We are two years into building a small Wikihouse intended as a holiday rental, and we have hit a serious problem.
What we found
During airco installation we discovered damp insulation and mold inside the walls. We brought in a building pathologist to assess the cause. Leakage was ruled out; vapour became the working hypothesis.
How we investigated
To monitor conditions inside the wall cavity, we installed LoRa sensors measuring temperature and humidity at three positions: outside, inside the wall, and inside the house.
Our wall construction (inside to outside)
- Underlayment – Finnish pine, 18 mm
- Bio-based insulation – flax, 250 mm
- Underlayment – Finnish pine, 18 mm
- Breathable waterproof membrane
- Furring strips
- White oak cladding
This follows the ‘Wren’ method.
What the measurements show
The flax insulation is highly hygroscopic – it absorbs moisture readily.
When outdoor temperature rises in the morning, the wall assembly behaves like a sponge: vapour is drawn in and retained. Indoor humidity holds steady at approximately 50%, which points to the vapour source being outside the building, not inside.
What the insulation supplier says
The supplier initially “signed off” on this construction. They now believe the outer underlayment board is sealing the insulation and preventing vapour from escaping outward. Their proposed fix is to perforate the outer underlayment to increase vapour permeability.
We plan to test this on one wall section. However, if the outer underlayment is the root cause, the same problem should be present in more Wikihouse builds, not just ours.
What we are asking
We are looking for any insight, experience, or alternative approaches that could help us resolve this without abandoning the project. All input is welcome.
Thanks,
Anton
Hi Anton. I’m sorry to hear about the moisture problem you’ve encountered, but thank you for sharing this in such detail, and for the work you’ve done to investigate the issue. It’s really valuable to see monitoring data alongside a thorough diagnosis.
This is not something we’ve seen before on any other built projects. From what you’ve described, we agree that the combination of the highly absorbent flax insulation and the Wren wall build-up may be contributing to the moisture accumulation you’re seeing. As WikiHouse has evolved, we’ve taken a different approach in the Skylark system, with a more vapour-permeable outer face. We are also now suggesting an external layer of woodfibre insulation as an additional measure to help keep the structure warmer and reduce the risk of interstitial condensation. Of course local climate conditions may also be a contributory factor.
Have you already looked into external airtight membranes (such as Proctor Wraptite) as a preventive measure? We no longer actively support the Wren system, so we’re not in a position to offer any guidance on remedial works. However, we appreciate you sharing your findings with the community, and we’d be very interested to hear the outcome of your proposed test and any further actions.
Based on what you’ve said, using this specific material build-up in a WikiHouse system can lead to serious moisture build-up, specifically via interstitial condensation inside the wall cavity, if an interior vapour barrier or airtightness layer is not installed.
While the wall is designed to be highly breathable from the insulation layer outward, the WikiHouse structural chassis introduces unique physics that complicate this specific layout.
Why This Setup Risks Moisture Buildup:
- Lack of an Interior vapour control layer (VCL): Warm, humid indoor air naturally moves toward the colder outside air (vapour drive). Raw 18 mm Finnish pine is vapour-permeable. Without an internal airtight vapour control layer (like a smart VCL membrane) directly behind the internal pine sheet, moisture from cooking, breathing, and showering will pass straight through the wood and hit the cold flax insulation layer, where it will condense into liquid water.
- The WikiHouse “Cold Bridge” Pockets: The WikiHouse system relies on interlocking plywood wedge joints, ribs, and boxes (the chassis). If you pack 250 mm of flax insulation tightly inside or around a CNC-machined WikiHouse chassis without a continuous internal air barrier, warm air will sneak into the gaps between the interlocking timber joints. When that warm air hits the colder exterior structural parts of the box, it reaches its dew point and creates hidden damp zones.
- Flax is Hygroscopic: Flax is excellent at absorbing moisture up to a point, but if the moisture inflow from the indoor living space is continuous, the flax will eventually saturate. Once saturated, its insulation values drop drastically, and it will rot the surrounding Finnish pine framing.
Possible way to fix the problem:
look at your vapour profile hierarchy.
To safely use the eco-material palette in a WikiHouse build without risking rot or mold, you must strictly follow the rule of “Tight inside, breathable outside”.
You should modify the layer sequence to include a vapour variable membrane behind your internal lining:
| Layer Order |
Material Layer |
Function |
| 1 (Inside) |
18 mm pine underlayment |
Decorative visual interior finish |
| 2 |
Smart VCL Membrane (MISSING LAYER) |
Stops indoor moisture entering the wall; allows wall to dry inward in summer. |
| 3 |
WikiHouse CNC Chassis Box (filled with 250 mm Flax) |
Structural core and thermal mass envelope |
| 4 |
18 mm pine underlayment |
Exterior skin of the structural cassette |
| 5 |
Breathable waterproof membrane |
Secondary weather shield; allows internal vapour to escape outward |
| 6 |
Furring strips / Battens |
Creates a 20 mm–40 mm clear ventilation cavity |
| 7 (Outside) |
White oak cladding |
Primary rain and wind barrier |
The “Wren method” layout your using is excellent from the insulation outward, but it relies heavily on the assumption that moisture cannot easily enter the wall from the room. By adding a smart vapour control layer directly behind your indoor pine boards, you will block internal moisture entry, keep your flax dry, and probably protect your WikiHouse structure for decades to come.
i hope this can help you. these are the things Ive found while preparing prodjects in TAS Australia. We are currently working on building modular kit homes her in TAS for people to build themselves and this is one of the most important if not the most important things to keep in mind.
Also keep in mind you will need to incorporate a HRV or ERV unit to keep moisture levels in the house down as installing a VCL or even a smart VCL internally will trap your internal moisture inside and that must get out!!
hope this helps some what and it be interesting to hear how you get on.
kind regards,
Nathanael Russell
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