Timber frame cabin

[stephendv]

After much reconsidering of our plans my wife and I have decided to build a timber frame cabin on our land in Spain, as a stepping stone to a brick and mortar house further down the line.  The design will be simple 12m x 4m and it should be moveable by truck or crane if necessary.  We've started the build and have the base section built from glulam, and am now considering wall design.
It basically comes down to 2 options, both of them using OSB as a vapour barrier with silicon sealed joints and a mixture of EPS and rockwool - I'm just not sure where to put the OSB and EPS...

Option 1 from inside to out:
Plasterboard directly onto 100mm studs - studs with 100mm rockwool - 18mm OSB taped to form vapour barrier - 40mm EPS on outside of OSB - 20mm battons - timber cladding.
Since EPS is on the outside, I don't think an external vapour is needed?  Also the studs will be on the warm side so potentially less thermal bridging.

Option 2 inside to out:
Plasterboard onto 40mm EPS - OSB taped to form vapour barrier - 100mm studs with rockwool fill - Tyvek vapour barrier - battons - cladding.


Thoughts?

[Baz]

The vapour barrier is to stop your sweat from condensing inside the wall. So the vapour barrier should be under the plasterboard inside to stop it right there. Moisture from outside is supposed to evaporate again when the sun comes out so ventilate behind the outside cladding.

[stephendv]

Yeah, but as far as I'm aware, having the vapour barrier further inside the wall structure isn't an issue, as long as the temperature is above dew point.  I.e. if it's warm enough, vapour won't condense so no issue.  Since option 1 has the vapour barrier after 100mm of rockwool, but before 40mm EPS I would think that it's warm enough?

[djh]

The vapour barrier is to stop your sweat from condensing inside the wall. So the vapour barrier should be under the plasterboard inside to stop it right there. Moisture from outside is supposed to evaporate again when the sun comes out so ventilate behind the outside cladding.

Vapour barrier inside is a good design for cold climates. In hot, humid climates it's a recipe for disaster. I don't know what climate Stephen is in but it might be worth checking designs from the American south or parts of Australia. Or of course local designs where he is.

http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810

[stephendv]

Interesting article.  Climate here is mostly hot and dry, but cold and dry in winter.

[EccentricAnomaly]

silicone.  battens.

Option 1 from inside to out:
...
Since EPS is on the outside, I don't think an external vapour is needed? 

As I understand it an external vapour barrier is only worth considering if the house is expected to be cooler than the outside for significant lengths of time - e.g., the US south east with air conditioning.

The normal barrier used on the outside of the insulation (e.g., Tyvek) is deliberately not a vapour barrier - it's designed to be more vapour permeable than the insulation behind it.   Its purpose is to stop the wind blowing into the insulation (e.g., in the gaps of the EPS).  I think you need to think about airtightness at that point.  Would taping the joins of the EPS be sensible?

[stephendv]

The normal barrier used on the outside of the insulation (e.g., Tyvek) is deliberately not a vapour barrier - it's designed to be more vapour permeable than the insulation behind it.   Its purpose is to stop the wind blowing into the insulation (e.g., in the gaps of the EPS).  I think you need to think about airtightness at that point.  Would taping the joins of the EPS be sensible?

Yep, I was referring to the Tyvek type stuff not a proper vapour barrier.  Not sure whether tapes are designed to adhere to EPS...  Maybe I should just go for the Tyvek anyway and stop trying to be a tight ar$e.  I've seen a silvered version which would be an extra radiant barrier against summer sun hitting the cladding.

[Justme]

We did some inter doodar calcs & in the walls & ceilings we dont need a vapour barrier (but are fitting one in the wet areas) but we do for the floor as its suspended over soil with an enclosed but vented space so it will have a build up of moisture over the wet seasons that cant be countered during the dry ones. We are using the silvered Tyvek on the walls but went for the thicker heavier Ruberoid stuff for the floor & roof pitch. The Tyvek tape will stick to virtually every thing like sh!t to a blanket.

[dhaslam]

Nice project.   Will  you  have an overhang on the south facing side or a balcony?      A well insulated  house could be very hot if windows are not protected from direct sun.    East and west windows should be small.  

[stephendv]

Won't have a roof overhang, but will construct a shade overhang over the entire south side.  All windows in the south (it's where the best views are  ) 

[billi]

   I can visualise already  , nice long evenings on the veranda with a Rioja 

Woodfirbeboard  for the insulation/heatprotect  are an option i think  ,  and not so big issues about moisture  breathing etc involved , cause it can regulate moisture better


like gutex as an example http://en.gutex.de/en/index_why.html


Billi 

[stephendv]

Justme, interesting that you don't need a VCL, what's your wall build up?  Billi, the wood fibreboards look very good, especially their added thermal mass which I won't get with EPS... plus EPS is vapour impermeable.  Would it be possible to just substitute fibreboards for EPS in my 2 wall designs? 

[billi]

stephendv

I know you  are good  in German    so here is their timber frame wall idea with timber cladding  or plastered exterior walls   http://www.gutex.de/de/Planen/Uebersicht_Konstruktionen/Aussenwand/more/Konstruktionen_fuer_die_Aussenwand/Holzrahmenbau

yes similar idea like your version 2   ,

they suggest  "GUTEX Multitherm" board on the outside of the Insulation board and then counter battens and cladding


Cannot say how that all compare price wise    , but i can help  shopping around

Billi

[Justme]

Justme, interesting that you don't need a VCL, what's your wall build up? 

As we dint need PP or Building regs we kept it simple.

Inside to out

Walls
15mm cedar T&G
100mm rockwool
Tyvek framewrap
25mm batten on stud
25mm counter batten
wainey edge 20mm thick cedar cladding

roof
15mm cedar T&G
300mm warmcel (over 100mm joists)
cold roof
100mm rafters
Ruberoid
25mm batten on rafter
25mm counter batten
cedar shingles

floor
24mm cedar T&G
VCL
30mm warmcel
170mm rockwool
Ruberoid
20mm rough sawn cedar planks
12"  to 36" void over soil
Void will be enclosed with loose fit cladding to vent