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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthordebbiel
    • CommentTimeSep 5th 2019
    I've got a 3.5m x 11m timber frame building to be used as an outdoor classroom (light use). The monopitch roof has ply with battens and onduline on top.

    The rafters are 400mm centres and are 190mm deep. What is the best way to insulate between rafters? Best insulation to use? Do I need to leave any air gaps, or full fill? Breather membrane/vcl? External insulation not an option, so what would be best for an internal thin layer to stop thermal bridging? The work will be done by volunteers, so I'm thinking Kingspan or similar for ease of use, but all ideas welcome.
    It will be finished with plasterboard internally - unless anyone has a better alternative?
    Thanks in advance.
    In the absence of any other replies, I'll put this up to be shot down...

    I try to avoid Kingspan being the default, but it's what people tend to know a wee bit about, so it's a good benchmark. No idea what insulation you have in floor or walls, or what Uvalue you're trying to achieve, so a little guesswork, and pragmatism, and recognising "volunteer labour"...assume you might think of 100mm kingspan:

    Option 1. polystyrene in 100mm and 50mm sheets, to give 150mm. That's about the same as Uvalue 100mm Kingspan, but not as expensive or dusty to work with (yes it "snows" but sawing kingspan is horrible). Push it so it against the roof sheet, to give some space below for cables, recessed lights, what ever. Squirt in expanding foam around the edges to hold it in place and stop air blowing around. Using expanding foam - less is more!!

    Option 2. Knauf earthwool loft roll 40. Either two layers of 100mm or one of 200mm (comes in both these thicknesses). That again will give about same Uvalue as 100mm kingspan. Easy to fit, not itchy, though a bit fluffy. Much cheaper than kingspan.

    Either way (I'd personally go for option 2), I'd add a VCL, just some cheap polythene, installed in the same direction as the ceiling joists, so the overlaps land on a joist. Staple it in place, and tape over the joints. Actually, if you run double sided tape onto the already fitted edge, it makes fitting the next run much easier, and gives the seal in one go. When the plasterboard goes up, it will nip the VCL sheets and keep them together anyway.

    Good luck.
    • CommentAuthordebbiel
    • CommentTimeSep 10th 2019
    Thanks for your reply GreenPaddy, and for your practical approach. I love the doublesided tape trick! The earthwool sounds our best option, it's not something we've considered. We've just built a house using a mix of woodfibre and sheepswool, but for cost / ease of use we'd defaulted to Kingspan on this, but I do prefer the sound of the earthwool. What do you think about the breather membrane? The walls of the cabin have breather membrane on the external side between the OSB and the cladding, but the roof has none. Too late to put it externally cos the roofing sheets are on, but would it be worth stapling it to the underside of the ply, either between the rafters, or wrapping it around the rafters so they are included? Fiddly I know, but I don't know how crucial it is. On the house we had a breather membrane roofing felt combo, but because this cabin roof is constructed differently we forgot the breather membrane, so now trying to make the best of what we've got. Thanks again!
    • CommentAuthorGreenPaddy
    • CommentTimeSep 10th 2019
    Don't bother with VPM internally, won't do anything. With the corrugated roof, there'll be air moving under the currugations and over the (OSB?) sarking, so that will help remove any moisture that gets to the OSB, from within or without. The VCL internally, will help reduce moisture getting to the OSB from within. You've suggested it's a light use classroom, so a lot less moisture liberated internally than a house anyway.

    You can beef up the Uvalue by going for the earthwool frametherm 35 or even 32. About 20% better thermally, but quite a lot more expensive. Using loftroll40, Uvalue of around 0.23W/m2K. With FT32 around 0.19W/m2K

    By the way, earthwool has a vapour resistivity of about the same as wood fibre and better than sheeps wool. Appears to be "relatively" green, but I haven't researched the reality behind headlines.
    AFAIK Earthwool is glass fibre with a brown colour and less itchy binders than GF used to use. Google says: "it is manufactured from recycled glass bottles, sand and other materials."
    • CommentAuthordebbiel
    • CommentTimeSep 11th 2019
    Thanks again! Now I'm wondering about using the earthwool in the walls. We were going to use Kingspan, but you're right, cutting is a pain. Does it have to be the frametherm in walls, or is the loft 40 ok?
    • CommentAuthorGreenPaddy
    • CommentTimeSep 12th 2019 edited
    The higher density wool, tends to stay in position better than the lower density loft roll, but if you pack it in carefully, it'll be fine. Any doubts about it slipping, and you could hammer in some long nails into the sides of the timber studs, leaving them mostly sticking out, so they grab the wool edges. Use up the spent paslode nails, or pulled 4" galv, of which there are always loads laying around after joiners have been on site. 3 or 4 nails on each stud would do it.

    What depth are the wall studs, I'm guessing 140mm? It might be worth investing in the FT32, as it's the last chance you have to insulate the place. If the labour's free, put some of that saving into the materials.

    I would add a foil faced VCl over the studs/wool (foil facing into the room), then nail 25x50mm battens onto the studs, to give you a service void. A classroom is likely to have sockets, switches added over time. Make life easy for yourself, and also the void stops electricians ripping huge holes in the VCL, to fit a little cable through it. Plus it improves the Uvalue with the foil/void lowE unvented cavity. Actually, you might want to use 25x100mm for the service battens on every other stud, so that the plaster board edges have a bigger landing area for fixing. Volunteers, might not be quite as accurate with the batten locations, so gives a bit more leeway.

