it got me thinking Will screed on PIR feel cold? Our extension floor will be 100mm or so of concrete, 100mm of PIR and then a 75mm screed - it meets the old floor that is just 100mm concrete (and its cold in the winter) so we awaiting a price to see how much to replace the old floor to match the new so we don't have half insulated and half not insulated. Also we will be fitting 25mm perimeter insulation - should we also fit this against the internal walls (that were previously external walls) as they are built straight off the footings? Perhaps I should of re phrased it 22 mill of chipboard on 100 mill of insulation will feel pretty warm under foot Screed on insulation is pretty standard and ok also Especially for tiling 1 Link to comment Share on other sites More sharing options...
nod Posted July 31, 2019 Share Posted July 31, 2019 1 hour ago, wozza said: Most of our double length garage floor has been removed because of this. Sorry to hijack the thread but it got me thinking Will screed on PIR feel cold? Our extension floor will be 100mm or so of concrete, 100mm of PIR and then a 75mm screed - it meets the old floor that is just 100mm concrete (and its cold in the winter) so we awaiting a price to see how much to replace the old floor to match the new so we don't have half insulated and half not insulated. Also we will be fitting 25mm perimeter insulation - should we also fit this against the internal walls (that were previously external walls) as they are built straight off the footings? Perhaps I should of re phrased it 22 mill of chipboard on 100 mill of insulation will feel pretty warm under foot Screed on insulation is pretty standard and ok also Especially for tiling Link to comment Share on other sites More sharing options...
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