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Fundamentals: Thin timber frame wall for mobile home

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  1.  
    • CommentAuthorstephendv
    • CommentTimeJun 28th 2010
     
    I'm planning a temporary timber frame mobile home for a plot in spain and need some help with the wall build up. Structure will be a 4m x 12m box exterior dimensions. The priorities for the design are: low cost, must be DIYable and thermal efficiency, in that order. After reading lots of past posts on this forum, I've come up with the following wall buildup, internal to external:
    1. EPS backed plasterboard (EPS because it's easy to cut out service channels) 90mm total, 70mm EPS
    2. OSB taped for airtightness
    3. 89mm stud filled with rockwool
    4. Breather membrane, preferably metalic to keep the house cooler in summer
    5. 25mm diagonal battons for ventilation gap
    6. Vertical wood cladding (thermowood)

    Gives total u-value of 0.26.

    Reason I put the EPS on the inside is to leave a vapour path open on the outside - is this correct? Is it OK to have the stud exposed to the air cavity?
    The majority of kitchen appliances will be on an inside stud so the only real "services" in the whole house will be a few wall plugs, lights and light switches. Should be doable by cutting into the EPS and using plastic channelling - is PUR also easily cut this way?

    Is there a better way to do this?
    • CommentAuthorTuna
    • CommentTimeJun 28th 2010
     
    Sounds very well specc'd for a temporary home. Is that lot really going to be cheaper than bunging down a 'shed' out of concrete blocks, or even just bringing a static home down there? How long is temporary and what are the seasonal extremes that you're going to be coping with?
    • CommentAuthorTimber
    • CommentTimeJun 28th 2010
     
    The wall make up seams fine to me!

    It is fine to have the studs 'exposed' in this way, no problems there as long as a breather membrane is used (which you are!), but you may find that the rockwool insulation between the studs pushes the breather membrane out. This should be reduced by the diagonal battens but might be worth thinking about.

    At what stage will the services be installed? I can just imagine it being a pig having to figure out where the services are then chanel them into the back face of the eps.

    Is it easier (and similar price) to use loose eps? it could be tacked into place, then services cut into face of eps and installed, then plasterboard over the top afterwards. Or using some 25 mm battens inside to hold the EPS and create a service void? If 25 mm battens to deep or costly, you could look at ripping strips of OSB into 50 mm wide 'strapping' and using that.

    Timber
    • CommentAuthorstephendv
    • CommentTimeJun 28th 2010
     
    The structure has to be mobile to avoid planning permission, the idea is to live there temporarily while: the main house is being built/the pound recovers/the Spanish property market stabilizes. "Temporary" could mean 2-5 years, after which we could still use it for guests or sell it on. Temperature varies between -7 in winter to 36ish in summer. Commercially available static homes are hideous
    :bigsmile:

    Checking online prices, OSB with backing is a bit more expensive than buying the two separately, but I thought installation would be faster to buy the bonded pieces. I imagined that I could glue the electrical channels onto the OSB then cut out the foam when installing the plasterboard+foam panels. ...but having never installed any of this stuff, I guess it could get tricky. If it doesn't add too much to the build time/skills required, then separate EPS, with channels cut into it and plasterboard directly on top seems to make more sense. thanks.
    • CommentAuthorSaint
    • CommentTimeJun 28th 2010
     
    Being a bit of a sandwich panel aficianado personally I'd have to ask why not go SIPs? Create a service void on the internal face and finish externally as in your suggested build up. I'd go for OSB skins with XPS as the core. Two Spanish XPS producers, URSA and TOPOX still have a product with a lambda of 0.029W/mK. They also make thick XPS boards, URSA more than TOPOX. Anything greater than 120mm would give you a better thermal performance than your spec, thinner, simpler to build and you should be able to source XPS at a good rate at the moment.
    • CommentAuthorstephendv
    • CommentTimeJun 28th 2010
     
    Can't easily install SIPs myself, will need crane + many more hands, and XPS would be more expensive than my wall build up for the same U value, wouldn't it?
    Thanks for those names, will definitely be using them for the main house build. But they don't have ready built SIP products, are you suggesting that I bond the OSB to both sides of the XPS myself?
    • CommentAuthorSaint
    • CommentTimeJun 28th 2010
     
