Home  5  Books  5  Magazines  5  News  5  GreenPro  5  HelpDesk  5  Your Cart  5  Register  5  Green Living Forum
Not signed in (Sign In)

Categories



green Building Press Book offers
Free UK delivery on all our books...

 Ecohouse 2 by Sue Roaf
Just £26.98

Ecohouse 2 by Sue Roaf


 Building With Cob
Just £25.00

building with cob


More great priced ecobuilding books here





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.




  1.  
    Does anyone have experience of using a cantilevered eaves board?

    To maximise airtightness & minimise thermal bypass, the main Masonite rafters & woodfibre sarking board will stop flush with the outer face of the inner leaf wall plate. A continuous sarking board stop batten will be fixed to the top of the rafters & a continuous perimeter board will be fixed to the outside of the stop batten, wall plate & rafter ends. A continuous eaves batten will be bolt fixed to the top of the outer leaf on a bed of flexible silicone sealant. 47x47mm eaves rafters will be fixed to the rear face of the eaves batten & through the perimeter board to the stop batten.

    An 18mm birch plywood eaves board will then be glue & screw fixed to the top of the stop batten, eaves rafters & eaves batten projecting unsupported around 200mm from the outer face of the outer leaf. The main load will be carried by the 50x38mm counter battens. So the cantilevered eaves board only needs to support itself, the lower edge of the first row of tiles & an arris rail to which the gutters & batten cavity vent strip will be fixed. Has this been done before? Any thoughts?

    David
    • CommentAuthorbeelbeebub
    • CommentTimeNov 24th 2009
     
    Hi david,

    Could you post a picture of what you describe. It seems a little like a scheme I've been mulling over for the eaves and verges of a bungalow.

    1. Tyvek membrane, held by counter battens (2)
    2. counter battens possibly 2 x 2
    3. Outer leaf "wall plate"
    4. Inner leaf wall plate

    The rafter (some 350mm deep Finnjoists or similar) rests on the inner leaf wall plate (4). The overhang is formed by the counter battens (2) which also hold down the Tyvek (1) and provide the vent/drain space under the tie battens. These counter battens pass over the cavity, rest on an angled timber "wall plate" on the outer leaf before cantilevering out past the outer leaf.

    The tyvek passes over the top of the cavity and is fixed in place with the counter battens being nailed into the outer "wall plate" and by the plastic "comb" (to stop insects etc getting under the tiles) between the counter battens.

    When I suggested it to the architect he was not keen, citing the lack of strength in the counter battens. Fair enough if they are 1" deep and 2" wide but 2"x2" should be stiff enough for 200-300mm overhang.

    As I said, this is only an idea at present.

    Apologies for the semi-hijack if this is not similar to the construction you've outlined.
    :bigsmile:

    B
      Eaves Detail.png
  2.  
    Hi B

    This is very similar to what I have in mind, except the plywood eaves board continues onto the overhang. This allows you to drain the Tyvek into the gutter, stop the counter battens at the first tile batten & use a batten cavity vent strip to support the base of the first tile, so there are no battens near the gutter "splash".

    I'm also using 350mm deep engineered timber rafters. I'm using a 60mm thick woodfibre sarking board to improve windtightness & provide a breathable warm roof. This stops above the inner leaf wall plate & is replaced by the 18mm plywood eaves board.

    I'll post pictures tomorrow evening when I can get to a scanner.

    David
    • CommentAuthorBenj
    • CommentTimeNov 24th 2009
     
    Hi David,

    Are you Constructing the roof on site? I'm trying to find a good supplier of Masonite type trussed rafters to reduce the labour component. Are you full filling with rockwool?

    Cheers,

    Ben.
  3.  
    Hi Ben

    Yes, I'm constructing the roof on site. The 2nd floor (Masonite beams & 22mm Weatherdek) will be built first & airtight membranes folded over the top before the rafters are fixed through the floor deck to the floor joists & blocking panels. The Masonite rafters & solid timber tie beams will be prepared off-site & the 2nd floor will form a working platform for putting together the A frames. They will be filled with Knauf Rafter Roll 32.

    I'll upload some diagrams later today.

