G and J

Bingo! https://www.ukelectricalsupplies.com/norslo-dry-lining-box-2-gang-32mm.htm

Good thought. In fact that will be the case in some rooms, especially bathrooms. But our big downstairs everything room thingy (I call it The Great Hall as I do enjoy irony) will need stuff both sides.

When I first watched this guys videos I thought, naaaah, can’t be that simple. We live and learn.

I’d identified that EPS based SIPs wouldn’t give any advantage over stick built early on, so I focussed on PU SIPs, and that, combined with the limited thicknesses available in the cement board SIPs lead me to discount them.

In that case I’m guilty of giving a more negative impression of our architect than is reasonable. We are scheduled in to start detailed design soon, and the discussions we’ve had have been those needed to get through planning, now thankfully obtained.

My word I’m such a bonehead. My head is so full of stuff explaining myself isn’t easy sometimes, there are so many disparate thoughts jostling to get to the front of the queue. The plan is to have an OSB or ply skin behind the plasterboard. This is personal preference (read OCD inspired folly, perhaps) but I like the idea of really solid walls I can hang stuff from anywhere, and maybe with better acoustic attenuation too. But I hadn’t thought about back box depth. 12.5mm PB, then 11mm OSB, then 11mm void = tight. OK, time to research back boxes….. And I have an ‘every millimetre counts’ attitude. The plot is 24’ wide, the house, externally, will be 21’. So to me and our already advanced layouts based on 1’ thick walls, it will make a difference.

The outside walls of our house are within a metre of our boundary on both sides. On the south side there is bang on a metre wall to wall with next door. On the north side it’s a whopping 1.5m. At the planning drawing stage the architect mentioned that we’d need to use fire resistant sheathing on the frame. Apparently part built timber frames make rather intense bonfires. 😕 I’ve not researched this, as on a few quick searches it appears credible, so I've accepted it as a constraint. He also was ok with larch cladding, but I think that will need to be treated for fire resistance.

I clearly need to research cement board prices more, I did my coatings on what I could quickly find in more than one place so as to reduce the risk of skewed results from a particular offer, so I readily accept I may have overestimated the stick build related costs. The SIP ones were from quotes capable of acceptance, so I’m more confident in them (but not necessarily in how comprehensive my list of things needed in addition to those quotes….) The target is a mostly two storey, mostly slate pitched roofed, mostly vertical larch clad, with some render, low risk house. Low risk means nothing ground breaking or bleeding edge, I seek the best options for us amongst the tried and tested. Sadly my curiosity demands I understand the less mainstream, but I bring myself back to heel soon enough. I hope. We soon start detailed design with the architects, and though I’ve a lot more on my knowledge shopping list, I’m already a much more informed decision maker.

Peeps talk about a service void behind plasterboard being 25mm or 50mm. If it’s only for cables, almost all maximum 2.5mm2 FT&E, is there any reason that void couldn’t be smaller? Say 11mm?

In truth I remained open minded, but to give a technology the best chance of being judged fairly one has to try hard to devise the best option. I started with the assumption that I’d be able to buy SIP panels with the outside made of cement board, but I failed to find a current supplier. One did say they used to do them but they discontinued them due to problems (unspecified). Our base case is exactly that, cement board sheathing our frame. It is a lot more expensive than OSB3 though.

I will readily admit I like the idea of SIPs and I did try really hard to assemble the best possible case for them. However the result is not favourable. It’s worse in my case as I have to have a fire resistant layer on the outside. The SIP companies I’ve consulted advise that that needs to be battened rather than fixed direct so that adds to wall thickness with little improvement in insulation. It negates the width vs u value advantage PU based SIPs provide. Even without this the extra cost of SIPs is significant. I think this is likely to be because there isn’t an active market in sufficient volume for what I would call diy SIP build, meaning from blank panels. I did find suppliers of technically suitable PU SIPs. (I focussed on PU SIPs as EPS SIPS yield a thicker wall given the above than stick built for a given u value.) But it’s not like the supply of OSB3, there simply isn’t the competition. The end result is I can source and have built for me a stick built frame for a lot less than I can source SIP panels for me to erect. Yes, I then have to insulate but that’s ok with me. I’ve learnt a lot about SIPs in this process, and I remain shocked by the way the industry is so dominated by companies that sell hard and clearly have very pleasant margins. I understand that even local building companies don’t build using SIP blanks, they get a specialist company to put up the kit and they take it from there. The supply is simply not there competitively (yet?) I now have a high regard for SIP technology, and I’m grateful for inputs from enthusiastic proponents - it’s helped me enormously in my data gathering - but my numbers are what they are. And as those numbers translate into Greggs sausage roll tokens (our build is just in walking distance - praise be!!) then those numbers guide me. My next research area should be diets, but it’s probably going to be warm vs cold roof and roof air tightness. 😕

Slander and calumny! Such wisdom cometh not from me, that was from Iceverge. It made sense to me but maybe I’m missing something. Again.

