Retrofit: Insulate behind joists?

[bakingjames]

I’m retrofitting our Edwardian end-terrace using wood fibre, and I wonder if there are any moisture experts on here…

 

Our house has a solid brick wall (220mm) with a solid sandstone outer layer (150mm). Where the joists sit, there is a gap of a few bricks and the joists sit about 100mm onto the brick wall. It’s just sandstone behind it.

 

This leaves about 120mm behind each joist (varies) of an air gap. I was intentionally NOT insulating between the floors, as I was hoping this bit of thermal bridging would make a difference the risk of the joists being too wet. We’re fortunate that practical moisture calculations (WUFI) have been done on similar constructions nearby so have largely used them.

 

However, given we’ve got this space, I wonder about sticking some insulation in this air gap behind the joists, keeping the ends nice and toasty. As we’re using wood fibre this would be highly hygroscopic too, potentially wicking moisture away from the joists.

 

What do you think? Would this make any difference? Any problems foreseen? 

[Redbeard]

When you say ' I was intentionally NOT insulating between the floors' I assume you mean 'the *walls* between the floors. If so this is what was done a long time ago, but is not common IWI practice now, which is to wrap the entire envelope (preferably externally but sometimes internally), including the interstices between floors.

 

I am a retrofit practitioner, but I cannot claim to be a moisture expert. I defer to WUFI when I can (if I can find a practitioner to do the calcs (leads appreciated!). Have you got pics or diagrams of your situation and your proposals? What thickness of WF are you using? Rigid, hard-to-the-wall, or flexi between studs?

[bakingjames]

On 26/06/2022 at 22:00, Redbeard said:

When you say ' I was intentionally NOT insulating between the floors' I assume you mean 'the *walls* between the floors. If so this is what was done a long time ago, but is not common IWI practice now, which is to wrap the entire envelope (preferably externally but sometimes internally), including the interstices between floors.

 

I am a retrofit practitioner, but I cannot claim to be a moisture expert. I defer to WUFI when I can (if I can find a practitioner to do the calcs (leads appreciated!). Have you got pics or diagrams of your situation and your proposals? What thickness of WF are you using? Rigid, hard-to-the-wall, or flexi between studs?

Redbeard, thank you for the reply! And indeed yes, I mistyped.

 

there have been nearby WUFIs on similar house constructions and published in the public domain - see ‘Niddrie Road Retrofit’. Their concern was the joists and they ended up building a new frame within the fabric of the building to support the joists.

 

60mm wood fibre was fine and we went for this - steico therm, hard boards, adhered with lime plaster and insulation fixings and lime plaster over the top. They are continuous from floorboard at the bottom to lath at the top.
 

Yes evidently a complete envelope would be ideal for heat loss but the worry in my head remains about joist moisture and rot. I’ve got access to between the floors when doing the radiators so could still go for this by shoving in some of the soft/spongy wood fibre, which I have plenty of.

 

the conclusion I’m coming to is that no/one really knows if insulating behind the joists is a risk and I’m wondering about doing a mixture, and installing some moisture sensors and take readings through the winter.