    David
    • CommentAuthortony
    • CommentTimeNov 25th 2009
     
    How about cedar for the fascias rather than ply (which tends to delaminate)
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2009 edited
     
    Presumably you will have a structural ridge beam to stop the rafters pushing the walls out but.... As the load comes on the roof it may still settle a bit in the middle so perhaps best keep the bottom end of the rafters an inch or more from the outer leaf? Otherwise you might get horizontal cracks a few courses down from the wall plate. Possibly at the top row of wall ties?
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2009
     
    You could also consider fitting dummy rafter ends on the outside to make it look more traditional?
  4.  
    B

    Below is a picture of the cantilevered eaves board detail.

    There are more details in the specification attached to the following thread:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=4699

    David
      EavesDetail.jpg
  5.  
    Ben

    Below is an outline of the A frames formed of 350mm Masonite rafters & solid timber tie beams. Note that the 2nd floor deck & joists provide the main tie beam. The 2nd floor is assembled first to allow the airtight membrane to be wrapped over the joist ends. The rafters ends are cut to the vertical to maintain the full insulation depth to the junction with the cavity wall insulation & to allow a windtight perimeter board to be fitted.

    David
      RoofStructure2.jpg
    • CommentAuthorBenj
    • CommentTimeNov 25th 2009
     
    Thanks David,

    I had overlooked the airtightness detail on mine thinking that a full fill cavity would be enough to stop draughts. Back to the drawing board i think.
    FYI I think this eaves method is used in Poroton block construction, maybe Viking House has done it before?

    Cheers,

    Ben.
    • CommentAuthortony
    • CommentTimeNov 25th 2009
     
    David, your eves detail looks flimsy! also the underside of the bottom of the bottom tile will be too low causing kicking of the ones above.
    • CommentAuthorFloyd
    • CommentTimeNov 25th 2009
     
    David, looks flimsy to me too. Ply moves a lot unless it's thoroughly fixed. Could you put the 18mm ply (or another layer of) under your mini rafters, on top of the outer wall plate, and run the mini rafters to the eaves? You'd have to lower the outer wall plate. Alternatively - is there room for much bigger counter 'batten' (2-3 inch deep) and do away with the mini rafters?
  6.  
    Tony & Floyd

    I agree it looks a bit flimsy. However, the plywood is continuous from the sarking board stop batten over the outer leaf wall plate. It is screwed to the sarking board stop batten, the 47x47mm mini-rafters & the outer leaf wall plate, & 50x38mm counter battens are screwed through it to the mini-rafters. So I think it will be able to take a high load before starting to buckle. The only load it needs to take is the bottom row of tiles & the arris rail to which the gutters are fixed.

    I'm trying to avoid putting any structure in the cavity, while having windtight barriers at the inner & outer leaves. I could beef-up the mini-rafters and run them over the outer leaf, but then I'd need noggins between the mini-rafters. This leads to a lot of junctions all of which would need making windtight, whereas leaving the plywood to oversail the outer leaf wall plate gives one linear junction to seal.

    Perhaps I should screw fix false rafter feet to the underside of the plywood, fitting tight to the outer leaf wall plate? These could be 100x47mm or more.

    Tony

    It may not look like it from my drawing, but the arris rail & batten cavity vent strip are sized to close the batten cavity, while keeping the bottom row of tiles in the plane of the roof.

    David
    • CommentAuthorFloyd
    • CommentTimeNov 25th 2009
     
    David, 38x50 counter batten - I thought you meant 38x19. I daresay it would hold up but I'm not sure I'd want to bounce up and down on it much :) I'd worry about the bottom fixing from counter batten to ply failing. I know snow doesn't bounce much, but it gets quite heavy!
    • CommentAuthorbeelbeebub
    • CommentTimeNov 25th 2009
     
    David,

    I've had the "it looks flimsy" thrown at me too!

    Can I ask the purpose of the woodfibre sarking? Is it because you don't feel the tyvek is windproof enough? I had assumed that the tyvek would be enough to prevent thermal bypass of the wool. If not I'll have to have a rethink!

    I've attached a PDF of my roof buildup and the construction method. Hopefuly this makes it a bit clearer.

    One of the funnies about my roof is that there are no ceiling joists to take the side loads from the rafters bearing on the wall plate.

    I don't want to hijack the thread, so I could tart another thread but it seems that my construction is very similar to davids so it may be constructive to share thinking!

    (it's a pdf, but with a .rar, so download then replace the .rar with .pdf, then open in a pdf viewer)

    B
  7.  
    Hi B

    Wow, that's going to take some time to absorb! The box purlins seem to be resting on the first floor. Why couldn't you use the first floor to brace the walls?

