Is this a cold bridge?

[Tennentslager]

My sisters extension drawings for BC from expensive architect.

[Tennentslager]

This is the view of the extension. 150mm concrete slabs over 170mm of insulation with ufh pipes clipped but she has a combie gas boiler.

How does that work?

[tonyshouse]

50mm sand blinding could/will percolate into the hardcore leaving a void 

 

the patio door sill is an unmitigated cold bridge 

 

more worrying is the sole plate and bottom rail of the timber frame sat on a dpc, I suspect condensation may happen on top of the dpc and if it does it will soak into the wood. Even without that problem the double plate is definitely a cold bridge. 

[George]

It is, but relatively minor. I don't think this would cause issues for building regulations or anything short of a passivhaus.  The balance an architect needs to strike is the building regulations, what a builder can build (i.e. what are they used to), cost and what they know works.

 

I've seen a lot worse and with 200mm PIR the average U-value of the wall will be very low. With an internal VCL I don't think condensation will be an issue.

 

Question the architect, if I had to mitigate it with minimal changes I'd change what the sole plate is sitting on to a course of thermoblocks.    

[Tennentslager]

Combie means a buffer tank right?

What size and is this a lot of hassle when she could add a long radiator easily. The ufh is only going in the extension as existing house has concrete slab which would have to be broken up.

Thinking extra pipe runs and cost. 

[Gus Potter]

A few thoughts.

 

Structurally in the main room you have concrete slab that floats about on some insultation but at the door you have the slab resting on the underbuilding which is a stiff point. As soon as the slab settles, cures, a lot of load will end up on the solid bit at the doors. Best to let it move independantly.

 

From a buidability view why not maintain the same detail all round as per the left side of the drawing. Then infill with timber /insultation to mitigate the cold bridge. Also doing it this way can cut you a bit of slack when it comes to measuring the doors for manufacture / door / threshold options. This can really save you as trying to set these tight slab levels with a local (Joe Blogs) builder and save as much as you can is brave. Cut yourself a bit of slack with the tolerances.

 

I've have been working on a rear extension with UF that only needs about 5 kW of heat as it's well insulated compared with the rest of the old house.. but I think I have got a combie that will modulate down to about 5- 6 kW so am hoping that I won't need a bufferton mitigate short cycling etc.  One of the keys I think is not to over size the boiler.