Where to put breathable membrane on a small timber frame building. Warm side or cold side?

Hi,

I am planning on building a small timber frame cabin , about 4.5m x 5.5m (15ft x 18ft).  It will be place on top of pre existing stone walls 1m tall, each wall 0.7m wide.  The walls will be lime pointed and the wooden cabin will attach to railway sleepers resting by weight on the stonewall.

So the wooden walls will only  be one 1m tall, and will not be connected to the floor.  I will be using similar framing of the top half of the attached drawing, except thinking of not having roofing boards on top of rafters.  Internal paneling will be reclaimed pallet boards, external cladding/siding will be similar, but cut to have a slight overlap.  Some sort of linseed paint or finish inside and out.  The plan is to have 100mm (4 inches) of rockwool insulation in the walls and between the rafters of the roof.  There are about 17m2 of wall and 40m2 of roof.
Heating by wood stove or solarization from windows.

The purpose of the cabin is to live there while we restore the large stone house beside it, and then to use it in the longer term for visitors.  So I want it to not get damp problems in a few years.
We live in Ireland, it is often rainy and cold, rarely freezes or snows.  For our american readers, it is like the coast in washington state.

My questions..............................................

The internet has a horrendous diversity of ideas on what to do in regard to what membrane to use and where to put it.  Could someone give me climate specific advice on this?  

My thinking at the moment is to have breathable membrane in the walls and in the roof, sealed with tape.

Roof,...from outside in......corrugated iron, on battens.  Between battens and rafters will be membrane.  Below membrane, rock wool, in between rafters.  Below rafters, pallet board paneling.  

Walls,.......from outside in.....pallet board cladding/siding....rockwool between studs....membrane.....pallet board panelling

I know this is membrane on cold side in the roof and membrane on warm side in walls.  Is this a daft or reasonable plan?

Any suggestions on membrane placement and/or specific membrane choice would be greatly appreciated.

Thanks

Liam

Someone more expert will probably chime in, but the basic premise here is reasonably straightforward. My climate is like Washington state, just colder..

There are several jobs that need to be done. The moist, warm air needs to be kept from contacting the cold parts. The vapour needs to be kept from contacting the cold parts; it can pass through some air barriers, though in practice I understand air leaks to be a much bigger issue than vapour drive.

And then rain needs to be kept off, and the insulation protected from wind.

I am assuming you want your taped breathable membrane to function as both air barrier and vapour retarder; no problem with that but it does need to be on the warm side of the roof as well as the walls to do these jobs.

The problem is there are some jobs left undone. Now you have nothing under the roofing as a backup bulk water barrier, and nothing under your siding to stop whatever bulk water makes it past the siding, and prevent wind-washing of your insulation. So, time for more layers...


There are oodles of different options for products and I suspect you'll have good choices available locally, they just won't be the same as are available here. You've got the key thing down in focusing on 'breathable'; specs for just how breathable should be available for each product.

There is obviously room to get fancier with layers but it all adds cost and time..

Moisture moves from the warm side to the cold side in a home.  From inside towards the outside.  Vapor barriers need to be placed on the outside of the wall. Typical American house construction goes like this:  Interior wallboard, insulated studwall, exterior sheathing, vapor wrap, siding.  There are variations on this depending on particular climate but this is typical for most of the country.  You are wanting that water vapor to escape and not become trapped inside the wall.  The water vapor barriers (TYVEK is one name brand) most use allow water vapor to migrate out, but keep liquid water from entering into the wall from the outside.

Walt, if I'm understanding your post I have to disagree.

Per foursevenfive: 'The theory is quite simple - in a heating dominated climate, for the most robust assembly - you want a vapor retarding airtight layer on the interior of your insulation and a windtight/airtight and vapor open exterior.'

The quote is from this page, which is describing a much more complicated and expensive wall assembly, but the basic principles still apply.

https://foursevenfive.ca/blog/vapor-open-exterior-without-sheathing-maximizes-outward-drying-potential/

More wall assembly diagrams here: https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

The tyvek you mention in your wall assembly is not serving as a vapour barrier in the sense that term is generally used. As you say, you want moisture to dry to the outside, so you want a waterproof, airtight, but vapour  *open* layer here. Usually called a weather resistant barrier(WRB). Tyvek is pretty vapour open, I seem to recall 58 perms.