    David
    • CommentAuthorbeelbeebub
    • CommentTimeNov 25th 2009
     
    David,

    That is an excellent question!

    The answer is that we have an unusual cross section. The ground floor rooms have a half flat bit at normal height before going up to the underside of the roof mid way. The section should clear things up.

    The ground floor rooms (in this case the east and west bedrooms) are in blue

    The upstairs 'office' space in a fetching shade of brown

    The two box purlins referred to in the pdf (pink) form part of the loft walls.

    They are supported by the end walls of the building (plus an intermediate wall).

    This arrangement give me:
    a) a greater feeling of height in the rooms
    b) a bit of "wow" factor (bit subjective this one)
    c) a slightly easier time sealing the usually tricky rafter/joist/wall area

    Unfortunately it does mean that there are no joists to take any out thrust loads directly.

    My plan involves a bit of stiffening of the wall plates to transfer the side loads to points that can take the out thrust loads (the aforementioned intermediate walls) and a roof structure that doesn't create out thrust. That's the last bit of the pdf, trying to get this across to builders/architects etc is rather difficult.

    :cry:

    Now i'm not sure about my chain of reasoning in the structure, so if anyone can spot a flaw in my reasoning shout!

    B
      Section.png
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2009 edited
     
    Posted By: beelbeebub Can I ask the purpose of the woodfibre sarking? Is it because you don't feel the tyvek is windproof enough? I had assumed that the tyvek would be enough to prevent thermal bypass of the wool. If not I'll have to have a rethink!


    I'm using Protect VP400 membrane (via Screwfix) on an outbuilding. By mouth I can just about suck air through it but I can't blow air through it - at least none that I can detect. I doubt it would allow any significant bypassing of the wool. I think it's also cheaper than Tyvek.
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2009
     
    Posted By: beelbeebub My plan involves a bit of stiffening of the wall plates to transfer the side loads to points that can take the out thrust loads


    Shouldn't need to do that if you have a structural ridge beam supported on gable or internal walls.
    • CommentAuthorbeelbeebub
    • CommentTimeNov 25th 2009 edited
     
    @Cwatters
    Thanks for the membrane tip, I'll look into it!

    The problem with a structural ridge beam is the span. it will span about 5 and 9 meters (5 from North Gable to intermediate wall, 9 from intermediate to south gable)

    As you can see from the section if we went for a deep ridge beam (e.g. 350mm) we would cut into the loft space and it would look odd.

    We realised that the loft walls could act as very deep beams and as we have deep rafters (the Finnjoist beams) we could support the rafters in the middle, leaving the ridge beam and wall plate to take out the resulting moment from the slight length imbalance. Rather like balancing a ruler with your finger halfway along.

    The purlin beams will be 1.2m deep by about 150mm wide. They will be made from timber flanges (3 off 2"x4" or 2"x6", bolted together with shear washers between). The web will be vertical lengths of 2"x4", again all bolted up with shear washers on, say, 400mm centers. The whole lot will be skinned on both sides with osb/ply, probably 10mm. The 1.2 depth is set by the widths of osb/ply sheets availble. This will create a very stiff structure (at least vertically).

    Hopefully my plan will mean that there is little or no out thrust. The stiffened wall plate is just a bit of string to add to the belt and the braces!
    • CommentAuthorCWatters
    • CommentTimeNov 25th 2009 edited
     
    Sounds like it should be ok.

    Got plans for a skylight? When we lived in Belgium we found a house which had a roof with strange shape skylight. The roof was a standard pitched roof but there was a skylight on both pitches that met at the ridge. I imagine it was a great room up there - sun all day long. Looked something like this. No ridge board or beam visible although there was a regular glazing bar along the top (eg it wasn't "frameless")...
      roof.jpg
  8.  
    B

    It's certainly an interesting design. Are you going to get a structural engineer to sign-off the drawings?

    It stikes me as odd that everyone agrees on the need to sheath a timber frame wall, but people rountinely question the benefit of a sarking board.:wink:

    To be honest, the woodfibre sarking board is not essential, but it is common practice in many countries & has a number of useful functions.

