Carp07 Posted April 29 Share Posted April 29 Hi, I’m not sure how to introduce but I’m just going to blurt it out… looking at doing a garden room and my architectural technician has designed a raft foundation for me but I still seem to be at a loss on how to go about it. the raft foundation design to me seems OTT for a timber frame garden room but then this is not my area of expertise so…. Advice please. to me it looks like 150mm insulation under the raft going to 100mm under where perimeter frame sits (with concrete going other way 100mm to 150mm). Any advice would be most appreciated Link to comment Share on other sites More sharing options...
JohnMo Posted April 29 Share Posted April 29 Here is what I did If you can buy a kit, mine is 70mm thick timber and fully insulated top bottom and sides. Way easier and not that expensive for what you get 1 Link to comment Share on other sites More sharing options...
kandgmitchell Posted April 30 Share Posted April 30 You are going passivehouse for a garden room? I'd buy a kit like JohnMo says and sit it on a reinforced slab. At 20m2 it'll be exempt building regs if you keep it away from the boundary. Link to comment Share on other sites More sharing options...
Carp07 Posted May 1 Author Share Posted May 1 It’s what my architect designed, personally I feel it’s over the top… you reaction says to me like “wow your going passivehouse for a garden room!” It’s 400mm away from boundary, spoke to building control and as long as fire ratings met of timber frame (so cement board cladding externally and 15mm fire board internally it fine). so you reckon a reinforced slab? With insulation under concrete slab but just say 120mm pir rigid board with concrete over top? Or slab then insulation on top with floating floor? Thank you for reply Link to comment Share on other sites More sharing options...
joe90 Posted May 1 Share Posted May 1 How much will it be used ?, if very often/long hours then maybe worth insulating but if not then heating it for short periods will be fine IMO. Yes that slab looks OTT to me. Link to comment Share on other sites More sharing options...
Iceverge Posted May 1 Share Posted May 1 Holy smoking woodfiber Batman! That is one hell of an install. U Value about 0.1 on the wall. Top shelf products from all the premium players. Gutex, pro clima. For giggles I'm going to price it up without the cladding and plasterboard as you'll need them anyway. Solitex contega = £5.90/M2 120mm multitherm= £39.44 225mm thermoflex =£39.77. Studs £8/M2 Finsa board £12.51. 50mm Thermoflex £8.76 Tape £3/m2 Service cavity battens. £2 Total £119.38 plus vat and with zero waste. GULP. I do like this type of wall but it is unfortunately just so so dear. Amusingly every single product was available from this website. https://www.ecologicalbuildingsystems.com/ I wonder how much homework your architect actually did and how much they copied from elsewhere..... 1 Link to comment Share on other sites More sharing options...
Carp07 Posted May 1 Author Share Posted May 1 Yes the whole thing the more I look online feels like a copy and paste job with like you say one site conveniently having all listed. Way over engineered too and is giving me a head ache. My view to the wall profile is as attached with 90mm rigid insulation inside timber frame then service batten & 15mm board. Building control said the architects drawing looks “robust” lol! I briefed him that I didn’t want to spend the earth too. Cost me £600 so far for his services which also includes submitting planning application. Back to the base, my original thoughts were to having 100mm mot, 20 - 50mm sand blinding, 120 pir insulation with 100mm concrete slab to top with which the timber frame would sit… But I don’t think this works and you need ffl 150mm above ground level which eats into head room to stay under 2.5m… so my current thoughts are as my drawing. Link to comment Share on other sites More sharing options...
Carp07 Posted May 1 Author Share Posted May 1 1 hour ago, joe90 said: How much will it be used ?, if very often/long hours then maybe worth insulating but if not then heating it for short periods will be fine IMO. Yes that slab looks OTT to me. I would have thought regular use for 2 - 4 hours at a time… want it to be able for use in summer & winter Link to comment Share on other sites More sharing options...
