The Bear Posted June 27 Share Posted June 27 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! Link to comment Share on other sites More sharing options...
Nick Thomas Posted June 27 Share Posted June 27 Hi. Great project! I recently put together a cold roof on 2x6 rafters for my own garden room. The full buildup, from top to bottom, was - * EPDM membrane * OSB deck * 2x6 joists * 50mm ventilated air gap * 100mm RWA45 rockwool * 25x50mm battens * 50mm rockwool * resilient bars * 12.5mm t&g pine You'd probably want an airtightness membrane too; I wasn't super worried about interstitial condensation in my case. Loss of head height below the rafters was a bit over 50mm (the lower layer of rockwool got compressed a bit between resilient bar and battens), and it was an absolute pig to fit from below, but I'm pretty happy with it. About halfway through the insulation I did find myself wishing I'd just gone for a warm roof to save on the effort, but like you, trying hard to fit within PD. The air gap needs to be ventilated; I've got a significant roof overhang, so I'm just putting some 70mm circular vents in the underside of that. PIR as the lower layer would probably be easier; I ended up using the rockwool and resilient bars as I was more concerned with acoustic than thermal insulation. Link to comment Share on other sites More sharing options...
Russell griffiths Posted June 27 Share Posted June 27 With a raised floor just be aware of rodents i would want a solid base beneath the insulation either plywood or cement sheet. just not a membrane then insulation, it will make the nicest rodent house going. 1 Link to comment Share on other sites More sharing options...
JohnMo Posted June 27 Share Posted June 27 Have you thought about wall insulation? You will need it, if heating in the winter. An important thing with these timber buildings is they expand and contract quite a bit. There should be nothing on the build that constrains this. So nothing without expansion and contraction room between timber sections, floor and ceiling, nothing strapped to wall without sliding fittings. My brother in law has huge cracks in his because of this. Link to comment Share on other sites More sharing options...
The Bear Posted June 27 Author Share Posted June 27 2 hours ago, Nick Thomas said: Hi. Great project! I recently put together a cold roof on 2x6 rafters for my own garden room. The full buildup, from top to bottom, was - * EPDM membrane * OSB deck * 2x6 joists * 50mm ventilated air gap * 100mm RWA45 rockwool * 25x50mm battens * 50mm rockwool * resilient bars * 12.5mm t&g pine You'd probably want an airtightness membrane too; I wasn't super worried about interstitial condensation in my case. Loss of head height below the rafters was a bit over 50mm (the lower layer of rockwool got compressed a bit between resilient bar and battens), and it was an absolute pig to fit from below, but I'm pretty happy with it. About halfway through the insulation I did find myself wishing I'd just gone for a warm roof to save on the effort, but like you, trying hard to fit within PD. The air gap needs to be ventilated; I've got a significant roof overhang, so I'm just putting some 70mm circular vents in the underside of that. PIR as the lower layer would probably be easier; I ended up using the rockwool and resilient bars as I was more concerned with acoustic than thermal insulation. Thanks for reply. Aiir tightness membrane is a good shout. I was wondering if the roof would just be an upside down floor! Link to comment Share on other sites More sharing options...
The Bear Posted June 27 Author Share Posted June 27 1 hour ago, Russell griffiths said: With a raised floor just be aware of rodents i would want a solid base beneath the insulation either plywood or cement sheet. just not a membrane then insulation, it will make the nicest rodent house going. 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.. Link to comment Share on other sites More sharing options...
The Bear Posted June 27 Author Share Posted June 27 1 hour ago, JohnMo said: Have you thought about wall insulation? You will need it, if heating in the winter. An important thing with these timber buildings is they expand and contract quite a bit. There should be nothing on the build that constrains this. So nothing without expansion and contraction room between timber sections, floor and ceiling, nothing strapped to wall without sliding fittings. My brother in law has huge cracks in his because of this. 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. Link to comment Share on other sites More sharing options...
