Lessons learned about insulating a suspended floor over a damp crawl space after 5 years of moisture data

Posted Thursday May 10, 2018 by Eric Fewster

ood moisture content of suspended timber floor joists in Hersey Street retrofit

It’s already been 5 years since we put in sensors under my suspended floor to find out what was going on with the wood moisture content. In early January this year, I decided finally to switch off the sensor data upload, as I felt after this time that we had enough information.

The final WME data is shown here – as you can see I’m still having some issues with one of the joists near the external wall (the red line), so here I have injected boron into the areas where WME was near or over 20%. Also one of the other sensors that had recorded high moisture levels previously seems to have spiked at one point (the orange line) – this is a dead air part of the crawl space with little air movement – so again I injected boron into joists in the vicinity.

So what have we learned after 5 years of monitoring and experimentation? We have tried different solutions to see the impact on wood moisture, waiting after each improvement to see if there was a difference. These solutions involved adding in a mechanical crawl space ventilation unit, and tanking the crawl space walls.

I think my conclusions from all this experimentation have been:

1. Where you have a damp crawl space environment combined with a suspended timber floor, you have an inherent risk of longer term wood moisture issues that can vary for different parts of the floor, especially when you want to insulate the floor and make it airtight. By damp crawl space, I define it as being where joist WME (wood moisture equivalent) is over 20%, where it feels muddy/humid on the ground, where you can see yellow fungus on the crawl space walls, and where you can see a visual difference in brick colour for bricks below and above the DPC (darker and lighter respectively).

2. In such a case, according to what I found at least, adding a couple of air bricks or even a mechanical crawl space fan, probably won’t be enough to mitigate the issue. In my experience, the issue was resolved only once I tackled the source of the moisture, which in my case was the crawl space walls. Be aware that a plastic sheet only on the ground and not on the walls (as was initially the case here) may not be sufficient to reduce the moisture content in the air.

3. Even when you can reduce the humidity in the crawl space by tackling the moisture source, you still have the risk of ongoing issues in some joists in certain areas of the crawl space – perhaps due to things like brick wetting from an external wall affecting a joist end, or from moisture tracking across mortar from below the DPC to the joist end, or due to lack of air movement around a joist which is in a dead air part of the crawl space.

4. So in conclusion, if I was doing it again I think I would rather take out the joists completely and install a solid floor. Sounds like it might be more hassle and more work, but if I’d done that at the start, I would have had peace of mind and it would definitely have been less work than what it has ended up being! For a solid floor, there are now good solutions where compacted recycled glass aggregate can fill up a crawl space while giving a very good u-value and being vapour-open. Bifoam is one option, which gives a u-value of 0.2 W/m2K with 365mm of compacted product – available from Ecomerchant.

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