Sill casting - what would you do here?

[VisionSet]

I'm in the middle of replacing the sill section of a bay window. I've knocked out the hole sill which cold bridged straight through.

I've replaced this with 2" aerated block as you can see and have started the form work to cast a new sill.

?pic=4dcijcjNow what would you do? As you can see, I have to avoid filling the cavity in the close up, and I have to not reinstate the cold bridge, so some sort of insulation between sill and blocks? Also I have not much depth to play with as you can see the window frame does not overhang by much, but I'll probably glue a weather bar along it. Does a DPC need to go in somewhere? I believe you usually fit a cavity tray kind of thing that is tucked into the top of my thermal blocks but this is custom and I won't find the right size. I can put 1200 DPC in somewhere - upturned against the blocks? Suggestions welcomed.

-- Mike W

[John Rumm]

I would have thought a can of expanding PU foam would be your friend here! Fill the top of the cavity, and the the gap under the frame. Once set, trim with a saw to your required levels, then shutter up your casting and pour away.

[VisionSet]

Ah yes, that would be perfect for the irregular void at the end of the sill, but against the thermal blocks, I'd rather use a thin sheet material - like

10mm. What comes that thin that would be suitable - I only need 0.5sqm. Preferably free! I hate that foam stuff, so messy.

[Andrew Gabriel]

Don't forget a half dowl along the timber base to form a drip grove in the bottom.

[visionset]

Well no I won't but does it really matter if you do forget. An angle grinder with a worn stone grind disk will easily put one in with a simple guide. In the past I've cast them with a section of flex cable clipped to the bottom of the mould.

[visionset]

This is the bit I'm most interested in opinions on...

[Phil L]

Affix a DPC in an L shape against the thermalite before you do anything else, any water that goes behind whatever cill you decide to add will run under the sill and away, provided you leave drain holes, TBH it's not imperative as the joint between window frame and cill will surely be sealed with plastic / silicone / beading / whatever.....unless you are thinking about damp 'penetrating' the concrete and into the thermalite?

[visionset]

Thanks Phil, I won't bother then.

[John Rumm]

Sash cord works well too.