    Yes you'd need OSB both sides to give you the structural strength. You could bond up your own panels, both URSA and TOPOX make 1200mm wide boards. Guess you'd be paying 15 €uros/m2 for 120mm thick XPS ? Hopefully less
  8.  
    Hi Stephen

    I'd use the 89mm stud with OSB on the outside and fix 100mm of EPS externally onto the OSB and then the Thermawood on the outside. The 89mm stud could double as a services cavity so you just need to plasterboard the inside.
    • CommentAuthorstephendv
    • CommentTimeJun 28th 2010 edited
     
    The house will need to rest on a rectangular frame (thinking either laminated beams or metal) that should be crane friendly, so the outside of the wall can't stick out too much from this frame. If I put the EPS on the exterior of the stud it would mean that the stud would have to be 100mm on the inside of the frame which I think is a structural problem.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 29th 2010
     
    I'd make up a thin stressed-skin box of ply or maybe OSB glued and screwed with only small glueing fillets, no stud frame. Then the insulation cd be an unbroken 'tea cosy' inboard or outboard of that, not heavily degraded by interruption by studs.
    • CommentAuthorstephendv
    • CommentTimeJun 29th 2010
     
    Fostertom, your's and Saint's ideas are similar - the problem for me is that without studs the frame has to be stronger and will probably have to be designed by an architect or at least someone who understands loads on wood. Also the joints between insulated panels and framing pillars could be more difficult to execute. Are there any references or books with more information on building with a stronger frame and insulated panels?

    The net is full of information on american style stick frame building and it all looks very DIYable for a person with my skills. I definitely like the idea of larger insulated panels or boxes with fewer thermal bridges - but seems more complex.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 29th 2010
     
    Stressed skin boxes are incredibly strong even when flimsy, but only material cost wd prevent you using thick ply, if in doubt. Boatbuilding sources of info? Really no need for 'frame' - all stresses in the ply. On caravan-scale, prevention of panel bulging wd be the challenge. That's where double-thin-skin SIPS wd come in - which cd be created in situ.
    • CommentAuthorTimber
    • CommentTimeJun 29th 2010
     
    I would suggest sticking to your origional plan. I see no issue with it!

    Plus, as you say, stick framing it will be a doddle if you knock up some plans and a cutting list in advance!

    A SIP type system is not (in my opinion) not an easy DIYable task. A standard timber frame system is a piece of cake. I would suggest designing and then building the whole thing as a flat pack kit then standing the walls up, rather than trying to frame the thing upright!

    Stick some ideas down on paper and post them up. I would be more than happy to give some more specific advise. Specifically wall to foundation/ground floor interface, wall/roof etc.

    Timber
    • CommentAuthorCWatters
    • CommentTimeJun 29th 2010
     
    Would it be easier to buy a pair of second hand portacabins and beef up the insulation on the inside?
    •  
      CommentAuthorfostertom
    • CommentTimeJun 29th 2010
     
    Here's something similar, effectively oval in shape, which allows v thin wall ply.

    SHOWER ROOM 'POD': Shower Room 'pod' is a stressed-skin unit, incl. the corner access extension, either
    prefab'd on or off site and craned in, or
    built in situ on a scaffold platform and levered into final position.
    Top and bottom shapes of 25 ply - may be cut from 3x5m sheets with plenty of offcut to make possible curved sole and head plates.
    70x38 studs fixed between.
    6 ply bent around their outside.
    Additional 95x50 floor and 70x38 ceiling joists.
    9 OSB to top and underside, so the flat top and bottom are effectively double skin.
    All glued and screwed airtight.
    P'b'd lining, insulation, heat-collect battening, cladding, EWI render to soffit to soffit and one curved side, see elsewhere.
    Supported on the timber beams underneath on spacer blocks.

    The stud wall and joist floor/ceiling voids filled with blown Warmcel, for hygroscopic benefit to the interior (via breatheable p'b'd lining); 60 PIR/PUR insulation unbroken 'tea-cosy' insulation outboard to top bottom and sides, underlayed with vap barrier, or else foil faced.
    • CommentAuthorstephendv
    • CommentTimeJun 30th 2010
     
    Thanks for the suggestions, besides the construction details for the pod, it might also neatly solve my plumbing problems :)
    CWatter, yep, that's something I'll consider if I can find the right sized portacabins in this area. I considered using 2 shipping containers for this project, but after factoring insulation and all the work to cut out the windows and doors I think it'll be cheaper and I'll end up with a more comfortable end product by doing the frame from scratch in wood.

