Where to start wall insulation?

[pilsbury]

Starting my block work tomorrow. Currently built to DPC with a weak mix fill to 225mm below. Should I start wall insulation straight off the top of the weak mix fill or leave a gap? How much gap?

 

[^woody^]

Down to the t̶o̶p̶ bottom of the floor insulation

Edit: bottom not top

 

[John D v2.0]

woody is the experienced one, but also bear in mind the further down you go, the less of a cold bridge you'll get. i.e. the cold can go under the wall insulation, thrrough 100mm of the inner leaf and it's above the floor inslation. Ideally make the path for heat as long as possible, here it has to go much further through non-insulation

 

[John D v2.0]

no idea why the image isnt' working, it's just a google image search for floor-wall junction detail with an arrow showing the shortest path around the insulation

 

[^woody^]

150mm below DPC is oftern recommended, but the concept is that you have a complete envelope of insualtion - so the wall insualtion goes to the bottom of the floor insulation, so depending on your design it may need to go further down.

But its not always practical, and I think it's a stupid idea to bridge the DPC, and especially have [quilt] insualtion below DPC where it can not only get constantly damp and sag, but allow water to be absorbed back up the wall and into the inner leaf. So any insulation below DPC relies on a clear cavity and weep holes to allow water to escape - something that might just not be practical.

You can compensate by running an upstand of 25mm celotex around the slab and screed instead from the bottom of the insulation to finished floor level - this is hidden by the skirting.

In reality though, the cavity wall insulation depth wont be checked on site.

 

[John D v2.0]

Indeed, it depends how far you want to go!. If you're building a passive house then any kind of thermal bridge is probably unacceptable, so you lay polystyrene and build a reinforced raft on top. Then you can literally have a continuous insulation. But in normal construction you're mostly just trying to avoid condensation risk which isn't as strict. OP, Look up limiting u-value if you're interested in the topic.

 

[pilsbury]

Thank you all for your input. I can safely put the wall insulation down to half covering the floor insulation and leave a 100mm gap between that and my weak mix infill. Seems a good compromise.

 

[John D v2.0]

Sorry didn't spot Woody's edit, yes bottom would be much better then you have a reasonable overlap