A research project carried out over 5 years monitoring how much moisture levels decreased in the crawl space under a suspended timber floor.
May 1, 2014: Crawl space ventilation unit trial
Since 22nd March, I have a crawl space ventilation unit installed at my house. I installed it because I have a problem under my suspended floor of high humidity levels coming from damp ground conditions coupled with inadequate passive ventilation through the air bricks. Over time this has resulted in damp floor joists, the WME (Wood Moisture Equivalent) of which is over the recommended maximum threshold of 18%.
The ventilation device comes from the US and is called Smartvent – it kicks in only when outside humidity levels are lower than crawl space levels. The relative humidity in the crawl space determines what moisture content the exposed floor timbers reach, so reducing ventilation on warm humid days means that this humid air won’t cool inside the crawl space (giving a higher relative humidity) nor cause wetting of the cooler surfaces (condensation), but only brings in drier air to do what it is supposed to do – which is to remove moisture from the crawl space and joists. This is all part of research that has been ongoing at my house since the end of 2012, which is being conducted by a PhD student and the AECB – hopefully it will in time inform better practice into retrofitting insulation into suspended floors.
January 21, 2016: Floor joists in crawl space finally drying out
I have been monitoring the moisture content of my timber joists since 18th December 2012, using Hygrotrac wireless sensors that transmit various readings to the internet, and finally we’re seeing the WME (Wood Moisture Equivalent in %) drop to below critical levels. The sensors are located at various points in the crawl space, including several in ‘dead’ spaces where air movement is reduced.
After more than one year of monitoring, it was clear that the air bricks on their own were not capable of drying out the timbers. I had managed to reduce evaporation in the crawl space by laying a heavy gauge DPM sheet on the ground, but this had not reduced the evaporation sufficiently as it was still coming from the crawl space walls. The air bricks themselves were also not enough to permit a decent change of air in the crawl space – on the side of the house facing the main street, there were several air bricks with decent air movement, but towards the rear there were only two, being also located only on one side (since the kitchen had been added in previous years with a solid floor). It was at this point that I had decided to install a SmartVent on 22nd March 2014 (see previous post).
After another year of monitoring, it was clear that the SmartVent was not making a difference to timber moisture levels (see graph above). It was apparent that too much water was still evaporating from the crawl space walls, and also from two original hearth foundations that the ground sheet had not covered before. A solution that I then decided to try out was to reduce the height of the hearths and then continue the DPM over these and up the crawl space walls, attaching it using battens and concrete screws to the crawl space brickwork just below the original damp proof course (see photo below) – this work was completed on 8th September 2015 (see photo below). Since then, wood moisture levels have started dropping dramatically and are well under 20% WME at present.
August 02, 2016: Suspended floor joist moisture update
WME (wood moisture equivalent) has been hovering both above and below 20% in a few joists during the past few months, while other joists seem to be doing better. The elevated grey line is the sensor in the brick sleeper wall, which is now behind the plastic covering the crawl space wall – so this brick wall is back to its original level (don’t know why it reduced after the walls were tanked).
March 31, 2017: Suspended floor timbers still drying out well
It’s good to see that the WME (wood moisture equivalent) is now below 20% for all joists except one (which is indicated by the red line – this is the only sensor in the joist near where it enters the external-facing wall on the street side). I suspect what is happening is that the joist end is getting a bit damp from the brickwork in the wall – either from driven rain and/or rain splashing from the pavement, or from moisture bypassing the DPM from the crawl space wall.
Notes on line colours in graph:
- The elevated grey line is the sensor in the brick sleeper wall – not sure why it keeps going up and down
- The bottom two lines are the two sensors inside the house
- The dark blue line (left hand side, third from bottom) is the external sensor