Cormac Foley Posted July 10, 2023 Share Posted July 10, 2023 Hi all, I'm hoping to get some advice on SIP panel foundation details. I'm going to put in a 25m2 garden office and want to have a solid floor so I'm going with a strip foundation. The last block under the sole plate will be a thermal block, this is where I'll also be running my DPM/Radon Barrier. I'll be bolting the soleplate through the block and the DPM, does this not defeat the purpose of the DPM under the soleplate? Is there any way to avoid this? Is there anything else I should or shouldn't be doing with the detail? (I've borrowed the initial design from another user, sorry, can't remember the name) Link to comment Share on other sites More sharing options...
cheekmonkey Posted July 10, 2023 Share Posted July 10, 2023 You shouldn't really need 100mm blinding, but otherwise seems the same as we're doing. depending if the floor goes in before or after the SIP you can 'double bag' with dpm - so we're going to dpm & wrap up inside the sip as floor goes in later & therefore the internal dpm wraps up the wall inside too. don't worry about the bolts through dpm/dpc. it's all out the ground anyway. can't quite see the dpc layer on screen but i believe that needs to be 150 above outside ground level, it's not clear but looks like it is lower Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 10, 2023 Author Share Posted July 10, 2023 Thanks for the quick response Cheekymonkey. Yeah, the 100mm blinding is a typo, should be 50mm. I was just running the DPM out over the block to act as the DPC. Just worried about the soleplate getting damp and having condensation issues (read a few comments about this issue) Do you think if I kept the DPM as shown in green in my sketch and then joined another piece of the DPM to bring it up the internal face of the SIP that would help? Link to comment Share on other sites More sharing options...
Iceverge Posted July 10, 2023 Share Posted July 10, 2023 What are the layers on the outside of the lightweight concrete block and the soleplate? Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 10, 2023 Author Share Posted July 10, 2023 PIR insulation and protection board. Haven't decided yet on whether to use a cement board or to put an EWI render finish on the PIR. Link to comment Share on other sites More sharing options...
Iceverge Posted July 10, 2023 Share Posted July 10, 2023 EWI on PIR wouldn't be a normal detail. EPS is better for this. I think you're better off ditching the strip foundation and just making an insulated slab from EPS. Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 10, 2023 Author Share Posted July 10, 2023 Thanks Iceverge, would you have a detail for this or could you point me in the right direction. Would it be a lot more expensive to do a raft/insulated slab? Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 10, 2023 Author Share Posted July 10, 2023 2 hours ago, Iceverge said: EWI on PIR wouldn't be a normal detail. EPS is better for this. I think you're better off ditching the strip foundation and just making an insulated slab from EPS. Would the PIR board in an EWI system be used at plinth level? Link to comment Share on other sites More sharing options...
cheekmonkey Posted July 13, 2023 Share Posted July 13, 2023 On 10/07/2023 at 15:19, Cormac Foley said: Thanks for the quick response Cheekymonkey. Yeah, the 100mm blinding is a typo, should be 50mm. I was just running the DPM out over the block to act as the DPC. Just worried about the soleplate getting damp and having condensation issues (read a few comments about this issue) Do you think if I kept the DPM as shown in green in my sketch and then joined another piece of the DPM to bring it up the internal face of the SIP that would help? i'd deffo double check the regs on DPC to outside ground level to be sure, our way around this is to bring block to finished floor inside & DPC outside, then bolt soleplate through above that. We will also run a 25mm insulation upstand at least 150 up inside the sole plate & SIP wall to avoid any condensation. basically i think bring your block up 50mm more & DPM/DPC works & then add upstand internally Link to comment Share on other sites More sharing options...
Redbeard Posted July 13, 2023 Share Posted July 13, 2023 PIR is very rare in EWI. Plinths are often done in XPS, but thereare 2 schools of thought here - the system providers say it makes everything safe because it is waterproof, and some say that if any water does get in the sandwich the (impermeable) XPS helps to trap it in. For this reason some (including me) use EPS below DPC. If relevant check your BCO is happy with this. Link to comment Share on other sites More sharing options...
Redbeard Posted July 13, 2023 Share Posted July 13, 2023 You appear to have an unmitigated thermal bridge at the sole-plate. You could just take your PIR upstand higher, perhaps. Link to comment Share on other sites More sharing options...
Iceverge Posted July 13, 2023 Share Posted July 13, 2023 Sorry I missed your reply. Here's one from another thread I drew up for a timber frame. On 10/07/2023 at 19:42, Cormac Foley said: Would the PIR board in an EWI system be used at plinth level? Nope. PIR doesn't fare particularly well in these circumstances. EPS is best. Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 19, 2023 Author Share Posted July 19, 2023 On 13/07/2023 at 16:18, Iceverge said: Sorry I missed your reply. Here's one from another thread I drew up for a timber frame. Nope. PIR doesn't fare particularly well in these circumstances. EPS is best. Thanks for the detail. A few questions: Just wondering if the raft foundation would be considerably more expensive than a strip foundation. What type of insulation would you put under the raft, XPS? What depth of insulation? What strength concrete in the raft? Fibre reinforcement or mesh? Is that a perforated drain in clean stone on the outside of the foundation? Link to comment Share on other sites More sharing options...
Iceverge Posted July 19, 2023 Share Posted July 19, 2023 EPS. As much insulation as you can. >150mm Concrete strength >30N at a guess. Mesh for a raft yes. Yes. It's a french drain. Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 21, 2023 Author Share Posted July 21, 2023 Back again, how wide would I want to make the boot of a raft foundation and would I be better off having an insulation break between the slab and the boot effectively having a ring beam under the SIP panel? I've seen this detail by Viking House and also Kore. Link to comment Share on other sites More sharing options...
Iceverge Posted July 21, 2023 Share Posted July 21, 2023 I wouldn't over think it. Your buildings loadings will be minimal. Go for a 100mm+ slab with some reinforcing mesh and pour it all as one. Any slight movements of the building will all go together then at least after construction. You could get a structural engineer to design it properly but would cost far more than a bit of extra concrete if you're apprehensive about the strength. Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 22, 2023 Author Share Posted July 22, 2023 Thanks, definitely overthinking it!!! Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 22, 2023 Author Share Posted July 22, 2023 (edited) Think I've finalised the detail for the base, is there anything I'm forgetting or getting wrong? Not 100% on the cladding extending down over the insulation and concrete board The air vapour barrier is shown in pink Edited July 22, 2023 by Cormac Foley Link to comment Share on other sites More sharing options...
Cormac Foley Posted July 22, 2023 Author Share Posted July 22, 2023 Would anyone have any advice on whether or not a SIP roof (flat roof) needs to be vented and if so, would you have a detail for it? Link to comment Share on other sites More sharing options...
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