fatgus

@Gus Potter Thanks for the screen shots... really appreciate it We've already got some labour costs in the spreadsheet, mostly because we're aiming for supply & fit for the big ticket items (for VAT reasons... 5% on supply & fit, 20% on supply only). Thankfully, labour rates here are not too outrageous if you choose wisely Once it's watertight, we're in no huge rush and looking forward to doing as much as possible ourselves... we've done it before, albeit on a smaller scale, and while we're admittedly getting older, being hands-on is one of the drivers for the project. This is bang on... There are quite a few aspects of our plan that many would question, but we are determined to have... a few economies elsewhere make the justification a bit easier

Thanks @Crofter 👍👍

One of the tempting routes is an i-beam stick build, designed by Cullen Timber Design, manufactured from their cutting list by a company not too far from us (Gloucester, I believe) and then put together by our builders. We'd use blown cellulose. Certainly no PIR. Fire is one of our main concerns as the external walls will be almost 80% timber frame and 100% clad in timber (it was originally 50% metal, 50% timber but that was scuppered by the planners). We want to mitigate risk, both for our peace of mind and to reduce insurance, but it's hard to find breathable FR boards. We think we have an option from Magply, but the fire risk is one reason we even considered reverting to block build.

Can never have too many sockets! Have to say, I’m looking forward to warm towels all year round. Our current bathroom rads only get hot when the rest of the heating is on, so in the milder months our towels are never warm. 1st world problems 😁

Hi @Crofter Can I ask where you sourced the Neo slider? I’ve just spoken with Aluplast and they said there are currently no UK fabricators of the Neo range. I’d be happy to import if it’s the right product, so wondering where you got yours?

At the moment, our Whatif? table goes from 0.1 to 0.2 for walls and 0.73 to 1.2 for windows. Even the worst case (0.2 wall/1.2 windows) is only an extra £3800 over 20 years compared with the best case (0.1/0.73). Your point about “as-built” vs “design” is really important. Based on the calcs, if we go for an “as-designed” 0.13-0.14 timber frame and in reality it only performs as well as a 0.2 frame, that should still be fine 🤞😄 I’ll take a look at your blog for sure Our intention is for UFH on ground floor only, with electric towel rails and possibly electric under-tile heating in the 1st floor bath/shower rooms. We’ll possibly install wiring to locations for bedroom electric rads “just in case”, only because it’ll be easy and cheap to do now.

Thanks @Crofter That sounds very promising... I'm going to see whether I can visit their showroom this week 👍👍

Thanks @Gus Potter 👍 I think we're actually approaching this from a very similar perspective. We're fortunate enough to be able to build the house we want and we're now going through the "sense check" before we commit to the key cost items that are difficult to subsequently change. I love a good spreadsheet and find it the best way, for me, to work towards a cost/benefit decision. That's led to this post really... e.g. I no longer see any point in having super low wall/roof U-values 🤷‍♂️ Windows are slightly different... We have quite a few and some are large (hence the possibility of low Uw values) so they have a notable effect on the overall heat loss. For example, the cost difference between Norrsken and Zyle Fenster is around £30k for a relatively small improvement in average Uw. Norrsken are lovely windows, but make zero economic sense for us. The window warranty is a very valid point. That said, the large glazing is hugely important to us. If units need replacing in 10-20 years, or if a future grand child puts a cricket ball through one of them, we'll deal with it then 🙂 Window detailing will very probably be done by us... We can take our time and make sure it's done right. Sorry Gus... I should have made our position clearer! We have planning consent and as of yesterday our pre-commencement conditions have been discharged. We have a lot of site clearance to complete, but we're hoping to begin the build in April. Refining the design is a bit critical, which is why we're looking at designing out anything that's unnecessary/futile...

Excellent… thanks @Tony L 👍👍

I recently visited a lovely home being built by one of the TF companies we've spoken with... the homeowner was very complimentary about the TF firm and to be fair the quality of the work looked great, but he was bemused by my questions about airtightness. I'm not sure it had even been considered. As we dig into this more, I'm thinking that a wall of 1.3-1.4 will be more than good enough provided we nail the airtightness.

👍👍 We've looked at the Internorm aluclad PVC for the wet rooms. It's a saving for sure and the Uw values are still great. Will take a look at that Veka option... thanks A while ago we looked at the Aluplast's Neo range, but at that time it wasn't commercially available. Might be worth another look. Also like the Rehau Artevo. The only possible hurdle would be planning... we're in a national park and the approved drawings state 'aluminium'. I think the Neo has an aluminium clad option... not sure about Rehau or Veka. More fun investigation!

Absolutely 👍

Fair point. In our calcs, I’ve used our current standard rate for the electricity, but have ignored the fact that we’ll also have PV & batteries, so provided there continue to be off-peak rates for charging the batteries in the months with low generation, we should be paying very significantly less on average. Our current average cost per KWh is around half the rate that I’ve used in the calculations (I was aiming for ‘worst case’) 🤞

At the low end, it’s 3G aluclad timber (e.g. Internorm) and at the higher end it’s aluminium, which we’re most unlikely to go choose. I’d say the bang/buck sweet spot would be the likes of Zyle Fenster, Rawington or Nordan, all of which are a lot cheaper than Internorm but are still around 0.8. That’s similar to our rationale… minimising ongoing running costs is important. This house gives us everything we want so we need to be confident that we’ll be able to afford to stay in it, come what may 😬

Indeed. I’m fairly sure ours will cost a little more than its value, but it’s the only way for us to get the house we want in the area we want to live. You never know what’s around the corner, but we intend it to be our last home so what it gives us is more important that how much it’s worth. That being said, once we’re gone I imagine it will be sold by our children (quite rightly) and I have limited desire to invest my hard-earned for the benefit of the next inhabitants That’s a good point… what are the things that you would strongly advocate, even if the strict cost analysis doesn’t perhaps appear to make sense?

