Replacing suspended floor, DPM detail?

[larry]

Hello everybody
New to this forum and I wonder please if anybody can advise.

1920s house, walls are solid except bottom is a dwarf cavity.
I started to pull up boards in one room to add insulation. Plan was to add some rock wool. In the end it looks like there is a bit of rot down there (not loads thankfully) so have added extra ventilation with telescopic vents

The joists were sitting on top of a timber that had been laid directly onto the footings. No fixings or any kind, just sat on top.
There were two sleeper walls, nothing underneath them and not really enough gaps to keep airflow once I add insulation..
There were bits of wood sitting atop the sleeper walls and the joists sitting atop those. Again, no nails, except a million nails used to keep floorboards fixed to joists.

Walls are soft brick and there was a lovely collection of damp brick dust around the footings.

I did a little digging by the footings in one place only and it looks like they are just sitting atop soil.

So my questions are.

1.. I need to rebuild the sleeper walls. In a honeycombed way? Could I use engineering bricks with holes in on their sides? Or even hollow concrete blocks? What do I need to add below? (Never built a wall before...)

2. Anything I can do to help keep my footings in good order in future... Add any of that moisture repellent cream??

3. I plan to add a DPM. Presumably I would sandwich this within the base of the sleeper walls... But how to detail with footings? We do have a DPC, would it be better to just bring this to the base of the footings so the bricks have a chance.to lose moisture in our new better ventilated sub floor space?

Thank you!

Pictures show footings as they are

 

[paulc]

3. I plan to add a DPM. Presumably I would sandwich this within the base of the sleeper walls... But how to detail with footings? We do have a DPC, would it be better to just bring this to the base of the footings so the bricks have a chance.to lose moisture in our new better ventilated sub floor space?

Late 1920s house here, not a million miles from you..
I wouldn't bother trying to incorporate a DPC within the sleeper wall. Instead, place a strip of DPM material between the bricks and wall plate (bearing timber) - This is how my floor is constructed, the original DPM being a bitumastic felt.

As for insulation, Kingspan type rigid boards will give you better insulation for the same thickness of fibreglass/rockwool, and you don't have to use netting to keep it in place. A few bits of battens nailed to the joists along with friction stops the stuff from moving. Downside is it is a bit more expensive, but as you only ever want to do the job once, a small price to pay in my opinion.

Oh, and if you need any imperial size facing bricks, J.Medler just outside of Norwich have a selection of different colours. I used them last year when I needed ~30 Norfolk Reds.

 

[larry]

Thanks John. Ship may have sailed on the rockwool decision.... I have a shed full of the stuff
Very helpful. What would you do with the DPM and the footings? (I.e. where would you 'end' the DPM?

 

[larry]

Ps why did I call you John? Clearly can't read. Apologies.

 

[Toby Newell]

Hello everybody
New to this forum and I wonder please if anybody can advise.

1920s house, walls are solid except bottom is a dwarf cavity.
I started to pull up boards in one room to add insulation. Plan was to add some rock wool. In the end it looks like there is a bit of rot down there (not loads thankfully) so have added extra ventilation with telescopic vents

The joists were sitting on top of a timber that had been laid directly onto the footings. No fixings or any kind, just sat on top.
There were two sleeper walls, nothing underneath them and not really enough gaps to keep airflow once I add insulation..
There were bits of wood sitting atop the sleeper walls and the joists sitting atop those. Again, no nails, except a million nails used to keep floorboards fixed to joists.

Walls are soft brick and there was a lovely collection of damp brick dust around the footings.

I did a little digging by the footings in one place only and it looks like they are just sitting atop soil.

So my questions are.

1.. I need to rebuild the sleeper walls. In a honeycombed way? Could I use engineering bricks with holes in on their sides? Or even hollow concrete blocks? What do I need to add below? (Never built a wall before...)

2. Anything I can do to help keep my footings in good order in future... Add any of that moisture repellent cream??

3. I plan to add a DPM. Presumably I would sandwich this within the base of the sleeper walls... But how to detail with footings? We do have a DPC, would it be better to just bring this to the base of the footings so the bricks have a chance.to lose moisture in our new better ventilated sub floor space?

Thank you!

Pictures show footings as they are

View attachment 10973

View attachment 10974

Kingspan can fall out due to expansion and contraction of wood, screw / glue / use strap.

I don't use plastic or felt, can be gaps where condensation forms and difficult to wrap forms. I use bitumin primer followed by attaching bitumin flashing tape direct to the timber. It does raise the height by 1.5mm but compresses down. Also the thickness fills in any irregularity, protects the dpm from puncture and it conforns to the shape of the masonary making a more secure fit that reduces movement.

 

[larry]

Thanks so much Toby. I was worried the same about rigid insulation to be honest, also the potential for leaving gaps between joists. Would be possible I guess with the new regularised joists I plan to install but would have been absolutely impossible with the previous joists which seem to be laid at completely random gaps!

I love the idea of using bitumen primer on the timbers and that makes a lot of sense.

I'm less clear on whether/why you might use bitumen or flashing tape on the masonry. I guess my key question is what am I actually trying to achieve? I mean that literally! - am I trying to keep the moisture in the ground in the masonry (stop it getting into the gap under the boards) OR am I trying to let the moisture get into the gap under the boards so it can be ventilated out... My worry is if I end up coating the footings and sleeper walls in something impervious whether that just keeps the masonry damper than it needs to be and leads to long term deterioration. (I can see the logic of a strip of something underneath the timbers).

Same question about the plan to lay a DPM over the soil as well - is this actually advised or does this just send the moisture to the masonry around the edge?

I hope that makes sense

 

[Toby Newell]

lol. Coat the timber, not the masonary! Sorry if that was not clear. Many dpms are applied to the masonary anyway but I think applying to the timber might be easier and more effective.

Yes, Kingspan is very efficient but unless you strap it can have a tendency to fall out. I have seen this many times. And yes, exactly! Anyone who has fitted as many noggins as I have will tell you there are plenty of joist that are skewed both horizontally and vertically.

You can carefully measure for horizontal angle but who wants to be cutting compound angles by hand for insulation?

Stapling a strap or two for each block stops them from falling out, I have also seen battens and chicken wire. I don't want to exaggerate, but I have seen 5 to 10% of blocks fall through before the floor has been laid, I am presuming more would fall out if guys use a hammer / impact driver and over time due to movement.

 

[larry]

Coat the timber, not the masonary! Sorry if that was not clear. Many dpms are applied to the masonary anyway but I think applying to the timber might be easier and more effective.

Thanks. Perfect, and yes that's what I'd understood you were saying. Applying to timber certainly makes most sense to me.