DKG Posted February 5 Share Posted February 5 I am looking to lay foundations for a large garden room in the next month and so have done some research and knocked together a drawing on what I have in mind currently Size 5.7m x 5.25m I wish to maximise internal headroom as I am limited to the 2.5m maximum and would prefer to avoid applying for planning permission, as it's relatively large I would also like to keep the roofline height low relative to the neighbour garden to avoid it looking too dominating as it's only 1m from the boundary and so hoping to have the top of the concrete slab approximately level with the outside ground, although realize this isn't ideal and plan to have a French drain around the perimeter and ensure any timber is at least 150mm above ground level I would appreciate it if more experience members here could provide any constructive comments on the design with any potential problems Link to comment Share on other sites More sharing options...
Moonshine Posted February 5 Share Posted February 5 (edited) You will get a cold bridge through the bricks, this is what I did, not that I am sayings it's right Edits, didn't see you had 30mm pir externally, no cold bridge but is the pir going between the vertical battens, if so its not going to work as you need ventilation for the cladding. Also will the external pir hold up especially at the base? Edited February 5 by Moonshine Link to comment Share on other sites More sharing options...
DKG Posted February 5 Author Share Posted February 5 23 minutes ago, Moonshine said: Edits, didn't see you had 30mm pir externally, no cold bridge but is the pir going between the vertical battens, if so its not going to work as you need ventilation for the cladding. Also will the external pir hold up especially at the base? I was planning on fixing the vertical battens over the top of the PIR to keep a continuous airgap I may need to consider how well PIR will withstand the British weather when external to the OSB but protected by the cladding I also haven't shown the breather membrane, which I'm unsure where is the best place to put it Link to comment Share on other sites More sharing options...
DKG Posted February 5 Author Share Posted February 5 The other option might be to put the PIR on the inside of the building, but I opted to put it on the outside down to the ground to prevent cold bridging where the concrete slab extends beyond the walls, which because I was concerned about having the threaded bolts too close to the edge of the concrete slab I wonder if PIR between the studs and OSB board would work or if there is any concern that the OSB boards wouldn't then provide the necessary rigidity Link to comment Share on other sites More sharing options...
Moonshine Posted February 6 Share Posted February 6 (edited) 11 hours ago, DKG said: I was planning on fixing the vertical battens over the top of the PIR to keep a continuous airgap I may need to consider how well PIR will withstand the British weather when external to the OSB but protected by the cladding I also haven't shown the breather membrane, which I'm unsure where is the best place to put it The other option might be to put the PIR on the inside of the building, but I opted to put it on the outside down to the ground to prevent cold bridging where the concrete slab extends beyond the walls, which because I was concerned about having the threaded bolts too close to the edge of the concrete slab I wonder if PIR between the studs and OSB board would work or if there is any concern that the OSB boards wouldn't then provide the necessary rigidity Ah now i see your plan, personally i don't like the idea of screwing through the PIR to support the battens and cladding, those screws will have 30mm unsupported and due to the weight of the cladding i would be worried about the screws bending / snapping over time. i think that based detail is a bit ugly / not robust, one with the PIR right down to the slab, and two the cement cladding board only supported at the top, i would be worried of it getting knocked and snapped. Also your breather membrane would be between your vertical battens and the PIR. I would look to put the PIR on the inside as then internally you would have a chance to cover the bricks at the bottom visually. There is a lot of PIR going into this build and i personally get the feeling its overkill for a garden room, second concern is framing in 4 x 2 strong enough? its taking a lot of roof weight? i did it in 5 x 2 and mine was 6m x 4.8m I wouldn't cast the threaded rods in at the same time as the concrete, i would come in after its cured and drill out holes and resin anchor the threaded bar in. You could do this after the bricks have gone in, but the concern would be getting enough resin down 150mm to slab for solid fixing. Your brick is going to have to make a few holes in bricks though. Edited February 6 by Moonshine Link to comment Share on other sites More sharing options...
Mr Punter Posted February 6 Share Posted February 6 I think you have labelled the vertical timber battens incorrectly. The arrow looks to be pointing to the 30mm pir. Regarding holding down, you could use https://www.metrofixings.co.uk/product/simpson-strong-tie-htt5-403mm-hold-down-strap/23994 fixed to studs and bolted to the slab internally. Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 14 hours ago, Mr Punter said: I think you have labelled the vertical timber battens incorrectly. The arrow looks to be pointing to the 30mm pir. Regarding holding down, you could use https://www.metrofixings.co.uk/product/simpson-strong-tie-htt5-403mm-hold-down-strap/23994 fixed to studs and bolted to the slab internally. Yes you are correct, the arrow is incorrectly positioned I'll take a look at those strap, although look a little expensive Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 14 hours ago, Moonshine said: personally I don't like the idea of screwing through the PIR to support the battens and cladding, those screws will have 30mm unsupported and due to the weight of the cladding i would be worried about the screws bending / snapping over time. I would look to put the PIR on the inside as then internally you would have a chance to cover the bricks at the bottom visually. There is a lot of PIR going into this build and i personally get the feeling its overkill for a garden room I was also concerned with screwing through the PIR. I was looking to avoid cold bridging but think I'll bring it back to the internal side of the studs I think perhaps 80mm between and 30mm over the studs is excessive, so might look at reducing that to perhaps 50-60mm between and 20mm over the studs Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 15 hours ago, Moonshine said: second concern is framing in 4 x 2 strong enough? its taking a lot of roof weight? i did it in 5 x 2 and mine was 6m x 4.8m Hadn't considered 5" x 2", but will certainly give that some thought Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 15 hours ago, Moonshine said: and two the cement cladding board only supported at the top, i would be worried of it getting knocked and snapped. I was not planning on leaving the bottom cladding board unsupported but hadn't worked out the details of how to support it. It was originally brought down lower to protect the PIR, so if I now move that to the inside the lower cement board will no longer be required Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 Link to comment Share on other sites More sharing options...
