TL;DR:
I’m seeking recommendations on how to effectively monitor moisture levels in joists during a long-term renovation project.

Background:

I’m in the process of planning underfloor insulation for a 1970s bungalow situated on a hillside. Alongside the insulation, I intend to install underfloor heating. Due to the hillside location, I have good access to the floor from both sides: the concrete base is approximately 1 meter from the underside of the floor at the roadside end of the house and drops to around 2.5 meters at the garden end.

Long term the goal is to approach Enerphit/Passivhaus levels where it is cost effective/sensible to do so. I have no intention of getting the place certified just want to make my home retirement ready i.e. minimise bills and maximise comfort (timeline is another 20 years till I retire)

The floor is supported by 100mm joists, and according to the "Guide to Best Practice Retrofit Floor Insulation – Suspended Timber Floors," these joists are considered a cold bridge, meaning they won't meet current building regulations. Specifically, a 4-inch joist is likely to have a U-value exceeding 0.7W/m²K, which does not satisfy the regulations.

To address this, I plan to insulate underneath the joists as well. My current approach involves extending the damp proof course (DPC) using a liquid DPC, both above and below the existing DPC. I’m considering constructing a sub-floor using thermal boards and backfilling with wood fibre batts (such as SteicoFlex) with a vapor control layer (VCL) on top of the joists. Since the thermal boards are breathable, I believe a breathable membrane underneath isn’t necessary.

One challenge I’ve identified is that the ends of the joists rest in a cavity, which seems like a potential area where moisture could accumulate and cause rot, even with breathable insulation this is an obvious cold junction.

I consulted with a RIBA retrofit architect who suggested limiting insulation to the depth of the joists and persuading building control that this is the only viable option. However, I’m hesitant about this for two reasons:

It doesn’t meet building regulations, particularly concerning cold bridging.
I aim to achieve a well-insulated floor with sufficient thermal mass to optimize the benefits of underfloor heating.
Alternative options I’m considering include:

Shortening the joists, filling the cavity holes, and retrofitting blockwork hangers.
Shortening the joists, filling cavity holes, installing a wall plate, and hanging the joists from it.
In all scenarios, the insulation strategy would remain the same.

The Question:

Given that this renovation project will likely take over 10 years and the floor will be completed in stages, I want to pursue maximum insulation and the most reliable method of supporting the joists whilst minimising the risk of rotting the joists. Any advice on installing a system to monitor moisture levels in the joists over time would be very much appreciated. As an engineer, I’m comfortable with the technical aspects of installation and monitoring but need advice on suitable equipment for this purpose and would happily learn from others mistakes!

Posted By: Mike1

Posted By: big_mikeShortening the joists, filling cavity holes, installing a wall plate, and hanging the joists from it.

I'd suggest that, if you do shorten the joists, this is probably going to be the easier method, just because it's easy to level up the 'wall plate' (pole plate where I've worked), rather than lots of individual hangers.

MIne are called ledger beams although ledger board seems to be more normal, though American? I thought wall plates went on top of a wall? But either way they certainly made things easy when it came to hanging the joists.

Thank you for your suggestions! Very much appreciated. I will have a think about your suggestions.

Posted By: wookeyspacing the wall runner/pole plate/ledger board off the wall with blocks of structural insulation (compacfoam, foamglas or almavert (I don't actually know if almavert is strong enough - check) would remove the thermal bridge entirely.

I agree this is a good idea. In my case the beams were on the inside of the insulation (a timber box beam filled with insulation at that place). The structural engineer specified the beam dimensions and the number and type of screws. I expect he'd want to specify (or check) any stand-off insulation as well.

The only thing you can do about the bolts is make them stainless (30% the thermal conductivity of plain steel) and or cover the heads with some insulation. But it's something only the most obsessive would worry about.
I feel the need to do some sums to actually quantify this, but I am supposed to be roofing right now, and the sun is out :-)

A typical detail I've received from struct eng'rs for projects with a floor supported on a wall runner is as follows:

- wall runner fixed with M12 hilti HAS anchors @ 250 cntrs, set in Hilti HIT HY270 resin.
- floor joist supported on joist hangers (Simpson Strongtie JHA450/47 - obviously get the appropriately sized JH for the wall runner height and the joist thickness)
- galv steel strap "L" shaped, fied to the wall runner and then along the joist, at 1200 cntrs (ie evry other joist if 600 cntrs, or every 3rd joist if 400 cntrs (this stops latteral movement)