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Green Building Bible, Fourth Edition
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    • CommentAuthorHoveTom
    • CommentTimeMay 23rd 2019
     
    Hi,

    New here and converting a 1930's bunglaow in Hove this summer. Building hasn’t started yet but will in a couple of weeks. My architect has drawn the insulation coming down the roof/ceiling and then down the Ashlar walls to join up with the floor insulation. This leaves a cold space behind in the eaves. The Ashlar wall isn’t high at 800mm so the triangle space behind isn’t large but being a bungalow conversion storage is a premium. I asked my builder if this was normal and if he could carry the insulation right down the roof/ceiling to join the floor. Then the eaves behind the Ashlar walls would be a warm storage space. He said this space is normally left cold to let the building breath. Is it? This would mean any access door into the eaves through the Ashlar walls would be a source of droughts and cold air into the bedrooms wouldn’t it?

    I plan on building some storage cupboards in front of the Ashlar walls but wanted to make use of every space with draws running all the way back in between the structural supports etc. Any knowledge greatly appreciated.
    •  
      CommentAuthordjh
    • CommentTimeMay 23rd 2019
     
    Posted By: HoveTomMy architect has drawn the insulation coming down the roof/ceiling and then down the Ashlar walls to join up with the floor insulation. This leaves a cold space behind in the eaves. The Ashlar wall isn’t high at 800mm so the triangle space behind isn’t large but being a bungalow conversion storage is a premium.

    I'm confused by this. Maybe a photo/scan of your architect's drawing would help? (downloadable attachment is probably best since you're limited in size of inline images. Or else post it to a site and put a link here).

    By 'roof/ceiling', do you mean a sloping ceiling (what is also sometimes called a skeiling)?

    I would expect an Ashlar wall to be on the outside of a building, so I don't understand how there would be space behind it. Is it a stub wall creating a gap at the lower ends of the rafters?What's it made from?

    Where exactly is the insulation? Internal, external or in between structural timbers? External insulation is generally best if possible, and you can get Ashlar slips if appearance is important.

    Posted By: HoveTomHe said this space is normally left cold to let the building breath. Is it?

    Not by people who are serious about energy conservation. That arrangement leads to all sorts of issues about continuity of insulation and airtightness in that area, and typically would mean that you can't use the space behind because the wall is part of the airtightness barrier.

    It sounds to me at this stage that I would be looking for a different architect. What qualifications and references does he have in this type of work? How fixed is your timetable?
    • CommentAuthorHoveTom
    • CommentTimeMay 23rd 2019
     
    Hi Djh,

    Thanks for your help. I’ll attach the architects drawing for that section. As it’s a bungalow conversion my bedrooms are in the roof space. So I meant the slanting ceiling of the bedroom which is also the roof of the house. Sorry, I haven’t got up to speed with the correct terminology of this site yet and this whole project is a steep learning curve for me!

    Find the plan attached.
      01250FA9-0E2D-4264-ADF4-FDC5045FBFC5.jpeg
    •  
      CommentAuthorfostertom
    • CommentTimeMay 23rd 2019 edited
     
    Ashlar means fine natural stone with very thin like 3mm joints, 'dragged' smooth after laying/jointing.

    Have you considered External Wall Insulation fixed to the outside of the walls and/or up and over the outside of the roof rafters - 150mm EPS in this case (the roof tiles taken off and re-laid)? Just checking! That would be the pukka job.
      P1010283med2.JPG
    • CommentAuthorHoveTom
    • CommentTimeMay 23rd 2019
     
    Really? I thought Ashlar wall is the correct name for the small structural wall or support?

    The whole roof will be new. I’m doing a small ground extension to make the ground floor square and then a new roof is going on with bedrooms and bathroom in that space.

    Most of the existing walls are original cavity brick built walls which had the cavity insulated in 2008 before I bought it. I’ll check the cavity insulation when the old roof comes off and if it’s collapsed or isn’t any good I’ll top it up or replace.