    Also add plenty of fixing pads, for shelves, cabinets, wall mount TV's, coat racks, etc. You can do this easily by nailing 25x100mm horizontally across the wall studs, in the service void zone before fixing the vertical battens. Doesn't add to cold bridging like noggins would.
    • CommentAuthordebbiel
    • CommentTimeSep 13th 2019
    I'd never heard of foil faced VCL - would that combat thermal bridging? When we were thinking of using Kingspan, we were going to put a layer of 20mm Kingspan across the internal face of the wall to try to help with that. Foil faced VCL sounds easier. We only need to achieve the Uvalues to get through building regs, as the building isn't residential and not getting lots of use. And yes, 140mm studwork. Thanks again!
    • CommentAuthorGreenPaddy
    • CommentTimeSep 14th 2019
    Shame it hadn't been 3mx10m, you wouldn't have needed building regs :confused:

    No, the foil VCL doesn't really do anything for cold bridging itself, it's just a slight improvement due to the cavity, and low emissivity surface.

    Do you mean 25mm Kingspan? Not sure it's avail at 20mm, at least not readily?

    According to Part L (I use Scottish Regs, so maybe double check this), for non-Dom...

    Roof = 0.18
    Walls = 0.28
    Floor = 0.22

    Roof- 190mm loftroll40 between roof joists does not meet BRegs at only 0.22. Under draw it with 25mm kingspan takes you to 0.17, so a pass. Under draw with 50mm kingspan gives 0.142. If you do under draw, then consider battening the ceiling as per the walls (we call that brandering), so again you have a service void for lighting cables, and an easy fix for the plasterboard. Of course you could just whack up some kingspan backed plasterboard, but then lights/cables will end up penetrating the VCL, hence my pref for service battens/branders. Again if you use 25x100mm every other joist, (as mentioned previously for walls) you'll have a big target for the plasterboard edges to land on, and get fixings in.

    Walls - you'll just scrape BRegs with 140mm studs filled with loftroll40, foil VCL, service void, at around 0.26. If you add a layer of 25mm kingspan, that takes you to 0.2. Add 50mm takes you to 0.16. Note, if you add the foil faced kingspan, then the VCL onto the studs need only be a polythene, since the the foil face of the kingspan will give you the low E surface.

    Floor - don't know what the build up is?

    Look at flat LED panels for lighting?
    The English (and Scottish) building regs/Stds have rather different insulation requirements, depending if this is a refurbishment of an existing building, or a completely new build, or a building being converted from one purpose to another - which is it?

    There's exemption for certain buildings (historic, small, unheated, ...)
    • CommentAuthordebbiel
    • CommentTimeSep 15th 2019
    WillinAerdeen it's a new build, non residential light use outdoor classroom approx. 3.5 x 11m
    • CommentAuthorGreenPaddy
    • CommentTimeSep 16th 2019
    Pretty sure that's what I checked against, having assumed it was a new build non-dom.

    I would also have assumed you would have agreed with building control office, what you would be building, and to what values, but since I've only built large commercial buildings in England, and since all the drawings, specs, and regulations must be signed off in Scotland before you make a start, I'm not familiar with the vagaries of domestic building control down south...get the impression it can be a voyage of discovery.

    Anyway, I suggest you confirm with your BC person what Uvalues he/she is expecting as a minimum (should be as I noted above), and then you're not taking any risk, and the BC officer knows what they're going to see when they get to site to inspect, at the pre-plasterboarding inspection point.
    • CommentAuthordebbiel
    • CommentTimeSep 16th 2019
    Looking back at the building reg application it was very vague (not sure how it got passed tbh!) - the building officer at the time seemed to take the view that it was a glorified shed so he wasn't really bothered! However, I know there has been changes in the dept and they are now using external building reg consultants, so I think your advice to doublecheck what they want is very wise.
    • CommentAuthordebbiel
    • CommentTimeSep 17th 2019
    Well, that was fun! I called building control with my application number to enquire what u-values they wanted me to achieve. The officer told me that different buildings have different values. I asked if he could be specific about my application. He suggested I look on the Kingspan website. I said that actually I had looked at their chart, but wanted to check if that was agreeable to building control, and also there was a phrase on the Kingspan chart that I didn't understand (best starting point, fabric only). He said he didn't know what that meant either! He then put me on hold for a bit and came back with .26 for walls and .18 for roof, and said it's all in document L, so I guess that's what he'd looked at while I was on hold.
    So that means I can go ahead with the earthwool / Kingspan combo as suggested by you GreenPaddy, thanks for all your help. One final question before I leave you in peace, on this set up would the polythene VCL go between the studs and the 25mm Kingspan, or on the internal side of the Kingspan?
    • CommentAuthorGreenPaddy
    • CommentTimeSep 18th 2019
    Ideally, you keep the VCL as close to the warm side as possible, however, it's an awful lot easier to install the VCL straight onto the studs, helps hold in the wool, then insulation board on top. That's fine in your case, as the interstitial condensation cals pass for that sequence, as the balance of insulation each side of the VCL are OK. In general, you have to be careful in locating the VCL versus insulation layers. For your case it's fine, and just use a polythene, not a foil faced VCL.

    The other point is that you get the benefit of the foil face of the insulation board, as it will face into the batten service void, not covered by the VCL.
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