Sunday 8 November 2015

making it up as we go

As we might have mentioned before, we didn't give ourselves a long planning head start before starting the build. This is forcing us to use the plan-as-we-go method a little more often than comfortable :)

We have been putting together our first timber order and getting quotes from local suppliers. This was hard because it required setting a lot of constructional details into stone and decision time is serious time. Also because some details affect other decisions and details, and so on. One example: 

Our initial wall build up from outside to inside was going to be this:
- Weatherboard (maybe western red cedar)
- Airgap with marine ply battens (the 'rain screen')
- House wrap (planning for Proctorwrap Enviroseal Commercial)
- Bracing plywood (AS1684.2 states that 7mm F11 provides enough strength)
- Timber frame with mineral wool insulation
- Tongue and Groove interior lining

Nice and easy. As seen on many Tiny House blogs. The only problem is building science. Once you start reading about vapour retarders and air barriers it becomes apparent that there could potentially be moisture problems in a small space with an inherently air-leaky surface such as T&G boards and with having the  most vapour impermeable surface, the plywood, on the outside of the building. Could there be a situation in winter, where moist air from the inside saturates our plywood by condensation (adsorption)? A dew point could possibly form on its surface when temperatures outside are a lot colder than inside. Are we overthinking this? Is Perth's climate forgiving enough not to have to worry about vapour retarders and airtightness? Will we even live in Perth with this house for the longest time, because Albany's climate, for example, could pose different challenges on the house?

First of all, reading up on this issue made us aware that air transported moisture is supposedly a much greater concern than diffusion through wall materials. The sheer volume of water penetrating into wall cavities in a normal house by air leaks compared to vapour diffusion is apparently massively larger. 

Secondly, reading up on websites like and, especially helpful, made us a little less nervous about the need for vapour "barriers" in this part of the world, but certainly increased the concern for airtightness of our walls. 

As soon as you start discussing airtightness of buildings, many people seem to argue that it must be unhealthy to live in a hermetically sealed space that "cannot breathe". Others, like those following ideas of the PassivHaus design, rave about airtightness to achieve energy savings. Personally, we like energy savings and natural, healthy spaces, and think that these don't have to be exclusive of each other. Airtightness in buildings is however such a huge topic that we can't do it any justice discussing it here in a little blog post, but what we arrived at for the moment is this: 

We like our walls to breathe in the sense that we like our walls to be able to dry out before they start to rot. Unintended air leaks however are not so great, as they not only increase energy needs for cooling and heating in a tiny house with little thermal mass, but also introduce moisture related problems. 

If you have a leaky house, as is the case with many older, traditional houses, moisture can easily move through the walls via cracks in walls and gaps around windows and doors, electricity plugs, light switches - you name it! The house we currently live in is so leaky that you can feel an actual draft in winter and it gets ridiculously cold, considering winters aren't even that harsh here in Perth. Then again, those old houses get crazy hot in summer! 
What we are after is definitely not a vapour barrier (think living in a plastic bag) but to allow for vapour permeability on the inside and outside of our wall. 
As most of the steam and moisture that could potentially enter the walls will be created on the inside of the building (cooking, showering, breathing,...), we want to achieve a fairly high level of airtightness on the inside of our walls, to stop this moisture getting into our walls in the first place.
We will try to relax and not worry too much about diffusion through wall materials and we'll just rely on the moisture-transporting properties of our timber and housewrap. 

How are we achieving this? Well, we really wish you could tell us! As we can't use the tried and tested drywall in our build as a vapour permeable air barrier, we are currently considering options like using a modern smart vapour retarder and airtight layer (Intello Plus or similar) on the inside behind our wall lining or just plain old thin plywood, which is fairly air tight (if meticulously installed!) and has smart vapour retarding properties as well. Another option we have seen would be to put the outside plywood sheathing onto the inside, which could address a lot of our concerns. However, we are both not really comfortable with relying on house wrap alone for protecting our walls from the elements, without having some sort of plywood behind it as a "back up" in case any rain does find its way behind the cladding and through a puncture or leaky seal in the housewrap. We will share our new wall layout later, once we are sure about it. Hopefully that will be soon, as we are expecting our trailer and first timber within the next week! AAH!

While we were considering all this, another idea came up and is really growing on us: Ditching a lot of the T&G lining boards and using wallpaper-covered plywood on some surfaces inside. We'll see.....

Another quick detail that we would like to share: In hindsight, it may have been more practical to order our trailer like this:
This detail shows the outermost steel beam and the subfloor and wall on top of it. The difference to our trailer is that the steel angle is welded onto the outside instead of the inside. The revised detail would allow for easy access to bolts, even after the whole floor has been insulated.

Unfortunately it's too late for us as our trailer is almost finished, but maybe someone else can learn from this :)

Let us know what you think about airtightness and vapour barriers/retarders in tiny homes - any experience or advice will be much appreciated!


  1. Insects breath trough the skin, but houses and humans exchange air normally through openings. If you have a perfectly sealead and isolated "Passivhaus" you will need some automatic ventilation system (or open the windows manually several times a day). It is not only moisture, but also CO2, smells, and so on.
    For designing the wall you might want to try the tool at

    1. Thx for the tip. I am going to try that tool. We'll seal the house but I doubt that it will be a _perfectly_ sealed passivhaus ;) ventilation systems are still something we are investigating. The zehnder comfospot 50 looks pretty perfect but we are not quite sure yet.