    1) A rigid surface which allows sarking membrane to be fully taped at all laps & prevents wind from working loose taped joints;
    2) A thermal bridge free insulating layer;
    3) A hygroscopic layer. Condensation can form under breathable sarking membranes under certain conditions, woodfibre can store this until conditions are right for drying & further aid rafter drying by keeping them slightly warmer than atmosphere;
    4) Secondary wind barrier;
    5) Tertiary rain barrier (secondary if used without sarking membrane);
    6) T&G boards avoid need for noggins bridging the main insulation.

    David
    • CommentAuthorbeelbeebub
    • CommentTimeNov 25th 2009
     
    @Cwatters;
    Wow, that Belgian roof sounds pretty spectacular! I've got 4 roof lights (Velux or similar) but nothing so dramatic. I think the local planners (and conservation officers) would do their nut! I'm in two overlapping conservation areas and an area of outstanding natural beauty so I'm really keeping my fingers crossed that I don't have to go with "consrvation" roof lights ( aka. black Velux's with a bar in the middle and a big price tag)

    @davidfreeborough:
    you raise some good points about sarking. I've not considered it simply because it's so uncommon around these parts. I believe in Scotland it's routine.

    If I may i have some observations/questions on the points you raised. I'm not criticising the sarking only trying to get a better grip on if it is something I should try.

    1) I'm thinking that my tyvek (aka sarking membrane) will be run from eave to eave up over the ridge (i.e. parallel to the rafters) rather than the more common horizontally (i.e. perpendicular to the rafters). Each lap between the runs of tyvek will be sealed by the counter batten that nails to the top of the rafter, forming a sandwich of batten, left tyvek, right tyvek, rafter. I had thought that this would keep the joints air and water tight, what do you think? I could tape the joints then nail them?

    2)It would act as a bridge free layer. In my design the rafters form thermal bridges through the wool. However the U value of 350mm of plywood is around 0.35 and the bridges are 10mm out of every 600mm. The U value of 350mm of wool is around 0.10 (using k=0.032 wool), so taking into account the bridges of the rafters I end up around the 0.14ish mark. Maybe I need to run the numbers for a thinner layer of wool, with a layer of wood fibre (k=0.04?) but with the slightly reduced thermal bridging.

    3)Would the wood fibre keep the upper surface of the rafters dry? I can understand that condensation may form in the outer (upper) parts of the wool. If it does and it is next to a rafter top the rafter top will get wet. Mind you the rafter top will be warmer than the wool as it is a better conductor. Wouldn't the wood fibre get wet? Is it better at absorbing the water and holding it until it can be evaporated than the rock wool? This is one aspect of wood fibre (and breathable/hydro construction) that I struggle with a little. My understanding is that the wool won't absorb water into the fibres, it will just hold it in droplets between the fibres. The water is then evaporated when the temperature rises and drifts out through the membrane. Obviously any water next to the rafter tops will wet and "suck in" to the wood. If there is a layer of wood fibre, does the water stay as liquid between the fibres? Does it get absorbed into the fibres? Does the liquid water get spread around the wood fibre? I'm not sure what's going on to be honest, only that various people tell me that it's either a good or a bad thing (the same thing!)

    4) If the tyvek works do I need a secondary wind barrier? To be honest I would feel a little better if there was a slight bit of low velocity air movement in the upper layers of the wool just to help move any water out.

    5) Can the wood fibre act as a rain barrier i.e. is it imperious to liquid water and happy to spend a lot of time damp? My understanding was that the primary barrier (tiles) was to keep 99% of the rain out. The only water that gets through is little splashes and dribbles of wind driven rain. The secondary barrier then allows this 1% to drain away. If my primary layer fails so that more than 1% gets through (say a cracked tile) then the secondary layer will keep me water tight but only until I repair the tile, it's not meant as a permanent solution. Ditto if my secondary layer fails, I need to get up there and repair it. If there is a semi water resistant tertiary layer is there not a risk that damage to the tiles or tyvek goes un-noticed for long enough for serious structural damage to occur? (As an aside, i rented a house where the loft water tank overflow emerged from the ceiling directly over the bath - the explanation given for this curious arrangement was a succession of previous tenants had failed to notice or report an overflow that ran down the outside wall until it had gone on long enough for serious damage to occur. During the refit the landlord decided to re-route the overflow to a place that couldn't be ignored!)

    6) Are you referring to the noggins needed between the rafters to prevent the twisting over? I've been pondering this exact problem. Are you saying that by having a ply layer under the rafters and a wood fibre layer linking the top of the rafters the need for shear noggins is removed?

    B
  9.  
    Hi B

    Good questions!

