Dunc

@JohnMo you've been very helpful in fondation design (thank you!) - I am using a modified version of yours for my TF build. As @G and J I'm wondering how to detail the threasholds - did you just omit the inner 140mm thermolite at the threasholds and set the doors over the outer block?

I don't understand how there could be no ventilation at all, unless the flat roof is a warm construction!

Thanks for the thoughts. The same guidance that limits the vent path to 5m also suggests that Mushroom vents don't work. To be fair to the architect, the planning drawings were done before TF manufacturer was selected. I imagine he expected a warm roof and a standard 140mm frame at that point....although having selected the cellulose fill TF it would have been nice for someone (architect or TF supplier) to point out the problem months ago. And if I'd done my research I'd have known cold flat roofs are generally challenging. I wasn't even aware of the cold vs warm thing until this came up...but then we pay professionals so we don't have to know this stuff, don't we? 😞 @Nickfromwales any details on the flat roof you know about? Size? Vent path length? It does seem that we'll have to change to a warm roof and have that installed/insulated by someone other than the TF manufacturer. Disappointing.

Our plans have a flat roof section of 5.8m x 2.5m. This is sandwiched between a 45 degree pitched roof on one side and a vertical wall on the other, along the long axis, such that ventilation and drainage are available only at the eaves on the 2.5m wide ends. The proposed build up is internal VCL, 421mm posi-joists with blown cellulose, OSB deck, firrigns (providing a minimum 50mm air gap), OSB deck, EDPM. I'm a bit stuck beween my architect who reckons this can't be ventilated appropriately, and the TF manufacturer who has provided an interstitial condensation analysis which shows the build up of doesn't pose a condensation risk. However they can't tell me what the length of the ventilation path is in the analysis (presumably there must be a limit) and just pointed me at NHBC guidance. All can see in there is section 7.1.10 which basically says "cold roofs are a bad idea; if you must do one the ventilation path should be less than 5m long". Anyone got a similar roof could you share details of how it's ventilated and why it works? Anyone point me at regs or guidance which would convince that the ventilation over the 5.8m length would be OK? I'm in desperate need of help here as this issue has only come up last minute and it's feeling rather catastrophic. thanks.

Not really, to be fair, the plot purchase was the major hold up (solicitors work to their own schedule it seems, no matter how hard they are pushed). As I understand it from our architect there were no really difficult questions or revisions on the warrant and nothing structural. Mostly on fire protection which was easily detailed on the building iteself. It is a little frustrating that the requirement to provde a reservoir of water for the fire brigade (because we're >100m from a hydrant) was not brought up until the last minute. I'd have expected the architect to be aware of this (given the location of the plot it's MILES from the nearest hydrant) and included it up front.

FINALLY! we have our Building Warrant approved. Nearly 2 years after making an offer on the plot; I really hope the rest of the project goes a bit quicker! 😁

Done. +1 with @Conor - very difficult to be specific about the "extra" value of eco features. Compared to what? Building regs? Some desicions may be influenced by other factors but end up being "more eco". Perhaps selecting a timberframe kit over brick& block for speed coincidentally changes the embodied carbon of the project, for example.

Are you certain they window size is limited to 1050mm (you used the word "tall")? Sounds rather close to the requirement that the bottom of an escape window should be no more than 1100mm above the floor. Nothing you can do about this requirement other than provide access to a protected stair instead. I'm not aware that the window size itself is limited to prevent falls, only that it either can't open more than 100mm, or a barrier is provided.

Which may be fine if you use a main contractor. If you manage the build and contract separate trades yourself this doesn't apply, based on my discussion with them. But I'd suggest it's worth a phonecall to confirm individual cases. Another thing to look out for is flat roofs - ProTek required a separate Insurance Backed Guarantee for the flat roof. Usually this is provided by the roofer, but obviously could limit your choice of contractor as not all will provide this. Possible to take out such a guarantee yourself but for £ several hundred.

Very similar to my quote, though I haven't taken them up on it yet. Check the T&Cs. ProTek and Build Warranty specifically exclude the first 2 years after completion and assume that this is covered by the main contractor's insurance. SelfBuildZone do not have this exclusion, from my investigations. I guess this only matters if you're not using a main contractor. Don't forget there will be site visit costs on top (another £1.5-£1.8k for me in the Highlands). Stage certs are only needed if you are drawing down a mortgage during the build - they demonstrate to your mortgage company that you have achieved a particular point in the build to allow them to release funds. After completion they're irrelevant because you have the final sign off, AFAIK.