Iceverge Posted February 7 Share Posted February 7 I don't see any issue with the PIR externally, in-fact I prefer it there, keeps the studs warm, and warm = dry, and dry = durable. As for too much insulation -Nonsense- you can never have too much. Remember its not only about energy usage. It's about heat protection, warm up time and heat retention when unoccupied too. Also buying insulation in multiples of 25mm is cheaper than more unusual thicknesses like 30mm or 80mm. I'm not a fan of the PIR between the studs, I'd much prefer to see mineral wool there. As a material of very low permeability you run the risk of it trapping any moisture that finds its way in. Mineral wool would be better. How about. Fiber cement siding. 25*50mm battens. 50mm PIR breather membrane 11mm osb (metric sheets) taped externally for airtightness. Very important. Rockwool batts between 97*42 studs at 400cc. 12.5mm plasterboard Skim If you're worried about the screws snapping add a few nails, or a long screw at an angle upwards, or attach the battens to the roof overhang for extra security. Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 I have amended some details Excuse the poor drawing (I'm experienced in Autocad but currently so using a Free cad program) I have increased studs to 120mm wide, reduced PIR to 60mm between studs and 20mm over (internally) I now have a cold bridge where the concrete slab extends past the building structure, so either need to bring the edge of the slab in so the bricks sit on the edge or accept the cold bridge or come up with a way to solve the cold bridge issue Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 11 minutes ago, Iceverge said: I don't see any issue with the PIR externally, in-fact I prefer it there, keeps the studs warm, and warm = dry, and dry = durable. As for too much insulation -Nonsense- you can never have too much. Remember its not only about energy usage. It's about heat protection, warm up time and heat retention when unoccupied too. Also buying insulation in multiples of 25mm is cheaper than more unusual thicknesses like 30mm or 80mm. I'm not a fan of the PIR between the studs, I'd much prefer to see mineral wool there. As a material of very low permeability you run the risk of it trapping any moisture that finds its way in. Mineral wool would be better. How about. Fiber cement siding. 25*50mm battens. 50mm PIR breather membrane 11mm osb (metric sheets) taped externally for airtightness. Very important. Rockwool batts between 97*42 studs at 400cc. 12.5mm plasterboard Skim If you're worried about the screws snapping add a few nails, or a long screw at an angle upwards, or attach the battens to the roof overhang for extra security. Thanks for your insights I will give it some thought tomorrow I have been reading many of your comments with interest in a previous similar discussion earlier last last Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 55 minutes ago, Iceverge said: How about. Fiber cement siding. 25*50mm battens. 50mm PIR breather membrane 11mm osb (metric sheets) taped externally for airtightness. Very important. Rockwool batts between 97*42 studs at 400cc. 12.5mm plasterboard Skim What are your views on stud sizes 95mm or 120mm? Link to comment Share on other sites More sharing options...
DKG Posted February 7 Author Share Posted February 7 (edited) @Iceverge - Is this more what you had in mind Cedral Lap instruction state a minimum airgap of 30mm, will a 25mm batten be sufficient? Do I require an airgap on the cement base board? Any comments regarding the 50mm thick EPS upstand around the concrete slab perimeter, do I perhaps need cement particle board outside of that EPS to separate the EPS from the pea shingle and to stop the EPS coming away from the sides of the concrete base and leaving an airgap Edited February 7 by DKG Link to comment Share on other sites More sharing options...
Moonshine Posted February 7 Share Posted February 7 looks better with the the cement board at the base rather than just the insulation and the loose cladding. Don't forget the insect mesh top and bottom of vertical battens Though i still have my own concerns about screwing through the PIR and those screws being unsupported for 30-50mm with the weight of the cladding. Link to comment Share on other sites More sharing options...
Mr Punter Posted February 7 Share Posted February 7 12 hours ago, DKG said: I'll take a look at those strap, although look a little expensive Maybe this sort of thing then https://www.tcfixings.co.uk/product/simpson-1200-x-30-x-2mm-light-restraint-strap-bent-100/16842 Link to comment Share on other sites More sharing options...
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