The Bear

This is kind of why I was thinking to leave insulating the walls to next year - see how much my wife complains over winter.. Thank you for the levity!

If you mean the very top layer, I went for EPDM, to go straight onto the roof boards. I'm prepared to push it, within reason, given that a) it's for my wife (women are short), and b) as the ceiling slopes towards the back, no big deal if she can't stand up straight against the wall. Note to self - measure wife.. I'm not sure how or whether that would work with the roof boards I've just finished nailing in.. I'm also unsure about making the walls airtight, and here's why: 1) By "airtight", do you also mean "moisture tight"? Reading your post above, they're not quite the same things, am I right? 2) It's a log cabin, which I am expecting to absorb moisture and move, despite my best efforts of.. 3) Hitting all the end grain with Osmo protector wax, and 4) Buying Osmo paint for the exterior 5) Wife not bothered about paint or finish on the inside 6) But I know what can happen when only one side of a timber is painted.. 7) So would it be sensible to put something - even a clear matt finish - on the inside? 8 ) Or would insulating the walls, with airtightness and vapour control layers, take care of this for me? Sorry, you can see I'm not quite out of the rabbithole yet!

Bottom pic shows my “built-in” vents. Although there’s a little variance between purlin levels, they were never going to be flush. Top pic shows similar at rear wall

@Iceverge Thank you for your comments. Like most other people on this site, I've gone down the rabbithole and become slightly overwhelmed. Warnings to KISS are noted. My saving grace may well be half-decent workmanship, fingers crossed. I do have a couple of queries tho.. Airtightness is all very well, and doable in the floor and ceiling. But Tuin also strongly recommends adding air vents at opposite sides, one high and one low. I can see the sense in this, keeping a small but regular change of air. But won't that negate me spending plenty of time and cash on taping the floor and ceiling? My understanding is lacking here. Or is it subtler? I want to keep moist air away from places it could condense and become trapped (within layers of the floor or ceiling), but inside the building overall, I want moisture inside to balance moisture outside? Cold roof seemed the best option for this cabin, what with height restrictions for permitted development. And as I've made progress on the roof, I see it also has "built-in" vents front and back.. I hammered those purlins down *hard* too - I'll post a pic shortly. To be honest, right now I'm still wavering on how to do this. I'm just so tempted to use PIR! But I still have some time to decide.. For the floor, the eco-build schedule does make sense - clear instructions, I can save some cash by using alternates to Solitex Plus and Proclima DA, and if I follow those instructions well, I can be confident jobs a good un. Even if I'm spending more and getting a lower U value than I'd have with PIR. For the roof, I have no problem with attempting a mirror of the floor, using a membrane either wrapped around the joists as per the floor, or individually fixed like this but fixed lower to keep my airgap. Then either jute and a membrane; or jute, membrane, PIR, t&g ceiling (however that would work). I also know I should, at some point, insulate the walls. But I need to get back to work at some point, so that might have to wait. I've also not thought about voids to run electrics through, hoping a solution would present itself while I'm working on something else.. On the whole, I am finding this fun, most of the time. Plenty to learn and the principles and benefits of good insulation are kind of fascinating. Very helpful to have people such as yourself on this site to bounce ideas off, or just say "stop overthinking and crack on!"

No worries! This is what I was thinking of.. Not sure how to put a vapour barrier in here yet (or why Ubakus doesn't flag this), but this is pretty much how my roof would look if I were to use thermo jute and PIR/PUR

I was thinking of the Ubakus models of slab insulation between roof rafters, then a complete layer of PIR beneath. I'll see if I can find a pic..

Christ, now I'm wondering how long it would take a determined rat to gnaw through 1mm of metal wire. He'll have his missus nagging him, umpteen mouths to keep warm.. he's gonna chew til he's through!

Even the government says PIR isn't breathable - so it must be true. TLDR: "Second, impermeable materials made from petroleum products: phenolic foam, PIR, PUR and EPS. These offer broadly similar performance, and do not breathe, which is an important consideration for moisture control" It's a bummer, given the difference in insulation value and cost compared to wood fibre etc., but I wouldn't believe a timber framing company with a vested interest. Even looking at the major suppliers for Celotex you'll find on Google, I'm not seeing anywhere in their blurb that it's breathable. You can draw your own conclusions.. This also makes me wonder why some people are using a layer of PIR beneath slab insulation. How does this work with moisture? I must be missing something, because I'd still love to use it for the cost and insulation benefits. Yes, too late for a different roof system. Many, many nails have already gone in.

Thank you - I've bought some panel mesh to try out - 6mm squares in 1mm galvanised wire. It's doable from beneath, apart from being a ballache and tearing my hands up.. You are right - I should also consider a curtain barrier. The list keeps on growing, but it has to be right!

Out of interest, would insulating the roof just be a mirror of the floor? So it’d be like doing a floor from underneath? Also, what are the thoughts on using a layer of PIR beneath the rafters, once they’re filled with slab insulation? PIR isn’t breathable, right? 🤔

@Iceverge when I know how much headroom I have from the rafters, I’ll post my thoughts on roof insulation and would be glad of any comments

I’m wondering about that! Probably the best solution, and more preventative than just running a skirt of it around the base..