    A second question; There will be a new section of block wall for the extension about 4m long. This also has a 100mm cavity designed in it. My architect has spec'd 50mm of celotex in there. I was advised by someone else to put 100mm of celotex in there and fill the cavity but my builder again said I need the gap for ventilation. I’m a little confused by this too as I understand the need for a cavity but know modern walls can be built without one?
    •  
      CommentAuthorfostertom
    • CommentTimeMay 23rd 2019 edited
     
    Wow if it's new roof then def grab the opportunity to do it properly! You have Planning consent for a given finished roof external shape? Then adjust the in-to-out location of the raftering to allow this configuration:

    1. Raftering - assume 100mm high
    2. Covered externally with 11mm OSB3 gapfilling-glued-and-screwed to the rafters' outer face.
    3. 15mm of plasterboard and skim internally, right down to the floor - the vertical that you call ashlar not necessary, or could be fitted later as eave cupboarding if you wish.
    4. 100mm insulation full-filling the rafter spaces.
    5. Laid over the OSB3, 150mm Expanded polystyrene (EPS - not extruded ditto XPS) insulation blocks glued on.
    6. 38 high x 50 wide downslope battening over the EPS.
    7. Breather felt 'draped' over the downslope battens.
    8. Tiles and battens.

    Items 1-7 total thickness 314mm compared with 153mm the way your drawings show - so worth it.

    Heat loss near enough zero forever. No need for any radiators upstairs - heat rises from below and doesn't leak away.

    Air leakage extremely low, easily and robustly achieved by the glued and screwed OSB3 - no fiddly membranes. No internal VCL required - it's a 'breathing' construction.

    Internal plasterboard is not an air or vapour barrier - can be punctured at will by electricians etc.

    If combined with External Wall insulation (EWI) over the outside of the masonry, will form an unbroken 'tea cosy' - very effective, uncompromised insulation.

    The EWI can be run down in a trench to the base of the foundations, a 'coffer dam' of perimeter insulation, means that your ground floor doesn't need to be disturbed for insulation.

    And more - check it out, you have the golden opportunity.
  1.  
    Posted By: HoveTomMy architect has drawn the insulation coming down the roof/ceiling and then down the Ashlar walls to join up with the floor insulation. This leaves a cold space behind in the eaves. The Ashlar wall isn’t high at 800mm so the triangle space behind isn’t large but being a bungalow conversion storage is a premium. I asked my builder if this was normal and if he could carry the insulation right down the roof/ceiling to join the floor. Then the eaves behind the Ashlar walls would be a warm storage space. He said this space is normally left cold to let the building breath.

    Nonsense, leaving the eaves space cold is a recipe for high fuel bills and a loss of storage space.

    Toms plan for the roof and insulation makes sense!

    I don't like board insulation between rafters or studs because it relies on the workmen paying great attention to detail to avoid gaps between the insulation and the timbers (even after foaming in). I would specify either glass fiber or mineral wool for that part.

    Re the new extension - I don't like cavity walls, the cavity is usually a cold windy place sucking heat out of the building - and in too many cases even after cavity wall insulation. Why not build the extension in single block with external insulation, quicker to build, cheaper and better insulated than the specification above. For the external insulation I would suggest a minimum of 200mm. Oh and IMO wet plaster is better than plaster board.
    •  
      CommentAuthordjh
    • CommentTimeMay 23rd 2019
     
    Thanks for the picture, (Hove) Tom. I agree with what (Foster) Tom wrote and with what Peter added.
    • CommentAuthorDarylP
    • CommentTimeMay 24th 2019
     
    @ HoveTom, That section can be improved by continuing the sloping ceiling insulation down to wall plate, to join with wall insulation layer.
    Less insulation, fewer thermal bridges, easier construction...
    cheers :bigsmile:
  2.  
    I too know ashlar as fine-jointed stone, but I googled it yesterday when I first read this post and apparently some call the (dwarf) stud walls in an attic 'ashlar walls'. Odd! (See https://www.candey.co.uk/project.html for example).
    • CommentAuthorgyrogear
    • CommentTimeMay 24th 2019 edited
     