+1 on the hob to oven distance being too far - carrying that heavy roasting tin across the kitchen and trying not to spill won't be much fun. Not keen on the bench seating. Fine for kids sliding in and out but older folks might not enjoy that? I'd put a corner sofa there and have the table over by the window.

thanks, @JohnMo some interesting points. I was looking at the NHBC diagrams (https://nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-15-ventilation-vapour-control-and-insulation/) Whether or not the roof covering is air permable, the suggestion is for a conitnuous ventillation gap which allows air flow. This seems quite different to gaps in the sarking which allow vapour diffusion.

I have a couple of questions on slating. They seem well-worn questions but I’m not sure I’ve found any specific answers. Nail direct to sarking vs batten/counter batten; Hooks vs nails; and slate size. Context for this specific roof: This is in Scotland. 45 degree pitched. Hybrid/Warm roof – cellulose between rafters (but no insulation above) the rafter. This needs to “breath”. In-roof solar PV in part of the roof (needs ventilation behind). Roof will have some Velux windows. New, regular sized slates (Cupa 3 heavy or similar). I gather that slating in Scotland is traditionally done by nailing directly to sarking boards (with a membrane between boards and slates). For this warm roof I would need to vent behind the sarking, so will have to stand the sarking off the rafters on battens, with eaves and ridge vents (rafter, counter batten, sarking board, membrane, slate). At the ridge, the membrane would have to be cut to permit ridge ventilation (i.e. the membrane can’t span the apex of the roof). This worries me because if the dry ridge vent fails then water could penetrate behind the membrane at the apex. Is this an unreasonable concern? Any way to mitigate the risk? In a battened roof (rafter, sarking board, low resistance membrane, counter batten, batten, slate) the membrane can cover the apex of the roof so any failure in the dry ridge just runs down the membrane and out at the eaves. Penetration of the membrane is limited to nails holding down the battens. So, what weather conditions does Scotland provide that would make this battened construction less desirable? Wind uplift behind the slates? Is the nail-direct construction just a hangover from using uneven sized slates? Then for fixing the slates, hooks look to be a nice idea for long-term maintenance. Any down sides to hooks? And if I need to go the “nail-direct” route can hooks be used in this context? And finally, various places on t’interweb seem to suggest that smaller slates are more common in Scotland (400x250 vs 500x250 down South). Other than tradition, any reason to use smaller slates (more fixings, more time)? Thanks.

What are the noggins supporting? If it's not a joint in the OSB deck could you a) remove them and placed further out - about 25mm outboard of the outer face of the cladding, providing an inverted L-shape type vent. b) use a multitool or reciprocating saw to trim the top off all of them 25mm below the deck (providing your 25,000mm2/m vent). New noggins placed a little further out if required for deck support. Or c) trim 25mm off the bottom of the noggins to create a gap at the top of the cladding providing a Z-shape type vent.

+1 Good luck finding any groundworkers with experience in insulated raft foundations up here; I couldn't. Econekt based in Glasgow were prepaired to travel to Inverness. They were very helpful and engaged but ultimately the cost was prohibitive for me. I've gone with strip footings and, with advice from @JohnMo and others here, I think I've achieved a pretty decent design.

Haven't used it but this looks like it might allow for >1 pipe/cable per hole and be re-penetrable https://www.filoform.co.uk/filoseal-re-enterable-duct-sealing-system

Thanks for the thoughts, everyone. Very helpful! Clarifications: @ReedRichardsTypo (I can't edit the orignal post). Companies I've requested designs from all use flow 45C and return 40C, deltaT 5C. @Chanmenie U values should be correct, MBC Timberframe "pasive" frame with blown cellulose fill...so yes, we should be on the way to passive levels for both insulation and airtightness. Form factor is reasonable but not perfect, 1.5 storey but with a lean-too entry/utility, and thermal bridging not perfect. @BeelbeebubYes, we are far north: Inverness-shire. We can see snow on Ben Wyvis this morning 🙂. Great point on future proofing the installation site! I'm really curious about the design temps. Only one company will share the details prior to me coughing up significant cash. This one calculated transmission heat loss of 2.373 kW and ventillaiton loss of 1.506 kW for a total of 3.879 kW (26 W/m2, 9 W/m3). I asked them to recalculate performance for flow 35C / return 30C and they sent the data below...seems to suggest that with a 26C temperature differential, a flow of 35C is not sufficient?