    1) In my opinion, running the Tyvek eaves to eaves & taping the joints is the only way to ensure its windtight without using a rigid board. However, this is less waterproof (due to lack of horizontal laps) and is difficult to install properly. The roofers normally start at the eaves and install board (if fitted), membrane (horizontally), counter battens & temporary tiling battens. This way they can move up the roof in stages & always have tile battens below to stand on.
    Normally, you can’t easily do this if you go eaves to eaves & want to tape the joints, but with your build up it should be possible. The only issue I see is that there’s a limit to how many tiling battens can terminate on any given rafter, so you’ll need to install a fair few rafters before installing the final tiling battens.
    2) If you’re using engineering timber rafters the advantages of the thermal bridge free sarking insulation are reduced, but it still allows a true warm roof where the rafters remain above atmospheric temperature.
    3) I’m not an expert on woodfibre, vapour permeability and hygroscopicity. As I understand it the woodfibre is capable of holding relatively large quantities of water until the conditions are right for releasing it. It has a greater volume, so the water content in the rafters is kept proportionately lower. It also has a greater surface area exposed to the batten cavity, so when conditions are right it can get rid of the moisture more quickly. You might be interesting in reading the following:

    http://www.naturalinsulation.co.uk/cms_items/20060607164406.pdf

    4) Mineral fibre insulation only works as advertised if kept in an airtight box. I live in a house with Tyvek sarking membrane & mineral wool between the rafters. There’s a gale going through the loft and I’ve been forced to insulate the ceiling below. Now maybe taping the laps & perimeter is enough to stop this, but I don’t trust the tape not to fail if the Tyvek starts flapping around.
    5) Neither do I don’t trust sarking membrane which is full of nail holes to keep the rafters dry, especially where the membrane is not draped between the rafters. The Isolair/Pavatherm+ boards are impervious to liquid water & will not rot if used with a ventilated batten cavity.

    http://www.natural-building.co.uk/pavatex_isolair_pitched_roof_system.htm

    I lived in a house in the south of France which didn’t have any sarking membrane. Large amounts of rain entered the roof space & lay on the (concrete) loft floor for days until it evaporated. Perhaps it was only 1% of the water hitting the roof, but it seemed like more when water starting coming out of light fittings! We hung tarpaulins to deflect the water away from the electrics.
    That said, I’m not convinced that you would spot a minor leak caused by failed nail hole in the sarking membrane on the ceiling below. It’s more likely that the rafters & insulation would just get wet (350mm of wool can hold a lot of water).

    http://download.proclima.com/en/ie/study.pdf

    6) I’m hoping that the combination of the following will avoid the need for noggins:

    To rafter upper flange: 60mm sarking board, sarking board stop batten, truss clips, perimeter board, eaves board, eaves batten, ridge batten, tile battens & gable ladder;
    To rafter lower flange: OSB lining, Fermacell lining, bevelled bearing plate, truss clips, lateral restraint straps (to noggins), 2nd floor, loft floor & loft insulation support battens.

    My latest thinking on the eaves is to combine CWatters idea with the cantilevered eaves board. So keeping the same structure add chunky false rafter feet to the outside of the outer leaf wall plate & use the cantilevered eaves board to tie it all together. This certainly shouldn’t be flimsy!

    David
    • CommentAuthordjh
    • CommentTimeNov 26th 2009
     
    "(it's a pdf, but with a .rar, so download then replace the .rar with .pdf, then open in a pdf viewer)"

    The third page crashes acroread
    • CommentAuthorbeelbeebub
    • CommentTimeNov 28th 2009
     
    @ djh: :sad:sorry about that, it's exported from Power Point 2007. If it's any help my "Foxit" reader (http://www.foxitsoftware.com/pdf/reader/reader-interstitial.html) works.

    @davidfreeborough:

    Thanks for the answers, you've given me a lot to chew over! Out of interest do you know what the price comparison of the woodfibre sarking is vs Tyvek? I'm going to try and find out but any guidance would be really helpful.

    1) I agree that wind tight is only possible with eaves to eaves Tyvek. I think that the joints should be water tight as the battens would clamp the two layers of tyvek together. I suppose some double sided tyvek tape at the joint couldn't harm.

    My build sequence is specifically designed to make installation as easy as possible on the premise that if the access is easy then it is easier to ensure a good job is done. It also allows for close inspection of everything before it's enclosed.

