The gap inbound of the OSB at the eaves needs filling?

Posted By: timmywoHow on earth do you subscribe to thread updates on here!?

You don't, in the sense you're thinking of. There is an RSS feed or the really low tech way that I use is just to open the All Discussions page - http://www.greenbuildingforum.co.uk/newforum/ - and that shows you what is new provided you are logged in.


I found the diagram and labelling too confusing to comment on, sorry.

You will certainly need to let moisture out at the outside because of the cold winters.

You may also need to let moisture out at the inside if you have air-conditioning and keep the place significantly cooler inside than outside during the summer. If you don't have air-conditioning then you should be OK with a VCL inside if you wish. Or breathable designs can be used whatever the circumstances, but need careful design.

Layer two must be permeable, so vapour can escape in winter and perhaps also in summer. You must have at least one permeable side so any water that does get into the wall somehow has a means to escape. If you have cold winters, that needs to be the outside.

Layer four is often impermeable (a VCL) in temperate climates but that can cause problems if air-conditioning is used. Because then vapour can enter through the permeable layer on the outside and condense on the cooler VCL inside.

A 'breathable' design avoids those problems but usually needs careful modelling to be sure that water doesn't accumulate within the wall. So-called 'intelligent' (variable permeability) VCLs can also help resolve such problems.

Posted By: timmywoThere will be no air-conditioning, only MVHR.

That's good; it simplifies things.

If Layer 2 allows vapor to escape out, would I need an air gap between Layer 1 (Exposed log face with chinking) and Layer 2 (House wrap)?

I can't answer that question myself. The answer may depend on local regulations (UK regs mandate air gaps in many cases, for example). Otherwise I think you would need to do (or have somebody else do) a condensation analysis using software such as WUFI. Insulation suppliers are sometimes willing to do them for customers and some consultancies will do them for a fee. Make sure they use dynamic simulation programs (BS EN 15026:2007) and not 'traditional' calculations, which are not very accurate.

I was hoping to avoid an air gap here so that the Mineral Wool would be right next to the logs and I could benefit from the R value of the logs. Also keeps the wall design simple.

You definitely need a drainage plane - that can be two sheets of membrane next to one another. But you may need ventilation as well depending how much water gets to the drainage plane and whether it is all year or there are drying periods etc. That's what the condensation risk analysis will show. As you say, ventilation would greatly reduce the effective insulation value of the logs. I don't know anything about log construction myself and there isn't a lot of it in the UK, so you might get more informed opinions from Americans or Scandinavians as well as people local to you.

I was thinking to be sure to keep the log wall dry on the inside that I would allow vapor to escape into the building, ie... House wrap > Mineral Wool > Internal wrap
Or, is that just asking for trouble?

Again, that is something the condensation risk analysis (CRA) would show. It depends on the various temperatures and the water content of the timber and other materials and how they all change over the course of a few years. That is what the CRA models.

There's a bit of discussion of some of the issues at https://buildingscience.com/documents/digests/bsd-105-understanding-drainage-planes

Hello all,

So after some more research and conversations, I have decided to go with the following wall design...

Layer 1: Exposed log face with chinking (20cm)
Layer 2: House wrap, Vapor Permeable
Layer 3: 20cm Mineral Wool Insulation, approx R32
Layer 4: Internal wrap, Vapor barrier
Layer 5: Wall finish

Main points to note...
Layer 2: Most House Wraps are Vapor Permeability in both directions, as this will behind 20cm of Wood that should be well sealed from weather limited water should get through so no need for a drainage gap. House Wrap will allow for Vapor movement

Layer 4: This will be a Vapor barrier to stop moisture from the house traveling to the outside, based on our location warm side of the wall will be inside or as we are shaded, outside and inside temp will be similar (so should rarely have the warm side of the wall on the outside that could result in moisture traveling from outside > in and getting trapper at this Vapor barrier)

I am still keen to get a condensation risk analysis performed, unfortunately finding someone with the required skill in Poland appears to be a challenge and seems nonexistent in the countryside (from my research, there appear to be many questionable building finishes here, in 20+ years we will see)