Sleeper Walls and Reducing Thermal Bridge

[MikeGrahamT21]

Hi,

 

Our bungalow is blessed with sleeper walls, and 100mm joists, and to increase to 200mm joists, would be far too much work digging out. At present the sleeper wall is simply created from bricks, with a DPC, and then the timber joists on top of that.

 

In an effort to get the thermal bridge as small as possible, would it be worth while taking the top row of bricks off of the sleeper wall, adding a new DPC on top of the remaining brick, and installing a timber for the joists to sit on?

 

Regards

Mike

[Mr Punter]

The void under a timber floor should normally be ventilated to the outside via airbricks.  This stops moisture forming under the floor.  I would not have thought that cold bridging from the joists would make much difference.

[MikeGrahamT21]

Yeah there are airbricks to the outside, and you are right, the bottom of the joists will be on show to this. So not much point?? Saves me work i guess if not, just trying to make it the best it can be

[Brickie]

There’s a product for sealing the gaps between the boards,I believe. 

I would imagine there’s a wall plate on top of the sleeper walls already,to receive the joists. It would be unusual for them to sit directly into brickwork (via the Dpc)

[Mr Punter]

If you have a ceiling height of at least 2350mm and you would be willing to lose 50mm, you could consider adding 30mm Celotex insulation on top of the floorboards with 22mm t&g chipboard on top.  This will make a huge difference compared to an uninsulated floor.  You would need to cut the door bottoms and remove the skirting and either replace or refit.

[MikeGrahamT21]

3 hours ago, Brickie said:

There’s a product for sealing the gaps between the boards,I believe. 

I would imagine there’s a wall plate on top of the sleeper walls already,to receive the joists. It would be unusual for them to sit directly into brickwork (via the Dpc)

 

This is only single storey house, but no it’s just brick, dpc and then joists, had a look on google earlier and found both variants so I guess it probably varies around the country and each build, which materials they had more of. We’re 68 bungalow. I’m kind of glad there is no wood there at moment as we are still stripping out all of the woodworm damaged stuff!

[MikeGrahamT21]

3 hours ago, Mr Punter said:

If you have a ceiling height of at least 2350mm and you would be willing to lose 50mm, you could consider adding 30mm Celotex insulation on top of the floorboards with 22mm t&g chipboard on top.  This will make a huge difference compared to an uninsulated floor.  You would need to cut the door bottoms and remove the skirting and either replace or refit.

If it wasn’t for needing to replace the wood I’d do this, but need to rip it all out, clean underneath and 100mm PIR in between joists, had a look at BG Plank system too, but it makes so little difference to overall u value it’s just not worth the expense. Going to put AVCL under floorboards too and stick it to masonry behind the skirting as an upgrade 

Edited April 24, 2018 by MikeGrahamT21

[Simplysimon]

@MikeGrahamT21, a vcl is really not required with the ventilation under the floor. if the dwarf walls are about 2m apart then 100mm joists are fine. good tightly fitted insulation will be fine. how are you supporting the PIR between the joists? your easiest option would be to nail a piece of ply/ osb about 80mm wide under each joist equally spaced and have the PIR sitting on this.

btw are you replacing joists as well as flooring? going regularised or o/s for joists as regularised will be 95mm and not 100mm.

[Mr Punter]

OK Consider ripping out all the timber and sleeper walls, compact the sub base, stone / crushed concrete.  Insulation then concrete or concrete, insulation, screed or chipboard.

[Brickie]

Or,if the floor is out (or mostly out,how about putting it back in 75mm lower (1 brick course)  & doing the system @Mr Punter suggested,bit with 50mm insulation instead?

[MikeGrahamT21]

14 hours ago, Simplysimon said:

@MikeGrahamT21, a vcl is really not required with the ventilation under the floor. if the dwarf walls are about 2m apart then 100mm joists are fine. good tightly fitted insulation will be fine. how are you supporting the PIR between the joists? your easiest option would be to nail a piece of ply/ osb about 80mm wide under each joist equally spaced and have the PIR sitting on this.

btw are you replacing joists as well as flooring? going regularised or o/s for joists as regularised will be 95mm and not 100mm.

Friction fit, with a bit of expanding foam in any gaps, theres not enough room below the joists to get the screwdriver drill under there to fix anything to the underside, plus the PIR will poke through as below, i've done 1 room already and that worked well, but I didn't know about AVCL's back then. I'm using the AVCL more for the airtightness factor, as you can never seal 100% without one in my opinion.

 

Yeah joists going too, i know the new ones are 95mm, so the PIR will protrude the bottom by 5mm, no point getting 90mm PIR as its more expensive than the 100mm.

[MikeGrahamT21]

14 hours ago, Mr Punter said:

OK Consider ripping out all the timber and sleeper walls, compact the sub base, stone / crushed concrete.  Insulation then concrete or concrete, insulation, screed or chipboard.

 

Yeah I had also considered this too, sub base is already concrete, so would just need insulation and then screed. I went away from this idea, as all of the heating pipes and electric cables run under there, would make it a large project and a much larger cost, so going to stick with the timber for that reason.

[MikeGrahamT21]

4 hours ago, Brickie said:

Or,if the floor is out (or mostly out,how about putting it back in 75mm lower (1 brick course)  & doing the system @Mr Punter suggested,bit with 50mm insulation instead?

 

Not thought of that, how about the DPC on the house though, as this is the same level as where the timber is currently sat, plus it would also decrease the air gap underneath the timber to around 75mm, below the 150mm min which is required, which is what its at currently.

[Brickie]

Are the joist ends bedded in the outside walls or sat on sleeper walls? 

If it’s the latter,then I don’t see a problem. 

If it’s the former,maybe you could chase out & fix them on hangers,with some Dpc taped round the ends for good measure. 

The ventilation could be solved by installing trlescopic vents on the back of the existing air bricks. 

[MikeGrahamT21]

In some places they are in outer walls, and in others internal walls.

 

Airbricks are doubles, so they are the same height as the 2 brick sleepers, and the bottom of the airbrick is the oversite concrete, so no room for manoeuvre.

[Simplysimon]

if pipes running below insulation, make sure they are well insulated

[MikeGrahamT21]

Absolutely, going with the thickest pipe insulation, the water bylaw stuff just to be sure, and to make sure we have hardly any heat loss from the CH too.