oliwoodings Posted May 2 Share Posted May 2 7 hours ago, Carp07 said: I would have thought regular use for 2 - 4 hours at a time… want it to be able for use in summer & winter I built a garden building for my wife's business, she uses it for 2-4 hours a day max all year round. I went waaaaaay simpler than your architect, and did it all within PD so no planning required. For floor I did 100mm reinforced raft over 100mm MOT, and then floated 100mm PIR over the top with floating subfloor on that. The walls sit directly on the concrete, with the PIR inside the perimeter. 2x4s infilled with 100mm PIR. Roof i did a hybrid design to save on headspace, which is fine from a condensation perspective given it's low occupancy. 2x7s with 100mm PIR pushed up to the top, leaving a 50mm service void, then a vapour barrier across the underside before PB. 18mm roof deck with EPDM. I got all the PIR from a b-grade insulation reseller to save some money - perfectly fine for a garden building. Take some of the money saved on the over specced bollocks and put it into an A2A heat pump. Then you'll be comfortable the entire year round. Cost us £1600 for a split multi-room AC install (our building is split into two rooms), best choice we made. 1 Link to comment Share on other sites More sharing options...
gaz_moose Posted May 2 Share Posted May 2 could you not just build it out of SIPS? i looked into garden rooms and found some bloke on youtube called oakwood garden rooms who did loads of videos on them then found they have a facebook page. most of the guys on there seem to use threaded rod concreted into holes in the ground for foundations. it does seem your guy is spending your money for you, is he some spunky young guy trying to carve out a portfolio. Link to comment Share on other sites More sharing options...
Carp07 Posted May 2 Author Share Posted May 2 (edited) Oliwoodings, yes that’s definitely what Ives been looking at for concrete floor… concrete then insulation on top. Did you do your slab level with ground then end up with 150mm ffl above ground? The walls this is also pretty much my thinking 4x2 insulated with pir, service batten then fire board inside and batten then cement board cladding outside. Oh and yes I’ve watched a few of those oak wood garden room videos. As I’ve had to go through planning (new build home) will have to see what they say about change of slab and wall design, as said before building control seems okay with it but planning could be different, maybe Edited May 2 by Carp07 Link to comment Share on other sites More sharing options...
oliwoodings Posted May 2 Share Posted May 2 2 hours ago, Carp07 said: Oliwoodings, yes that’s definitely what Ives been looking at for concrete floor… concrete then insulation on top. Did you do your slab level with ground then end up with 150mm ffl above ground? The walls this is also pretty much my thinking 4x2 insulated with pir, service batten then fire board inside and batten then cement board cladding outside. Oh and yes I’ve watched a few of those oak wood garden room videos. As I’ve had to go through planning (new build home) will have to see what they say about change of slab and wall design, as said before building control seems okay with it but planning could be different, maybe Yes I did the foundations level with the ground. Only mistake I made was oversizing the slab compared to the wall perimeter due to changes in layout. This meant bits of my slab stick out under the walls, creating drainage issues. So, just make sure the slab is sized correctly and built correctly! Link to comment Share on other sites More sharing options...
Carp07 Posted May 3 Author Share Posted May 3 I'm still unsure to be honest on slab design, either insulation above or below concrete slab (obviously original plan is below albeit OTT)... so either 150mm insulation below slab or 100mm insution above (i know its different types of rigid insulation also). I thought with slab (with insulation above) you are suppose to go 50mm bigger than where you plan to sit your wall plates for strength or does that not matter as its reinforced concrete slab - plus the external batten then cladding will take up the 50mm anyway. Im just looking to get max headroom and i know you suppose to bring your finished floor up 150mm above ground (got to stay below 2.5m) The wall design i'm definitely going to go with insulated 89mm timbers with service batten on inside. Link to comment Share on other sites More sharing options...
Carp07 Posted May 3 Author Share Posted May 3 In my head how insulated above slab will look though maybe move wall to edge of slab so got to decide which Link to comment Share on other sites More sharing options...
Carp07 Posted May 3 Author Share Posted May 3 Link to comment Share on other sites More sharing options...
Carp07 Posted May 3 Author Share Posted May 3 Link to comment Share on other sites More sharing options...
Carp07 Posted May 3 Author Share Posted May 3 Two drawings showing (ignore the first can’t seem to edit it away) under slab insulation and over slab insulation (dpc under timber frame) Link to comment Share on other sites More sharing options...
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