Crofter

I don't know how much you can change their offering, but you could swap that 120mm polystyrene layer for a thinner layer of polyurethane foam (probably about 80mm for the same performance, but that depends on the exact materials in question). This would allow a more efficient footprint and thus some savings on roof/foundation costs relative to floor area, and would also make fixing your cladding considerably easier. I wonder how that buildup handles moisture transfer. The insulation and render appears to present a fairly impermeable barrier on the outside of the house. I designed my own buildup to become more porous as you work your way from inside to out. But cleverer people than me can probably comment on that one. If you didn't have the layer of render, I presume you would still need something on the outside of the insulation- perhaps a breather membrane. That would certainly be far cheaper and easier than render. Then you simply apply vertical counter-battens over that, fixed through the insulation using long screws, and horizontal cladding battens over that. You might be able to fix those battens using short screws into the counter-battens, but only if the counter-battens are meaty enough. Mine were only 25mm so I used more long screws, which passed through both sets of battens and into the studs. Fitting the cladding itself is pretty straightforward- a coil nailer and some stainless nails helps. But watch out for the corners and where you meet any openings, it's quite easy to make a mistake in these places. Are you thinking of board-on-board, or some other cladding profile?

Yes that's a very valid point. uW rather than uG is the most important figure. I'm tending to weed out any companies that solely cite WER values, although I believe that A++ is a passable level of performance. (You'd be amazed at the number of companies who claim that 'A rated' is 'the highest possible rating'... have they never heard of A+, or A++, or are they just lying?)

OK so I've chickened out and have asked a local joiner to help me take the measurements. Better safe than very very sorry! Btw I've had a few quotes come in now and it appears I can get everything in 3G for about £2k (supply only). The big name window company who were visiting the other day quoted seven times that amount, for 2G of unspecified performance. OK so that was supply + install, but still! All they seemed to want to talk about was finance options- probably a sign of where they make much of their profit margin...

You can't assume that the final roof covering will remain 100% watertight- you have to plan for what happens when water gets under it. A continuous horizontal batten laid directly onto the sarking would not allow for any drainage, so the purpose of the counter-batten is simply to space the tile batten off the roof, and give a drainage gap. 25mm seems the usual size but building regs (in Scotland at least) allow for 12mm minimum, IIRC. If you really wanted to save a few pennies you could use little squares of ply or something as a spacer at each fixing point, but even someone as parsimonious as me would find that a bit stingey!

Surely you change the spec of the build and not pay for render? What's the wall buildup?

I'm replacing the old timber single glazed windows in my 1970s house, and whilst I am happy enough with the idea of fitting the new ones myself, what terrifies me is that I might measure up wrongly and end up with thousands of pounds worth of windows that don't fit!! In my new build it was all very easy really because I bought the windows and then built the house around them. What's the best way to measure up- and just how accurate do I need to be? Do I take the visible internal size and add a bit. Or likewise from the external size? Or, for sake of peace of mind, should I just remove all the surrounds now and measure directly? Might not go down too well with SWMBO! Whilst on the subject, I think self-install is going to be less hassle than dealing with big name double glazing companies. I've barely started this project and I'm losing the will already!

Thanks, I'll give them a go although their online quote is giving some pretty hefty prices. Hope they can bring that down a bit over the phone!

Re-visiting this one. Just waiting to hear back from Craig, but in the mean time any suggestions at all for windows a bit closer to home? Modern uPVC have been good to deal with so far but the delivery is a bit of a killer.

The only advantage I can see in using planks rather than sheets is that they are individually easier to work with, and you can miss one out every so often to make it easier to climb up the roof whilst building. Sheets are cheaper, quicker, and give considerably better racking strength.

If it's a modest amount of heat required, a vehicle heater can be a good option. The Chinese have recently started cloning the well known Webasto/Eberspacher type heaters- I fitted on to my boat and so far it's been great. They're designed to work off a low voltage and happily burn red diesel. A great option if you are off grid. Random example off eBay: https://www.ebay.co.uk/itm/Vehicle-Air-Diesel-Parking-Fuel-Heater-Warming-12V-5KW-LCD-Switch-for-Truck-Boat/292805810337?epid=23027418008&hash=item442c9618a1:g:jtsAAOSwH2Vb4ZKS:rk:2:pf:0

Don't nail on your battens, especially with a nail gun- you won't know how many of your fixings have missed the studs, and you also have much less control over the direction. Screwing obviously costs a bit more, and takes a bit longer, but IMO it's the only way to do it right. If I was being a perfectionist, I would stick every batten under a pillar drill and drill perfectly perpendicular pilot holes at the correct centres... increases your chance of hitting the stud, and prevents the battens from splitting... just a thought

@Thedreamer you would attach the plasterboard to the battens, not to the studs. In my ceilings, I had 100mm PIR to screw through and, yes, some of the screws missed the wood- but you get your eye in. Just make sure you have decent and comfortable access when working and it's quite easy to check for square. Less so when leaning off one side of a ladder...

