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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.
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