    I Would Not Say That That Site Is Any Criterion - He Also Uses First-Letter Capitals Everywhere, Which I Have Never Seen Before Neither...

    gg
    •  
      CommentAuthordjh
    • CommentTimeMay 24th 2019
     
    Plus although he claims a lot of reassuring memberships etc, there doesn't seem to be an address on the site. Another reason to run a mile.
  3.  
    The only reason for reference to that site was that it came up in a Google search for 'Ashlar walls' with a picture of attic stud walls. That's all. I had not even Noticed The Capital Letters!

    :bigsmile:
    • CommentAuthorGreenPaddy
    • CommentTimeMay 26th 2019 edited
     
    Hi HoveTom,

    I can see where you're going, but I wouldn't have started from here...

    maybe your architect is family or a friend, so at risk of being insulting, it's not a very imaginative use of building materials, well not for this side of the millenium. Single skin block wall with EWI (as suggested above); 100mm for the slab is a bit mean, why not put 150mm insul with the conc slab on top (could use fostertom's beloved EPS at 200mm still better than 100mm cellotex) and save on the screed. That's how I do all my new builds.

    Go for fostertom's fab roof, or maybe use a knauf wool rafteroll 32 and bulk up the rafters - doesn't need accurate cutting and foaming. Why does he show a 50mm ventilation cavity under the tiles, if you're presumably using a fully breathable membrane. I haven't installed a roof cavity for over 10 years, for new roofs, though sadly I do keep seeing it on drawings. 140mm (full fill) rafterroll 32 would give the same Uvalue as 90mm cellotex, but be much faster to fit and less likely to have big gaps around undercut boards (unless you foam all edges).

    Forget about insulating down the dwarf walls (joist hangers, call them what you will), take it down the "combe" nice Scottish word for you (pronounced "coom") follow the sloped ceiling right down to the wall head.

    The Dormer looks very mean on insulation too. Add a layer of cellotex on the outside too, then fix your tiling battens over that. There's so much timber in the structure of dormer that there's hardly any space between studs, except on drawings which always show only insulation.

    Please let us know your architect's reaction to all this... give us all a wee smile.
    • CommentAuthorDur
    • CommentTimeJan 3rd 2020
     
    Hi - not sure if it is done to add to an oldish thread but it is closely related so here goes...

    We were going to insulate to the knee / Ashlar walls then saw this and other threads so have started figuring how to insulate to the eaves.

    It should be just about possible albeit a nightmare to do. But assuming we can get it in there I now realise that we will need an internal layer on the dormer, either side of the windows and so this will have to carry down and beneath the window or will look odd. I guess it is also needed under the window because the rafter ends would not be covered otherwise.

    So, is there any sense in putting the insulation between the rafters down to the eaves and then putting the second "underneath" layer on the face of the knee wall instead? Or is that just the worst of both worlds?
    BCO has okayed 75 mm between the rafters and 60 mm underneath but I am swaying towards 75 mm underneath. BCO said just insulate the knee walls (actually he called them Ashlar walls) because it will be much easier.

    Hopefully photos will help...
      IMG_20200102_182806-S.jpg
    • CommentAuthorDur
    • CommentTimeJan 3rd 2020
     
    and...
      IMG_20200102_182910-S.jpg
    • CommentAuthorgyrogear
    • CommentTimeJan 4th 2020 edited
     
    Posted By: Dur(actually he called them Ashlar walls)


    Would appear to be a deformation of the French "esselier" or "aissellier":

    ashlar:
    n.
    1.
    a. A squared block of building stone.
    b. Masonry of such stones.
    2. A thin, dressed rectangle of stone for facing walls.
    [Middle English assheler, from Old French aisselier, "board", from aissele, from Medieval Latin axicellus, from Latin assis.]

    Per Dicobat, "aisselier" or "essellier" is a sloped brace between a tie-beam and a wall, transmitting the load dropped by principal rafter via a short stud (''jambette'') (which Interglot (curiously) calls a 'knee wall').