The vapour barrier is to keep vapour from entering the wall from the inside. If it gets past this layer, the last thing you want to do is unnecessarily impede it from getting back out!

In your described assembly, the actual air/vapour barrier is either omitted, or the 'interior wallboard' is an appropriate substance, suitably detailed to be airtight and vapour retarding. (For example, I've used plywood with Vana Tescon tape to meet these criteria)

Hope that's reasonably clear.

Hi Dillon,

Am I reading your first post correctly as suggesting that I should have two breathable membrane layers in both the walls and the roof?

This wouldn't be a huge problem, as enough membrane (tyvek equivalent) for the whole cabin is cheap enough here, just 70 euro.  Lots of other parts of the budget are much bigger, eg 450 for the corrugated iron, 450 for insulation.  

Does the quote from foursevenfive suggest that the outer layer should be much more vapour permeable than the inner layer?

The reason I am asking for advice on the net is that wooden houses are very rare here and even if I had the money to hire a carpenter, he/she would probably never have built a timber frame and timber clad house.

Thanks

LL

liam loftus wrote:Hi Dillon,

Am I reading your first post correctly as suggesting that I should have two breathable membrane layers in both the walls and the roof?

Yes. Specifically an air and vapour barrier on the inside and a weather resistant barrier under the siding. As you can see from the foursevenfive wall example the inner layer especially can be taped plywood or similar, but obviously with your palletwood plan a rolled membrane would be a better fit. Just keep.in mind you're trying to make a single continuous airtight barrier out of the inner layer, ie seal all joints including that between wall and roof vapour barrier.

I'm a big fan of rain screens, which is really just a vented air gap between the WRB and the siding. Lets the wall dry out from windblown rain much easier.

liam loftus wrote:
Does the quote from foursevenfive suggest that the outer layer should be much more vapour permeable than the inner layer?
LL

Yes, that is desirable; this is the intended role of tyvek, having a moderately high perm rating, ie being fairly vapour permeable. Hopefully there is another product available for the inner layer which is less vapour open. In my area this has often been plastic sheeting, which is *not* desirable, being entirely impermeable!

I'm no expert but have been researching this subject myself as I have a lot of moisture problems in my 95 yr old frame house.   My understanding is that it's most important to not let moisture move from the inside of the house into the walls.   Therefore - air circulation and VENTING to strategically vent  interior moisture to the outside is critical.  For example bathroom fans and a kitchen range hood fan.    

Matt Risinger is a builder of large, high end homes in Texas, and uses a lot of expensive poly (ick)  products  but he goes into great detail of the principles of ventilation and insulation specific to different climates in his videos - here's one

Here's my opinion, based on building a cabin and a few other insulated structures.  I put an air tight barrier (plastic sheeting, visqueen, poly) on the inside of the walls and roof and tape it at all joints.  This is to keep moist warm air in the building so it doesn't get into the insulation and condense.  This also keeps wind pressure from forcing air through the walls into the house.  

On the exterior of the walls I use what we call "housewrap".  One brand here is called tyvek.  It is water proof but vapor/air permeable.  Air can slowly enter and exit it so it allows moisture that does get into the insulation to "dry to the outside".  It's also waterproof-ish so when wind driven rain gets through the pallet siding, it can't get into the insulation.

The roof needs a different material than the exterior of the walls.  Walls only have to handle incidental water.  Roofs see rain all day long.  I believe over here when you put on metal roofing for a residential space, you would put plywood sheathing (or pallet boards) under it with tar paper on the sheathing.  That way when you get a leak at a screw hole, the water will hit the tar paper and drain out instead of soaking into the insulation.  We have a product called "ice and water membrane" that is often used where ice dams or leakage is even more expected.  It's thicker and sticky and seals around screw/nail holes better.  The sheathing is needed to support the membrane, support the roofing a bit better (for walking?) and to block noise from rain/hail hitting the roof.  

I'm not sure if you'd need roof venting in your climate, hopefully a Washingtonian will chime in on that subject.  