    Timber, thanks for the offer, I _will_ take you up on it!
    :bigsmile:
  17.  
    I have a similar problem up here in the highlands. My solution is to use 4x2 uprights on the inside of the mobile, a second 4x2 on the outside and bolt both together through the wall. This increases structural integrity and prevent the walls flexing during a blow. The roof gutter overhangs about 40mm so another 4x2 is fixed to the outside upright and this goes higher than the roofline. I use 6x2 timber to run along these new uprights. There is now a solid frame running along both sides of the building. To achieve a slope one side is higher than the other.

    The trusses are 6x2 timbers laid across the supports at around 600mm centres where possible and will end up being clad with osb. The walls are externally insulated with 100mm polystyrene (limited funds) fitted between the uprights and held in place by more osb sheet. The roof will eventually be covered with epdm or profile sheet seconds.

    All of the major construction is fastened using 10mm coach bolts & coach screws and I could if nescessary strip the whole lot off in a week and tow away the mobile so I am not breaking any rules on that side. I have lived in this for 2 years and although not complete it has made a substantial difference.
    • CommentAuthorstephendv
    • CommentTimeJul 2nd 2010
     
    What are you using for the base/chasis? I'm considering a frame forming a rectangle of 12m x 4m using engineered laminated beams (no idea of thickness required, 20cm?). With 4m trusses across at 60cm centres.
    Alternatively a steel welded frame like in the first photo here:
    http://www.casasnatura.com/esptecnicasblu.html
    • CommentAuthorTimber
    • CommentTimeJul 2nd 2010
     
    Have a search for New Zealand House Trucks!

    Timber
    • CommentAuthorThe Crofter
    • CommentTimeJul 2nd 2010 edited
     
    Unfortunately none of this work was planned so I do not have a nice flat concrete pad to work from. The external uprights go straight onto the surface which is typical sutherland gravel after the area was cleared. I am relying on the skeleton to transfer loads straight down and not stress the mobile. My trusses are at 400mm (I am literally doing this work at the moment so just changed the truss pitch) centres which means I can use 12mm osb for the roof and then profile sheet.

    When everything is finished I will fit an osb "skirt" round the base and this will be covered in gravel from the excavation. This effectively anchors the whole structure so I am confident it will put up with 100mph winds.The whole site drains well so all this should last a long time.

    One of the reasons for doing this is to stop the roof from flexing in a blow. The wind lifts the alloy skin and it ripples down the length of the mobile. Scary to watch!. A feature of the new roof will be a set of baffles to disrupt the wind flow and prevent lifting.

    Incidently I am also using the frame to cover the existing windows except for two fire exits. New "windows" will be made using polycarbonate sheet which is incredibly durable even when flexing. Sort of double glazing !.
  21.  
    <blockquote><cite>Posted By: The Crofter</cite>
    One of the reasons for doing this is to stop the roof from flexing in a blow. The wind lifts the alloy skin and it ripples down the length of the mobile. Scary to watch!. A feature of the new roof will be a set of baffles to disrupt the wind flow and prevent lifting.
    </blockquote>

    Hi Crofter, I think I'm going to have to go down the same route as you - it's the middle of July and I am watching and listening my roof rippling already in light gales and I don't like it. I'm on the Western Isles right on the west coast so full force of the Atlantic and so far only a earth berm to shield the caravan.

    Would it be possible for you to take some pictures showing exactly what your setup looks like? Any tips very much appreciated!

    Thanks, Willie
    • CommentAuthorplanthead
    • CommentTimeJul 19th 2010
     
    Hi i have just purchased a static caravan and am on a residential council site.
    I want to do similar on this to improve the energy efficency .I am thinking of using Eps on the exterior exsisting walls from QuadLock as have friends who have used the company and method for building homes.
    As i am a single woman i am not sure how to re roof it (i will obviously get my son who is a joiner in to do the trusses)but we dont know who to conntact regarding the making up of them and wether we can come directly off the exsisting roof with a lightweight system.
    I also want to incorporate solar tiles and 3 solar tunnels (for natural light)
    does anyone know how to work out the weight and wether i will need supporting corner battons like you are doing?

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