    Per Dictionary of Crafts & Trades, an "aiessllier" was a (mediaeval) splitter of shingles (bardeaux) or aisseaux (wooden roofing tiles). Traditionally, a French shingle was 61 cm long (= 24 inches).

    FWIW

    gg
  4.  
    Couple of questions Dur...

    has the roof been replaced, ie. with a breathable membrane, or is it an original roof with felt or a limited breathability membrane?

    What aspects of under-drawing the rafters down to the eaves are making it a nightmare (having to cut the sheets smaller to get between the joist hangers? (remove a couple, to get the boards in, then replace?)

    Cutting around the joist hangers? (just underdraw both sides of the joist hanger, then fit rectangles of the insul between the joist hangers, in the gap.)

    At the eaves, start to fill the little triangle first, so that say 200mm of the ceiling is insulated, and nicely foamed in to the corner with the wall head and the rafters/sarking. Then put the rafter insulation in, landing onto that initial piece. Much easier than trying to make the larger rafter insulation fit into that little triangle piece.

    Make sure you fill ALL edges of the insulation board with expanding foam, to give a uniform, no draft, layer, that won't fall out as the insulation boards shrink a little over the next 6/12 months.

    To hold the whole lot securely (at the eaves bit behing the joist hangers, fix two rows of 25x50 timber battens, like train tracks, horizontally over the surface of the under-drawn insulation, fixing through batten/insulation, into the rafters.

    Presumably you will be fitting battens over the surface of all the underdrawn insulation, for fixing the plaster board?

    Below the window, which ever way you fit the insulation (rafters or between joist hangers), you'll have an insulation weak point at the cill, where as you point out above, the sawn rafters will be solid timber in the insulation zone. If you want to avoid that, you will have to have a deeper cill, which you could take to floor level, just at the window.

    Having said all that, you could fill the ground floor ceiling area from wall head to joist hangers with 400mm wool, insulate between the joist hangers vertically, and then a continuous layer over the joist hangers, as per your orig design. If you've no services in the eaves area, and you ensure there are no week insulation points, and you take your time and make a careful job of it, I can't see that there's much difference between the two options.
    • CommentAuthorDur
    • CommentTimeJan 5th 2020
     
    Thanks for your comments GP.

    The roof has a a breathable membrane on the front of the house (which has been retiled) and no membrane on the back but both front and back have 18 mm sarking as you can see in the pics. We are putting 75 mm between the 100 mm rafters and 75 on the underside of the rafters (ie inside face). The plan is to put the plasterboard straight onto the second layer of PIR .

    We have been having a go at it today and it is perhaps not as tricky as I feared,
    We figured that we have to cut each piece in half so the bottom piece can go in first and then the top will go in.

    I was intending to foam all the joints (especially as it seems that every piece of timber is twisted/bowed etc etc).
    I'm a bit worried that the foam could block the 25 mm air gap.

    Sorting out the triangle on top of the wall first makes more sense than after - which is where I was going.
    I'm thinking to stick that piece onto the top of the wall with foam so try and make a proper connection so would appreciate thoughts on that...
  5.  
    I asked about the breathability of the roofing membrane (having seen the blue packing straps), as the highly breathable ones (and NO membrane would probably rate as highly breathable) do not require a vented cavity, as your sarking boards are self ventilating. Your BCO will likely refute that, but he would be incorrect in that belief.

    That means you do not HAVE to have the 25mm air gap insulation/sarking, or at least if you can't be bothered trying to change the setup (and BCO's mind) there's no need to worry about partial blocking of that.

    How were you planning on fixing plasterboard through the 75mm insulation? That's a lot of big nasty 120mm screws, which will be tricky to hit the rafter, and more likely to rip the plasterboard edges? I personally overlay battens, to give a fixing for the plasterboard (we call that brandering up here), and gives a much more successful fixing arrangement. You also can see the fixing timber, rather than trying to locate the rafters every single screw.