    However I would also agree that the WF board should be better at water proofing with the T&G joints. One question though, does the presence of a number of joints not increase the air penetration? I notice that they recommend dry jointing above 18deg pitch, are the joints really tight or would it be better to add a smear of glue anyway?

    2)good point

    3)I'm still trying to get to grips with the water movement in buildings. I understand the need for allowing water to leave a structure. It seems to me (at the moment) that a lot of worry is directed at the movement of water through a structure (people saying that if this doesn't happen the house will become really unhealthy) when the bulk of water/VOC etc transfer happens via air movement in and out of the building. my parents have a house that was not built with any particular attention breathability (thermal effectiveness was the primary goal). The RH hovers around 40-50% throughout the building, pretty much throughout the year (dipping in v.cold dry spells). There is no buildup of smells or dust even in the little used rooms and despite my mum's dislike of using the cooker hood on anything other than super low. I should point out they have MHRV installed. If I was building an air tight house with no MHRV I would not be surprised if it got smelly and damp.

    4) Do you think the wind is getting through the Tyvek it self or through the gaps. I do agree that tape on it's own is likely not to last especially if the Tyvek starts flapping. My current (probably to be updated) design has every edge of the tyvek mechanically clamped so I'm reasonalby confident of sealing the edges but would Tyvek let air through the membrane? As a side not my parents Tyvek roof tends to crackle and pop in light winds, it's like being on a sailing boat! There's also a certain direction and speed of wind that occasionally causes a "farting" sound to come from one section of the roof :shamed:

    5)We have a wood shed that's covered with interlocking tiles and has a low pitch (about 15 degrees). Even in bad rain and wind there doesn't appear to be any drips coming though. However I agree that some water will get through at some point. if it's the torrent you describe in france I would think there is something wrong with the roof! You're right about probably not noticing a failed nail hole. I would hope that whatever system I use (WF or Tyvek) could cope with a single failed nail hole


    The info you've provided is really useful: Thanks!:bigsmile::bigsmile:
    • CommentAuthorCWatters
    • CommentTimeNov 28th 2009
     
    Posted By: beelbeebub
    3)I'm still trying to get to grips with the water movement in buildings. I understand the need for allowing water to leave a structure. It seems to me (at the moment) that a lot of worry is directed at the movement of water through a structure (people saying that if this doesn't happen the house will become really unhealthy) when the bulk of water/VOC etc transfer happens via air movement in and out of the building.


    Personally I feel ventilation is essential but that the breathability of the structure itself is much less important. I've nothing particular against breathable design but it has to be done right. My concern is that it has the potential to become "fashionable" and for people to insist their builder omits vapour barriers on structures that are not designed to be breathable causing a risk of interstitial condensation.
  10.  
    Posted By: CWattersPersonally I feel ventilation is essential but that the breathability of the structure itself is much less important.

    I agree, I think the benefits of breathability are often overstated. For me the benefit is that it allows the building to be tolerant of water vapour which finds its way into the structure, whether from the outside or the inside. I'll still be using a vapour barrier on the inside because I believe its the only robust way to make it airtight.

    Below is a close-up of the latest eaves overhang detail. The false rafter feet have been replaced by tilt fillet blocks between the eaves board & the tilt board.

    David
      EavesOverhangDetail.jpg
  11.  
    Posted By: beelbeebub1) One question though, does the presence of a number of joints not increase the air penetration? I notice that they recommend dry jointing above 18deg pitch, are the joints really tight or would it be better to add a smear of glue anyway?
    3)I understand the need for allowing water to leave a structure. It seems to me (at the moment) that a lot of worry is directed at the movement of water through a structure (people saying that if this doesn't happen the house will become really unhealthy) when the bulk of water/VOC etc transfer happens via air movement in and out of the building.
    4) Do you think the wind is getting through the Tyvek it self or through the gaps.


    1) The joints are quite tight. However, as you might have guessed by now, I'll be going for the belt & braces approach of woodfibre sarking board & Tyvek (or similar). The Tyvek will be taped to give the windtightness. The woodfibre should stop it flapping around or making farting noises.
    3) For me breathability is just about making a robust design which will stand-up to the unexpected. You still need to make it airtight & ventilate it. The cheapest air barrier is 0.2-0.3mm polyethylene and this also acts as a pretty good vapour control layer.
    4) I think the wind is getting through the laps & around the eaves & gable perimeters.

    David
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press