JohnMo Posted June 27 Share Posted June 27 Thread I did last year if any help Link to comment Share on other sites More sharing options...
joe90 Posted June 27 Share Posted June 27 2 hours ago, JohnMo said: An important thing with these timber buildings is they expand and contract quite a bit. Yes they do, I built a “log” garage and no vertical timbers should be fixed to the horizon timbers except one end, door and windows have contraction gaps to allow for this. If you need to fix vertical anything sliding fittings at one end. Mine is 45mm logs and although only a garage and not been through a full winter yet it stays quite warm so I think with a small heater yours will remain quite comfortable. 1 Link to comment Share on other sites More sharing options...
Nick Thomas Posted June 27 Share Posted June 27 20 minutes ago, The Bear said: I’ve thought about it. But that’s also a reason why I picked the 70mm log option, over say 40mm. My walls are 450mm thick 😅. Straw bales, not logs, but definitely not the right option for maximising space. A quick google tells me a 70mm log wall has a U-Value of 1.41 W/m²K. Going naively, since your log cabin is 5.0M X 3.8M X 2M (ish), you can say you've got about 35M² of wall, about 19M² of floor, and the same again for the ceiling. So at 0 outside and 20 inside, you'd lose ~1kW through the walls. If you went the 150mm rockwool approach in floor and ceiling, the U value is about 0.26, so you'd lose ~145W through those in total. Insulate the walls a bit, and the heat demand could be more like 300W. Link to comment Share on other sites More sharing options...
JohnMo Posted June 27 Share Posted June 27 15 minutes ago, Nick Thomas said: My walls are 450mm thick 😅. Straw bales, not logs, but definitely not the right option for maximising space. A quick google tells me a 70mm log wall has a U-Value of 1.41 W/m²K. Going naively, since your log cabin is 5.0M X 3.8M X 2M (ish), you can say you've got about 35M² of wall, about 19M² of floor, and the same again for the ceiling. So at 0 outside and 20 inside, you'd lose ~1kW through the walls. If you went the 150mm rockwool approach in floor and ceiling, the U value is about 0.26, so you'd lose ~145W through those in total. Insulate the walls a bit, and the heat demand could be more like 300W. Plus glazing, door and ventilation heat losses. Doesn't sound much, but you need to tomes by by 75x4524 (kWh per day) or let it get cold over night and panel heater takes a fair while (hours) to reheat. If it takes a kW to heat that's close to £6 a day on a cold day. Link to comment Share on other sites More sharing options...
Nick Thomas Posted June 27 Share Posted June 27 4 minutes ago, JohnMo said: Plus glazing, door and ventilation heat losses. Doesn't sound much, but you need to tomes by by 75x4524 (kWh per day) or let it get cold over night and panel heater takes a fair while (hours) to reheat. If it takes a kW to heat that's close to £6 a day on a cold day. Get triple glazing and the windows could perform better than the walls 🙈! I'd definitely want some insulation in addition to the logs, myself. Apparently a good, lime rendered straw bale wall hits U=0.171. Probably overkill ^^. Link to comment Share on other sites More sharing options...
The Bear Posted June 28 Author Share Posted June 28 16 hours ago, Nick Thomas said: My walls are 450mm thick 😅. Straw bales, not logs, but definitely not the right option for maximising space. A quick google tells me a 70mm log wall has a U-Value of 1.41 W/m²K. Going naively, since your log cabin is 5.0M X 3.8M X 2M (ish), you can say you've got about 35M² of wall, about 19M² of floor, and the same again for the ceiling. So at 0 outside and 20 inside, you'd lose ~1kW through the walls. If you went the 150mm rockwool approach in floor and ceiling, the U value is about 0.26, so you'd lose ~145W through those in total. Insulate the walls a bit, and the heat demand could be more like 300W. 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.. Link to comment Share on other sites More sharing options...