That’s very kind of you - I am pleased with the base! But I went for the Tuin kit, both for convenience and to bypass my tendency to overthink. Without the pressure of taking time off work, I’d have happily had a go myself. But my wife is starting her own business, so needs must. And honestly, given that the ground screws weren’t as straight or level as I’d hoped - hence the triple laminated perimeter - plus the inevitable moisture and bow/cup/twist issues with the 6x2’s, just finding the racing line on the base has not been simple. So right now I’m glad I went for the kit. And if I can get the insulation right (more or less) to make it an efficient building, I’ll be happy.

I’ve had a read of your thread, and your building is very similar to mine. However, I’m still getting my head around the roof insulation! Think I’m good on the floor - although the gentleman who mentioned rodents has given me another good point to consider. I will go back over your thread a few times and see if more sinks in! Thank you.

I appreciate your bluntly honest assessment of my calculations - seriously! I’d assumed an eight hour working day, with two hours worth of energy to get up to temp in the morning. I’d thought if this was compressed into an hour beforehand to get up to temp, and OAT would rise during the day anyway, it might average out over the day. Mindless optimism.. The issue with sitting wall insulation now, is mostly time and money. I’m taking time off work to do this, and of course it’ll take longer than I expect, as nearly everything does. It may be more realistic to get the building up, so my wife can use it over summer and autumn for her business, return to work for some funds, then insulate walls before it gets too nippy. Also, looking at JohnMo’s thread, walls look more complicated than I expected. I’m just getting my head round the layers to do the floor and roof properly! I’ll probably feel better about it once I make more progress. I spent two hours yesterday just getting the rear first log straight and level, but hopefully that will pay off.. My best mate said I’d learn a lot doing this, and he wasn’t wrong!

I will have a read, thank you.

Heating costs just doubled 🤦‍♂️ probably my maths..

Hmm, that is interesting, thank you. So, if 1kWh is about 25p, it’d cost maybe £2.50 a day to reach and maintain room temp on a freezing day? £50 a month? Wall insulation would pay for itself in the long run then. Knowing my wife, and her patience levels, a season of cold starts would be the best way to motivate her; she will not want to wait right now for me to fit wall insulation..

I’ve thought about it. But that’s also a reason why I picked the 70mm log option, over say 40mm. My current thoughts are to just see how my dear wife gets on over the winter! See if she spends a fortune on heating.. That approach would also give time for the building to settle, and do the majority of its expansion and contraction in the first year.

Ugh, that is a good point.. though fitting any kind of solid sheet underneath now would be tricky. I had wondered if a decent mesh around the perimeter would be effective..

Thanks for reply. Aiir tightness membrane is a good shout. I was wondering if the roof would just be an upside down floor!

Hello, first-time poster here. I’ve lurked a bit, now I’m taking the plunge.. I’m building my wife a garden office: ground screws, timber base, 70mm Mia log cabin from Tuin. Here’s where I am so far: Tuin offer their own insulation kits: basically, marked up Celotex and fixings, 40mm or 50mm for floor and roof. I thought that with 6x2 beams in the base, it should be possible to improve on that for the floor. And with rafters below the pent roof, to keep within permitted development I’d go for a cold roof, even if it meant losing a chunk of ceiling height. My key aim is, this needs to be a comfortable, 365 days a year working office, that will last up to 20 years with proper maintenance. Looking at insulation, PIR was my first choice. But the internet has shown me, not necessarily the best or easiest to install. Some of my timbers, naturally, have a twist or bow, so I can’t be sure I won’t have air gaps. And though I’ve used expanding foam often for work, that also seems potentially a bit hit and miss. Though I am minded to buy a single sheet of Celotex to see just how tricky it is to cut accurately. Despite myself, I’ve come round to the idea of this approach: Mostly on the basis that a well-fitted slab insulation would perform better than a poorly done PIR job. Although it would cost more, my thinking is, if I took care with the fitting, at least I’d know it was airtight. The roof is another matter. This will sound daft, but I don’t know the exact floor to ceiling height I’ll have yet. It’ll depend on the flooring I use, and also input from my wife! I have a rough idea from the plans, that base to ceiling height will be 2092mm. But that doesn’t include height lost to flooring, or the exposed rafters, which are 145mm deep. I can see now why Tuin recommend a warm roof, perhaps. But I don’t want any hassle from an unexpected planning visit. The good thing is, I’m still at a stage where I can make decisions on these bits, though the clock is always ticking. From looking at previous posts here, I was wondering about slab insulation between the rafters, with an air gap above, then some PIR below, like this, and maybe with a thin tongue and groove plank ceiling. But I can see that will depend on how low that brings the ceiling. Still, I thought if I asked for advice now, I could be turning it over while the office goes up. Grateful for any and all helpful comments!

Hello, I’m David. I’m a time-served decorator, specialising in “particular” clients after a high-end finish. I'm here because I’m taking time off work to build my wife a garden office, and a bit of lurking tells me this would be a great place to seek advice on insulation. What a minefield.. Here’s where I am so far!