    Insulating at the wall head - you'll want a piece of insulation that's say 200mm deep, so that when the between and over-rafter insulation of 150mm comes down at 45 degrees, you've got at least that depth of insulation to land on.

    I think I'm seeing in your photo - a facia board, then a gap, then the outer wall head with a wall plate on top, which the rafter tails land on. Then the cavity with the inner wall head and wall plate, then the GF ceiling. Would that be right? I'm slightly perplexed by your architect's drwgs which show an insulated cavity. Are you stripping the inner wall, and installing cavity board insulation, then rebuilding a new inner blockwork skin?

    It's important were cavities and insulation in the walls are, so that you get continuity of insulation envelope up the roof line. If you could confirm the wall build up, that would be helpful. The eaves/wall head area is a really important area to get right, or you'll have a load of cold air washing away all you heat, and negate the good work of the rafter insulation.
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2020
     
    Posted By: GreenPaddyI asked about the breathability of the roofing membrane (having seen the blue packing straps), as the highly breathable ones (and NO membrane would probably rate as highly breathable) do not require a vented cavity, as your sarking boards are self ventilating. Your BCO will likely refute that, but he would be incorrect in that belief.

    Do you have a reference that could be quoted to help change the BCO's mind? I think you're right but all the evidence I've seen suggests that ventilation is required in England at least. Unless you have the supporting evidence of a proper condensation analysis to demonstrate the lack of risk.
  6.  
    This happens to be one product I use, as it's designed, (and I think manufactured??) very local to me.

    There is a stated vapour permeability for membranes below which they become "low resistance" (can't recall the number right now), and allows them to be used with the softwood sarking boards, as they are considered self ventilating (6" boards happen to be the standard roof covering in Scotland for weather conditions).

    I might be slightly more uneasy with OSB sheets as the sarking, but the attached docs I think state it's ok. Dur's photos showed sarking boards, hence my comment.

    I've been building roofs this way, with fully scrutinised building warrant approval for over 10 years, so it's accepted practice, but a as we know, house building is not know for doing anything other than what was always done.

    I also do my own cond calcs, which give the same verdict, but I'd trust BBA certs over my cond cals every time.

    www.proctorgroup.com/assets/Datasheets/Roofshield%20-%20LABC%20Fact%20Sheet.pdf

    www.proctorgroup.com/assets/PerformanceSpecs/Roofshield%20Performance%20Specification.pdf

    www.proctorgroup.com/assets/CAD%20Details/A%20Proctor%20Group%20-%20Roofshield%20Scottish%20Warm%20Roof%20Slates%20and%20Softwood%20sarking.pdf

    I've done a number of warm roofs over the years, a la FosterTom style (or similar), so not here to offer which is necessarily better, but if you have a new cold type roof, might as well get an extra 50mm of a cheaper, more env friendly insulation in the roof.
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2020 edited
     
    Posted By: GreenPaddyI might be slightly more uneasy with OSB sheets as the sarking, but the attached docs I think state it's ok. Dur's photos showed sarking boards, hence my comment.

    The certificate says:

    'Where plywood/OSB sarking is used on a “cold” roof, cross ventilation is required to meet the requirements of Approved Document C. Ventilation would not be required if using traditional square edge softwood sarking or the roof is constructed as a “warm” roof.'

    edit: PS the other drawing refers to itself as a 'Scottish warm roof' but has timber on the cold side so wouldn't count as a warm roof in most parts of the world!
    • CommentAuthorDur
    • CommentTimeJan 6th 2020
     
    We have a 9" solid wall with a 2 x 3" plate on the outer half so no cavity. The drawings earlier in the thread were the OP - picked up the same thread as similar subject...

    It looks like a 200 mm deep piece of 100 mm PIR with an angled notch where there is a rafter will work OK. It will sit a little on the plate and the whole of the inner half and extend a little back over the ceiling. I will have to knock the plaster off the laths to get it in level but I think that would be OK. It should be OK to get a good insulated joint. Not so sure about taping the joints but will look at that when it comes to it.

