EWI, new porch and thermal bridging

[dr. t.m.thorn]

I'm trying to upgrade my G rated solid walled house, running on bottled LPG (ouch!). I'm on the way to EWI, but before I go there, I need to build a porch for both airlock and hallway purposes. Plan A was to build said porch in thermalite blockwork and put 100mm phenolic on top when I did the rest of the building. My thought was that the porch would both then be a warm airlock and a nice place to sit and watch the view (nice view)...and that it would then be part of the thermal envelope of the house and avoid any thermal bridging.

I've just set out the final sizing for the porch in bamboo canes on the front of my house and it looks ridiculously large. So I need a plan B, which ideally reduces the outer dimensions, without compromising the inside too much (tardis, anyone?).

Some have said that I should just construct the porch in blockwork (and render, like the rest of my house) and forgo the EWI, but I'm concerned that that will then represent a thermal bridge to the solid walls of my house, which will then be encased in EWI. Any thoughts?

I did wonder about the possibility of building it out of blockwork and using something as a thermal break to build into the house walls - But I don’t know what sort of product I could use – ideas welcome?

I’m also not going to use thermalite blocks anyway because everyone I know says that they crack badly – my builder has a preference for Durox – any opinions on those or thoughts as to alternative?

Another interesting suggestion was to use clay blockwork with a good u value, but I think these are 300mm and I’m trying to reduce the thickness of the walls.

Any thoughts very welcome – especially as my neighbour wants to build the same porch and he’s applying for planning permission next week so it would be nice to get his straight

Thanks very much,

Tania

[fostertom]

Simple - EWI the house as if the Porch isn't there (it isn't yet) then build the Porch against it structurally freestanding, of whatever's easiest.

Don't forget to dig a trench and run the EWI right down to base of house founds, ideally backfilled with Leca as a french drain. That way you can prob not have to insulate your existing ground floor because the path-length of heat going down thro the slab, down to base of founds and up again to garden surface will be so long as to form a pretty good insulation in itself - and in addition adds all that underfloor thermal mass into effectively your interior.

For discussion - this can work even with suspended timber ground floor with voids under (you can block the through-ventilation airbricks) if you treat those voids as 'rooms' supplied with dry air by your MHRV systwem (but exhausting direct to outside so as not to feed their cooled air back to the heat exchanger).

[Kate de S]

I'm not a designer but I like Tom's idea - you could use timber, or maybe even glass? You would of course have to be strict with yourself/selves about it being part of the garden, not part of the house 

[fostertom]

Well, it could still look like a conventional attached porch - or not!

[Dave Howorth]

An external porch outside the thermal envelope also simplifies the requirements for the doors. The outer door needs to be secure and weatherproof (driving rain proof threshold details etc) but doesn't need to worry about thermal bridging. The inner door needs to be insulated and airtight but doesn't need a waterbar or stormproof design etc.

[Paul Buckingham]

My only concerns with building the porch as a stand alone on to the EWI is that the junction between the two could potentially crack with movement and you would have to cut into the EWI above to install the flashing for the roof. Other than that I do like the idea!