First question has to be- what do you think the market is in your area for holiday lets? Party size, length of stay, seasonality, and of course the going rate for accommodation. Occupancy rates will make a massive difference to your income. Here in Skye, the market has shifted in recent years- people offering weekly lets are beginning to struggle, whereas those offering shorter stays are doing very well. The traditional model of a Saturday changeover is much less lucrative, so if you work full time you need to employ changeover staff to cover the mid-week cleaning. Perhaps less of an issue if you are retired, of course. Out of curiosity, what do you see as being the advantages of going BC exempt? It was a path that suited me quite well, being almost 100% DIY, but for a more typical project where trades are employed it loses much of its benefit.

I used normal 25x50 battens, with a large central screw into the stud or rafter, and then the plasterboard screws either side. I didn't have any problems with this, but a meatier batten wouldn't have hurt at all.

At the risk of causing a bit of thread drift, upgrading to triple glazing can be a very cost effective way of increasing the thermal performance of a building. Many people don't realise at the start of their build just how much heat is lost through even very good windows. Upgrading to 3G is a relatively low impact way of making a big difference.

Frametherm can give better real world performance because it can fill the studs more tightly. It's pretty hard to fit PIR into studs without leaving some gaps, which would need to be foamed. If you have the option, blown cellulose looks excellent as it gives a better R value than wool, whilst properly filling the frame.

Most of my boards were just paper faced (I got them as seconds at about a third the normal price). I did tape the ones that were foil faced. So I had to rely on a separate membrane to act as vapour barrier. It's just a heavy grade of polythene but I don't see any reason why it doesn't also provide airtightness. Caveat- my build was buildings regs exempt so I didn't actually do an airtightness test- on a 'real' house you might need or want to aim a bit higher.

35mm would probably have been enough to use non-mutilated back boxes. And of course it opens up the option of using metal boxes behind the plasterboard. My wall buildup is 150mm glass wool between the studs, and 50mm PIR on the inside of that. Roof was the same but 100mm PIR instead.

My service void was only 25mm and it was a bit of a PITA. All my back boxes had to have the plastic lugs cut down to prevent them hitting the back of the void. Given that you need to leave big chunks of it empty, I didn't see much point in insulating the void. An extra 25mm, on the wrong side of the vapour barrier, doesn't make a huge difference to a well insulated build up.

I have it on excellent authority from our resident Welsh plumber that white CT1 goes yellow after a while- so for aesthetic reasons he suggests using clear CT1 with a white silicon over the top if necessary. Btw I used Forever White myself and am perfectly happy with it, but time will tell...

So... I've picked up a cheap 2nd hand single room MVHR unit off eBay. It's incomplete, hence the low price. The major bit missing is the control unit, but upon opening it up it appears that there are just two small computer-type fans inside, each wanting 4w at 12v. I've powered it up off a car battery and it works just fine. Plan is to wire it into the bathroom lighting circuit... yes I know there will be no hysteresis... but it will get me up and running easily. I can pick up a cheap 12v transformer off eBay, like this: https://www.ebay.co.uk/itm/DC12V-LED-Driver-Power-Supply-Transformer-6W-12W-36W-48W-60W-Strip-MR16-G4-CCTV/272777976696?hash=item3f82d59b78:m:mITRanGIZh6GiuSn_zCgkFw:rk:11:pf:0 Question is- is there anything to be gained or lost by buying an oversized transformer? They all cost the same anyway.

As Joe90 says, roofing felt is a better way of preventing condensation. You may still get water condensing on the underside of the sheets, but the breather felt will prevent it from dripping back down into the garage.

There is some risk if no breather membrane, although a garage is likely to be very well ventilated anyway. To the OP- I would get a price for corrugated steel, it may not cost any more than bitumen sheets and you will save money on the rafters because it needs considerably less support.

Welcome to the forum! When you're half way through the project, try to remember that incredible sense of enthusiasm that you have right now- you will need it by the sack load to get through this So go on then... we need the juicy details... whereabouts are you, what are you hoping to build, and how far along have you got so far?

Good news (hopefully): https://www.bbc.co.uk/news/uk-scotland-highlands-islands-46685387