    I hear what you are saying about the 120 mm screws but if I add battens, that's another 25 mm into the room and it is not big to start with. I had been wondering if there is such thing as an insulated screw.

    I am slightly sympathetic to the BCO request for the 25 mm gap. It gives a sort of hybrid ventilation. We have seen some damp on the inside of the sarking after very heavy rain and wind before Christmas. The tiling is effectively new and my suspicion is on moss etc etc in the valleys either side of the dormer and the porch. I have been and cleaned so waiting for more heavy rain to see.
    • CommentAuthorJeff B
    • CommentTimeJan 7th 2020 edited
     
    Quote: "I had been wondering if there is such thing as an insulated screw".

    Dur - I had a similar debate when internally insulating the dwarf walls/skeilings in my dormer bungalow. In the end I simply took the pragmatic approach! How many screws are we talking about and what is the surface area of all those screws compared to the surface area of insulated wall - I'd venture to suggest it is a minute proportion. Purists will argue that whilst that is true, the thermal conductivity of the screws is many times that of the insulation and battens. However the screws will be plasterboarded over which gives some additional cover - I think you just have to use your common sense here!

    Edited: I should have explained that I did use 25mm battens over the Celotex and screwed through those into the rafters. The gaps between the battens were filled with 25mm Celotex and plasterboard put on top, screwed onto the battens with usual bugle headed 30mm plasterboard screws.
    • CommentAuthorDur
    • CommentTimeJan 7th 2020
     
    Thanks Jeff
    I guess the battens are at 40 cm centres with that method?

    So I guess...
    Pros - half the number of long screws which also = less chance of missing the wayward rafters (perhaps not a big deal)
    - thermally better

    Cons - more battens required (!!)
    - more difficult to vapour seal maybe
    - a little more expensive.. 50 mm + 25 mm is c. £3.50 a sheet more than 75 mm only

    I'll contemplate!
  7.  
    Dur, the battens would be run horiz at 600 centres, and vert at 1200 (that way your 400 centre rafters will coincide at 1200's.
    • CommentAuthorDur
    • CommentTimeJan 7th 2020
     
    That makes sense except that the rafter spacings are all over the shop but that is just one more little challenge!
    • CommentAuthorJeff B
    • CommentTimeJan 7th 2020
     
    Dur - I marked the ceiling and the floor in-line with where the centres of the wall/rooftimbers were (I had to drill a lot of pilot holes as the timbers were not always at the same precise centres as you have found and of course they were covered with plasterboard!) and then I was able to know where to screw the battens onto the Celotex. The Celotex (50mm) was held temporarily in place with a few blobs of adhesive and vertical lines marked with a pencil before manouvering/screwing the battens in place.

    The 25 x 50mm sawn timber battens were horizontal only, at about 40mm centres. I did not worry about an additional vapour seal as the Celotex is aluminium foil faced on both sides and I used foil backed plasterboard for the final finish, which in turn had several coats of vinyl emulsion on top, which itself provides an additional vapour barrier. Not forgetting of course that there was a second layer of Celotex (25mm) between the battens! These were a friction fit between the battens and I used 100mm wide self adhesive aluminium tape to cover the edges and the battens all in one go. Hence there was no need for vertical battens as the plasterboard sat firmly against a bed of Celotex!
    • CommentAuthorJeff B
    • CommentTimeJan 7th 2020
     
    All-in-all, yes, it was a time consuming job. Yes, it was more expensive using both 50mm and 25mm Celotex but it worked out well in the end. The dwarf walls were easy but attaching the quite large plasterboard sheets to the skeilings was a bit of a challenge trying to keep them in place whilst screwing them to the battens. I remember my good lady wife holding them steady with a broom (with the broom head wrapped in a towel to prevent marking the boards!). Plaster-boarding the dwarf walls first, meant that most of the weight of the skeiling boards was supported by them and prevented the latter from slipping.
   
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