I’m under the impression that it takes very little to constitute the above. I wholeheartedly applaud you waiting for the right one. As for kits, that I’m less convinced about.

We are about to self build for the second time in 35 years. I think it’s a brilliant thing to do, but… I also think it’s a stressful thing to do, big sums involved, either lots of people involved or if not then megatons of work for yourselves, maybe a bit of both, it’s a long slog, and it’s hard sometimes to imagine the end result and thus be confident on all the myriad decisions along the way. And I don’t think it’s an easy way of boosting your equity either. All these downsides….aren’t I a ray of sunshine! Well I am actually, because I would also say that if you, throughout the process, keep in mind why you are doing it, then you are likely to end up with a home that’s much more suited to you and yours than you could ever purchase. Where you know (and hopefully love) every nook and cranny, and although this is a bit of an ego thing, you might also have an individual, unique house you can be proud of. When we were planning to buy our plot, over three decades ago, I went to several builders for quotes for the whole thing. One of them asked why we weren’t doing it ourselves. I answered that it’s because neither of us are in the trade. He laughed, called me a rude name, and said that even if you’re in the trade you have to learn all the other trades, so I’d have one more trade to learn than others would. “So what?” he asked, and in hindsight he was so right. Plot finding is v hard though, especially in our lovely but overcrowded country, it’s taken us a third of a century to find our second plot, so good luck in your search.

That’s a timber frame cavity wall, and an excellent u value to width ratio if I’ve understood it correctly. May I ask why you didn’t do that?

@JackOfNoTrades was that with the cement board sheathing directly onto the OSB3 or was it battened first?

Woo hoo! I’m celebrating, I finally managed to produce a sensible looking ubakus wall build up dooberry thingy. Now I need my homework marked….. (should I be worried about the red colour on the drying reserve measure? It goes away if I turn off the PIR layer).

Goodness, thank you for sharing that. So your OSB skin is on the inside and is your main airtight layer in your wall. Not sure we can go that route due to fire resistance - I’m still not sure how I do address airtightness. Your framing timbers are I’m guessing circa 5.1m and your floor is hung from the side of them. Was there a specific advantage in doing that over what I think is called a platform floor? (I mean where you build downstairs, the floor sits on that and then you build upstairs directly on top of it.)

It accounts for how all the high energy hard sell is paid for for the SIP providers. I’m guessing the same is true for frame kit providers too. In our case the fire resistance requirements put SIPs at a disadvantage but I think that there simply isn’t a mature market for blank SIP panels like there is for other building materials, otherwise the cost of a SIP self build would be closer to stick built. With markets, volume is such a dominant factor. I think you’ve shared your wall build up before JohnMo, but I can’t find it. Would you mind sharing it again?

Well, I thought I’d feed back on all my investigations. Revising my earlier SIP figures, to include a cement board skin, the costs for the frame, not including barrier membranes, battening, cladding, PB or skim, roof trusses, and in the case of the SIPs, also excluding the attached garage (no idea why, but they all left this off): 1. Get a company in to do the lot and it’s just over 104k. Game over. 2. Have a different company make it in factory and deliver it for me to erect and it’s about 56k. 3. Buy blank PU SIP panels and build it myself, buying in joists etc. from other suppliers will cost about 39k. 4. Stick build it (excluding labour), assume cement board skin and osb3 skin, 140mm rockwool in frame, 50mm PIR inside that, 18k. Crumbs. I think my way forward is clear then….

We are keeping the larch down the sides for aesthetic reasons, and we are told we can, albeit treated for fire retardancy, but that we have to frame with cement board rather than OSB3. This is from the architect rather than the SE, as we haven’t got an SE yet. I think in my castings it would be prudent to assume both OSB3 and cement board, just in case… Thank you for sharing that.

We are replacing a bungalow which is on a narrow site which thankfully doesn’t taper front or back. Whilst we are extending the footprint a bit at the back we are under no illusions that the external finish has to be no nearer our boundary on either side than the current bungalow. But yours is a point well made.

Always good to understand your exit strategy, however heartbreaking….

Don’t know what a covenant consent thingy is, but… Presumably you’re settled with the fallback position of being limited to that outline planning permission (I didn’t know that still existing), or if not, you’ve an exit strategy of selling the plot if you can’t get what you want.

Ummm, did the parish council support your planning app?