Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    • CommentAuthorBruno
    • CommentTimeSep 14th 2014
    Hi all,

    I'm new here and, although this topic has been covered in previous threads, I can't find suggested eaves details online that fit my planned spec, and I wonder if anyone can point me in the right direction, because I don't want to reinvent the wheel if a consensus solution has already evolved.

    My setup is as follows:

    Detached house, with rooms in the roof
    Aircrete single-skin 215mm block walls, with 200mm EPS EWI
    Hollowcore concrete floors
    Airtightness target: <1
    Roof to be insulated over-rafters (to meet EWI), probably with an inter-rafter contribution too
    Proficient airborne soundproofing - since we're on a (minor) flightpath
    Roof U-value target: 0.11
    No requirement for a significant eaves overhang
    Roof finish will be clay tile
    Hipped roof, with a central table-top flatroof (with a good fall designed in and a top-quality membrane)
    [Other misc details, in case it helps: square footprint, bay windows to front (with hips over), two dormers, and several south-facing rooflights]

    To recap, I'd like to find any commendable eaves details that complement the above spec. (They must exist)!

    If I have any specific related questions, they would be:

    1) Should we cut the rafters short at the wall plate or carry them through, and accept the thermal bridge? (And does anyone have any quantitative suggestions as to what difference this 'actually' makes, thermally)?
    2) If we stop the rafters at the wall plate, do any roof boards exist that allow direct fitting of fascias and gutters? (I'm hesitant to pursue a solution involving cantilevered mini-rafters and such).
    3) Traditional rafters or engineered timber?

    All comments gratefully received!
    • CommentAuthortony
    • CommentTimeSep 15th 2014
    I would carry rafters through and over sail blockwork by 300mm to allow 100 soffit after your EWI.

    You could use 50x75 sprockets and fix tour fascia to that, I used cedar for fascias, doesn't need treating or painting and does not rot.

    Traditional, but you will need an engineers design for the roof, I would go hollow concrete floor again
    • CommentAuthorBruno
    • CommentTimeSep 15th 2014
    Thanks very much Tony

    Any recommended strategies to limit fascia depth (courtesy of the ERI)? Are tapered rafters something that have been (or could be) used successfully?
    • CommentAuthortony
    • CommentTimeSep 15th 2014
    Yes or better cut the underside off level to suit a 100 wide soffit with a 120 or 145mm fascia, join the wall and roof insulation seamlessly.
    If its a warm roof then you'll probably have counter battens between the over rafter insulation and the tile battens. These can be extended out over the EWI with an inverse taper (i.e. getting thicker towards the end) to form false rafter feet. This way the rafters can be cut flush at the wall plate, you won't need to cut the EWI around the rafters & the roof will appear very thin. It's also a lot easier to make windtight.

    You can fit a fascia & sloping soffit to the false rafter feet or form traditional open eaves with a section of sloping soffit/sarking board on top of the false rafter feet at the overhang. Ventilate the batten cavity with a continuous section of over-fascia vent either on top of the fascia or on top of the sloping soffit/sarking board where it will be hidden by the gutter.

    • CommentAuthorowlman
    • CommentTimeSep 16th 2014
    Posted By: Bruno,.................No requirement for a significant eaves overhang.

    What is your rationale for this? Is it simply you don't like it, or is it site constraints that prevent it.
    • CommentAuthorBruno
    • CommentTimeSep 16th 2014
    Thank you David. Great feedback. I like the inverse taper on the counter battens idea and am now running with that.

    I've made no decision yet on which insulation product/s to use. From what little information I've provided (airtightness target: 1, maybe even 0.6, MVHR, good soundproofing, U-value target: 0.11), is there anything that makes you think there is a 'standout candidate' for the roof insulation for this build? Let's start with the 'money no object' solution please (and I'll work swiftly backwards from there)!

    Hi Owlman, site constraints I think is the answer to your question. [Aesthetically, I'd plump for an overhang and exposed rafters. I included that comment to indicate that (a) I might be willing to forgo such an overhang if that meant being able to achieve a superior result, in terms of overall integrity and thermal performance, for example, and I guess I was also thinking: (b) even though the suggested spec might hint that we have Passivhaus pretensions, there are no vast swathes of south facing glass that might require the eaves to perform a significant summer shading function].
    • CommentAuthortony
    • CommentTimeSep 16th 2014 edited
    Money no object then go aerogel or vacuum insulated panels, I would use best bang for the buck, inorganic quilt, fg, eco or rockwool.
    With insulation you're normally paying for the thermal conductivity (lambda or k). In other words you pay more to achieve a given thermal resistance in a thinner overall construction depth or a higher thermal resistance in a given overall construction depth. So the key question is: how deep are your rafters?

    If you have 300-400mm deep I beam rafters then you have enough depth to use relatively cheap mineral wool.

    If you have more standard 220mm deep solid timber rafters then you could go for 220mm low cost mineral wool between the rafters and 200mm low cost EPS over the top.

    I haven't done the calculations, but if you have 150mm deep rafters then you might need to use graphite loaded (platinum) EPS, polyurethane or polyisocyanurate.

    If you end up needing phenolic or aerogel for anything other than localised cold bridges then you probably need to re-think your construction buildups!

    • CommentAuthoralistairm
    • CommentTimeSep 19th 2014
    We are currently building a new dwelling with EWI. We have no overhang with the attic trusses and have carried the insulation straight up outside of timber frame to meet roof. We have then fixed a wall plate along top of wall and fixed timber triangles to carry the sarking and slates. The external block wall will be built up and have a block on the flat to close the cavity which will also create the fascia for afixing the gutter. The block on flat will also support the overhang.
    I'm a bit confused. Could you draw a picture?

    I think you are saying that the attic trusses sit on a wall plate which sits on the top of the masonry wall. I assume the timber triangles are fixed to the outside face of the attic trusses to allow the sarking & slates to project over the EWI.

    So what does the block on flat sit on? Where is the cavity which it is closing? Does it project out over the EWI? Is it not a cold bridge from (warm) masonry to (cold) outside?

    • CommentAuthoralistairm
    • CommentTimeSep 19th 2014
    Its a timber frame house.

    140mm kit clad with 50mm insulation 50mm cavity then 100mm blockwork rendered.

    Block on flat closes the cavity.

    Nothing penetratres the EWI except fixings.

    Picture is during construction, roof to be loaded before external leaf is built.
    • CommentAuthortony
    • CommentTimeSep 19th 2014
    Not a good idea to close the cavity! introduces a thermal bridge.
Add your comments

    Username Password
  • Format comments as
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press