I'd second Dillon's love of a rain screen or air gap between the exterior siding boards and the housewrap.  When water gets behind the siding, it would really help to let the back side of the siding dry out.  If it stays wet, it will rot.  The air gap can be a "rain screen" product or battens.  Battens may still hold water against the siding at those spots which could still lead to some spotty rot.

Arguments could certainly be made to build the house to older, leakier building standards that stood the test of time.  The more the house can breath, the less likely mold/rot is.  The wood stove will need to run more but maybe that's a trade-off that's worth it.  I'm actually not sure how to build with 1800's methods AND insulation.

Dillon, you and I both live in heating dominated climates.  Up here in AK we. in typical construction, actually have a moisture barrier on the inside of the insulation as well as a vapor barrier on the exterior.  Typically goes in this order:  interior wallboard, 6mm poly sheeting sealed at all penetrations and seams with sealant (this is for all exterior walls and ceilings), 2X6 insulated studwalls, exterior wall sheathing, house wrap such as tyvek,(which allows water vapor out, but keeps liquid water from entering from the outside), then exterior siding.  In a heating dominated climate we are trying to keep moisture out of the wall cavity.  We do not want any moisture to reach the frost point inside the wall because it will then condense, saturate your insulation and render the insulation null and also allow for mold, rot and other not so good problems  to manifest.  If/when moisture condenses inside the wall cavity it typically will not dry out due to our cooler overall climate and the lack of time needed to dry and the sealed nature of the wall cavity.  Attics are typically done similarly but without the tyvek.  Most attics are vented either with gable vents or ridge vents to allow the moisture vapor to escape.  Do a google search on the cold climate housing reasearch center which is located in Fairbanks AK.  Lot of good info on house construction in cold and arctic climates.

I know this is a dead thread, but google brought me here, so it lives as a zombie to the world.  As such, I'm adding something which wasn't mentioned.  I'm an engineer with some experience with waterproofing and extensive experience dealing with problems people create.

Everything in this thread is excellent and correct regarding where the two barriers are.  Something that people don't realize is that, in most systems, that the tighter you make the air flow in the system core (between the barirers), the more you isolate potential problems.  This can be very bad.  An example is spray foam on the back of the exterior plywood.  It will almost be a moisture barrier itself.  Even if you have your ourter barriers and construction breathable, putting that in made it so that air and watervapor load will only be able to transport through the wood at some abysmal rate.  A small leak will be visible from neither the interior nor exterior until the construction starts to disentigrate.  This applies to any insulation you build tight enough; air does not flow through batt insulation at any appreciable rate (though liquid water may, which at least lets you know something is wrong earlier).  In old construction, that wood got baked dry every winter.  If you contain the moisture between the two barriers, you will eliminate a condensation problem, which is only really important because your house is insulated well enough so that you don't have zero relative humidity in the cold weather, but you run the risk of trapping the air/water.  The air in the wall cavity has to be free enough that any moisture which makes it in there is intentionally lead back out through the exterior.  One way to acheive this is to have appropriate air gaps.  Here is an example:  
Cathedral ceiling stating at interior = paint, taped & plastered GWB, vapor tight barrier, flush contact at joists/rafters and 1/4" minimum gap between insulation and back of board, battinsulation in the lower part of the joist/rafter depth, 4" ideal air gap over the top of the batt insulation separating it from the roof plywood (this is to prevent ice damming as much as anything else), pylwood/decking, watershedding roofing membrane, shingle roofing.   [note that modern code may require more insulation than this will allow given the depth of the wood elements, but that the best way to address this is to install foil faced rigid insulation as your air barrier on the inside (foil inward), or you switch to all rigid insul on the outside of the roof, frequently i switch to metal roofing if i need rigid on the outside so as to avoid another layer of plywood... you can also leave the wood exposed if you go that route, depending on fire code.]

The same basic concept applies to walls... don't prevent air movement between those membranes!  Air has to make it through your layers in the plane of those layers.  Generally you have planes of insulation with intentionally allowed air movement on both faces of the insulation, but most particularly on the outer face.  If you are using batt insulation and don't smash it up against the plywood, it should have adequate air paths for some evaporation on the back side of the plywood.