We’re in the process of getting quotes for our build. The u-values for the timber frame range from <0.1 to 0.15 and for the windows the average ranges from 0.73 to 1.1. I’ve been playing around with Jeremy Harris’s heat loss spreadsheet and have created a whatif? table looking at the 20 year impact of wall & window u-values on likely heating cost (assuming the mean minimum OAT, so hopefully a worst case). Of course, I’ve made some other assumptions (floor/roof u-values, MVHR efficiency, airtightness etc) and my calculations could be wrong(!) but it looks as though the difference between the ‘best’ option (0.1 walls, 0.73 windows) and the ‘worst’ (0.15 walls, 1.1 windows) is about £8k in total over 20 years. The cost difference between the two options is many times that, so the payback is probably >100 years 🤷🏻‍♂️ So I’m thinking we should forget about 0.1 for the walls, maybe aim for 0.13-0.14, and save another significant chunk by going for middle of the road windows (0.8ish). I guess there are other considerations, but I’m not sure I see the point of aiming for super-low numbers… What am I missing?

Fair point. That’s our main concern. I think aluclad is a better option. The main reason for looking at aluminium was that we’ve struggled to find one supplier of aluclad that can do all we need without some compromises, so we’d either need to to use more than one supplier or, well… compromise 😁 A bit concerned about mixing suppliers, particularly on the same elevation 🤔 Deceuninck (and others, I’m sure) can do it all, but we’ll look more carefully at the frame construction 👍

Have thought about it. Might revisit Then I shall respectfully remain fatgus 😂 Aerobarrier is really interesting. Have encountered it a lot while reading this forum and it would seem to be well worth the money 👍 👍👍👍 to posi joist roof & blown cellulose. Norrsken is on our list. Love the windows, but wish they weren’t such a premium over most of the others. I suspect we’ll go for something more middle of the road like Zyle Fenster, Nordan or possibly aluminium like Deceuninck, although I’d prefer aluclad. We’ll soon be sending our final schedule out to the window candidates, so we’ll see what that throws up. Thanks for taking the time to comment everyone… it’s much appreciated!

We’ve had some samples of cedral and a few composite alternatives. Other than possibly millboard, there are none that really appeal, which is a shame 🫤

Thanks

The existing roof is flat and will be removed entirely, so the overhang can be tailored to match whatever we do with the existing walls. We’re considering larsen trusses on the existing walls so we can maximise the insulation and try to maintain a kind of continuous envelope of insulation around the entire building. We’re aiming for passive-ish levels and airtightness is a priority, along with the insulation. We’ll have MVHR, ASHP, UFH, triple glazed windows/rooflights. Incidentally, despite the username I am also Nick and am in Wales 😁 We were going for block construction, but it’s taken a long time to get to this stage and timber frame would appear likely to make the process quicker and easier in so many ways. Twin wall isn’t definite, but seems appealing… more insulation, easier to blow said insulation, less cold bridging and thicker walls. We’ve had a few quotes that seem reasonable, although we’re still looking at the details. We’ve discussed the fire risk with one of the TF suppliers and their suggestion was to use cement board behind the cladding. I can’t help feeling that the original plan of block build would make more sense 🤔

We’ve finally been granted permission for our extension and hope to get started in the spring, subject to discharging a few pre-commencement conditions. Our plan was to go for block build with 300mm cavity (a bit like Denby Dale), with EWI on the existing (block) building, but we’re now considering a pivot to twin-wall timber frame, mostly because of speed and precision. The original design had wood cladding on the ground floor and metal on the first. Planners didn’t like the aesthetic so we switched to 100% wood. On the intended block build, that seems to be fine, but on a timber frame I understand that insurance can be problematic and, if we were to sell, mortgagability might be an issue. Obviously, some of the building will be the old block structure, but the surface area of the extension walls is something like 3x that of the retained exterior walls. It would be great to know if anyone has faced a similar choice, built a 100% wood clad timber frame house or have any other insight on the subject Thanks!

Thanks @craig

We're getting a quote from Craig 👍 Internorm & Norrsken are probably our two top contenders at the moment, but they're pricey. I think the Internorm seals seem 'better' than Norrsken, insofar as there are more of them, but we'd ideally prefer outward opening which Internorm don't do. Nordan is currently number three, but is notably cheaper. Some of our fixed windows are quite large and a few manufacturers can't do them in one piece. For aesthetic reasons, we also really want our main lift & slide to be three panel but a few of the suppliers can only do two or four, or a two with a fixed side panel which doesn't look right. This really is proving quite tricky!

That’s probably quite feasible. I can see us using different companies for windows & sliders anyway. The openers are mostly first floor, with ground floor being fixed, sliders or patio doors, so slightly different aesthetics probably wouldn’t be too noticeable.