The Bear Posted June 28 Author Share Posted June 28 16 hours ago, JohnMo said: Plus glazing, door and ventilation heat losses. Doesn't sound much, but you need to tomes by by 75x4524 (kWh per day) or let it get cold over night and panel heater takes a fair while (hours) to reheat. If it takes a kW to heat that's close to £6 a day on a cold day. Heating costs just doubled 🤦♂️ probably my maths.. Link to comment Share on other sites More sharing options...
The Bear Posted June 28 Author Share Posted June 28 16 hours ago, JohnMo said: Thread I did last year if any help I will have a read, thank you. Link to comment Share on other sites More sharing options...
Nick Thomas Posted June 28 Share Posted June 28 4 hours ago, The Bear said: 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. It was a very naive calculation, but it gives you a ballpark to see how you feel about the costs upfront. Retrofitting this stuff later can be a pain ^^. Also remember that it's an assumption based on when it's 0 outside; seems the mean in January in Reading is actually 5°C. Getting up to temp, as @JohnMo noted, would require extra heat going in above the 1.15kW - that's just what's required to *maintain* the temperature difference (i.e., how much heat is leaving the building, and so, how much need to be replaced to maintain steady state). One benefit of better insulation is that the same size of heat source will get the building up to temperature faster in the mornings. It does take time to install, but, it'll probably be faster if you do it in the first place rather than retrofitting it ^^. Link to comment Share on other sites More sharing options...
JohnMo Posted June 28 Share Posted June 28 We were typically using 5 to 9kWh each day via a panel heater during Nov (we got down to about -7. However we are now hooked into to house heating system. Link to comment Share on other sites More sharing options...
Iceverge Posted June 28 Share Posted June 28 @The Bear Have you already bought the kit? You've made a fine job of that base, I don't think a stick build would be beyond you. Link to comment Share on other sites More sharing options...
The Bear Posted June 29 Author Share Posted June 29 On 28/06/2024 at 11:12, Nick Thomas said: It was a very naive calculation, but it gives you a ballpark to see how you feel about the costs upfront. Retrofitting this stuff later can be a pain ^^. Also remember that it's an assumption based on when it's 0 outside; seems the mean in January in Reading is actually 5°C. Getting up to temp, as @JohnMo noted, would require extra heat going in above the 1.15kW - that's just what's required to *maintain* the temperature difference (i.e., how much heat is leaving the building, and so, how much need to be replaced to maintain steady state). One benefit of better insulation is that the same size of heat source will get the building up to temperature faster in the mornings. It does take time to install, but, it'll probably be faster if you do it in the first place rather than retrofitting it ^^. 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! 1 Link to comment Share on other sites More sharing options...
The Bear Posted June 29 Author Share Posted June 29 On 28/06/2024 at 11:43, JohnMo said: We were typically using 5 to 9kWh each day via a panel heater during Nov (we got down to about -7. However we are now hooked into to house heating system. 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. Link to comment Share on other sites More sharing options...
The Bear Posted June 29 Author Share Posted June 29 On 28/06/2024 at 12:16, Iceverge said: @The Bear Have you already bought the kit? You've made a fine job of that base, I don't think a stick build would be beyond you. 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. Link to comment Share on other sites More sharing options...
Nick Thomas Posted June 29 Share Posted June 29 34 minutes ago, The Bear said: I appreciate your bluntly honest assessment of my calculations - seriously! Hah, I meant my calculation was naive, not yours ^^. You're doing great, tackle the jobs you have time for and come back to the rest when you can. 1 Link to comment Share on other sites More sharing options...
JohnMo Posted June 29 Share Posted June 29 33 minutes ago, The Bear said: rodents has given me another good point to consider. Could you just staple small hole chicken wire under you floor frame? 1 Link to comment Share on other sites More sharing options...
The Bear Posted July 4 Author Share Posted July 4 On 29/06/2024 at 14:38, JohnMo said: Could you just staple small hole chicken wire under you floor frame? I’m wondering about that! Probably the best solution, and more preventative than just running a skirt of it around the base.. Link to comment Share on other sites More sharing options...
The Bear Posted July 4 Author Share Posted July 4 @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 Link to comment Share on other sites More sharing options...
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