@JohnMo are you suggesting that a 4kW pump would be fine because the heat load is being over estimated, and my 1.7kW figure is correct. Or that even if heat load is ~3.9-4.8kW a 4kW pump is sufficient? @Beelbeebub not built yet but rather late to make a subtantial change in the overall plan!

Apologies for yet another "how big does my ASHP need to be" question; I've read a lot but understand very little. ASHP to drive UFH for a 1.5 storey house, well insulated (U=0.1/0.12/0.1 W/m2K roof/wall/floor) and airtight (<0.6ACH) with MVHR. JHarris spreadsheet heating demand 1.7 kW (-5C out/21C in, to match UFH design quotes). Various quotes from ASHP/UFH designers suggest heat losses and pumps sizes: 4.8 kW loss/8.5kW pump, 4.7 kW loss/5kW pump, 3.9kW loss/5kW pump. All seem to be using -5C out/21C internal temps, flow temp 45C, return temp 5C SAP calc indicates 4.45 kWh/m²/yr, if that's relevant. 3 key questions: 1. Have I done something wrong in the JH spredsheet? 2. Can the ASHP be too big? i.e. supplier recommending 8.5kW pump for a load of 4.8 kW (or 1.7 if I'm right!)? 3. If the 4.7 or 3.9 kW load is correct and paired with a 5kW pump, is there enough overhead for DHW duty? Bonus Q: One supplier suggested that fitting a buffer will "deal" with an ASHP that is oversized. Does that sound right?

How much effort/time do you want to put in? If you have plenty of these, pretty much any standard construction technique can be made passive. Are you sure that you/your contractor can do the airtightness work well? If you want it wind & watertight & near-passive as fast as possible the MBC twinwall seems a no-brainer. I don't really see how either the KTS or the MBC HP "systems" are different from any other 140mm stud timber frame; they're just variations on the same theme you could achieve using a stick build on site or purchase from a variety of other suppliers and they will all need significant design detailing and on-site airtightness work. I would expect the floor area as drawn in the initial design reflects the INTERNAL wall (plasterboard). When your frame supplier draws up their plans they build outwards from that so your usable floor area stays the same, but your foundation footprint gets bigger for the MBC twinwall.

@Dave Jones that diagram appears to be for a single skin (e.g. 140mm timberframe) where the only horizontal insulation is 25mm PIR + the marmox block. In the case under discussion the build appears to be 2 skin blockwork with cavity insulation. If the cavity is insulated, the thermal bridge of interest is downwards to the foundation blockwork. @JohnMo's maths shows 440mm deep aircrete blocks will do just as well (if not better) than the marmox block.

@SBMS the architectural drawing appears different to your initial coloured plan: the insulation seems to be UNDER the slab on the architectural drawing?

Yes: radon membrane under slab, continues horizontally under the aircrete the turns up inside the cavity join the horizontal DPM throgh the wall. I'm at the design stage too and haven't specified the insulation for the cavity, so reading this with interest to see what options are available. Mine is twin wall timber frame with cellulose fill, so my above ground cavity insulation is a separate consideration. Because of this, I'll probably run the radon membrane under my foundation cavity insulation then up the inner face of the external block to keep both insulation and aircrete block dry. General question for you & others: I'm not sure on the value of 2x300mm aircrete blocks? With insulation above the slab the top block is pretty much continuous with this and limits both the horizontal and vertical bridges. What does another lower insulating block add?

The way I looked at it very crudely [hope I don't embarrass myself here!]: Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock). Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m2K/W. The U value (1/R) is 0.5 W/m2K. Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/). Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m2K/W. The U value (1/R) is 0.6 W/m2K. So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC? I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

Good info. I'm currently contracted with MBC...it's been generally good but a couple of points which anyone building in Scotland may find helpful: 1. They can't supply Rationel/Velfac windows in Scotland (someone else has the contract for that aparantly). This isn't made clear on the website. 2. The groundworks team don't want to travel to NE scotalnd to install a raft, even though the timberframe side of it is fine up here.