- Report
- FR-014
- Published
- Written by
- Marta Quintela
- Reading time
- 02 min / 392 words
Moisture in timber facades after three winters
Sixty-four probes in the larch facade of Hallmark Library show where three subarctic winters actually left their moisture.
Section 01
What we measured
Hallmark Library has sixty-four moisture probes set into its larch facade, at three depths and on every orientation. They have reported every fifteen minutes since the building was closed in, in October 2023. Three winters later we have a continuous record of how a ventilated timber facade behaves in a subarctic climate, rather than how the hygrothermal simulation said it would. Most of the facade behaved exactly as modelled. Moisture content rose slowly through the dark months, peaked in March as the snow began to melt, and fell away within weeks of the first dry spring wind.
Section 02
The north-west corner
The exception was the north-west corner. In the second winter the outer boards there stayed above twenty percent moisture content for six weeks, well beyond the model and close to the level at which decay can start. The point cloud from the cladding stage explained it within an hour. A flashing above the corner had been fixed four millimetres lower than drawn, leaving a gap that let wind-driven snow into the ventilated cavity. Nothing in the drawings was wrong and nobody on site had made an obvious mistake. The building was simply a few millimetres different from its drawings.
Fig. 014.1/ Moisture content, north-west corner
Unit / %
| Period | North-west corner | Facade average |
|---|---|---|
| Nov 23 | 14.1% | 13% |
| Jan 24 | 18.2% | 16.1% |
| Mar 24 | 17.4% | 15.6% |
| May 24 | 13.2% | 12.5% |
| Jul 24 | 11% | 10.8% |
| Sep 24 | 12.1% | 11.5% |
| Nov 24 | 16.3% | 14.1% |
| Jan 25 | 22.5% | 16.4% |
| Mar 25 | 21.8% | 15.8% |
| May 25 | 14.2% | 12.6% |
| Jul 25 | 11.5% | 10.9% |
| Sep 25 | 12% | 11.6% |
| Nov 25 | 14.6% | 13.2% |
| Jan 26 | 16.8% | 15.9% |
| Mar 26 | 16.2% | 15.1% |
| May 26 | 13.1% | 12.4% |
Section 03
What changes
The flashing was refitted in May 2025 and the third winter peaked at seventeen percent, back inside the model. We now scan every flashing, sill and junction before the cladding goes on, and compare it with the model while a fix still costs a ladder and an hour rather than a scaffold and a season. The probes stay in for the life of the building. The lesson is not really about larch or snow. A facade is only as good as its smallest junction, and only measurement finds that junction in time.
Section 04
Three winters in numbers
Across all three winters the facade spent 3.4 percent of its probe hours above twenty percent moisture content, and almost all of that came from one corner in one winter. The south and east elevations never crossed the threshold at all. The west came close twice, during driving rain from the coast, and dried out within days both times. That pattern matters more than any average: a ventilated larch facade in Umeå is robust, provided its junctions are built as drawn. We now send the probe data to the facilities team of the library every month, so they see the same picture we do.
Fig. 014.2/ Weeks above 20 percent, winter 2024 to 2025
Unit / wk
- North1 wk
- East0 wk
- South0 wk
- West2 wk
- North-west corner6 wk
