Foundation Redesign

[Phaedrus]

The site I've acquired has construction already started.  The picture below shows the footings and blockwork/brickwork to DPM level (150mm above finished ground level).

 

 

I wish to change the construction to timber frame and have been looking at how to get the best fabric first approach I can within the constraints already imposed.  The section below shows what my neighbour who is building in timber frame has specified.

 

I'm concerned that this isn't good enough with insufficient insulation in floor and walls and cold bridging potential.  Would appreciate your suggestions on how to get close to Passivehaus standard from here without ripping up the entire building footprint.

 

 

 

[Mr Punter]

From the photo you only have about 250mm wall thickness.  If you are going with brick outer leaf you would lose a fair bit of space to internal wall insulation.

 

Actually the cavity may be a fair bit wider - just the angles and the higher brickwork playing tricks.

Edited March 2, 2021 by Mr Punter

[Phaedrus]

 

Just now, Mr Punter said:

From the photo you only have about 250mm wall thickness.  If you are going with brick outer leaf you would lose a fair bit of space to internal wall insulation.

 

Yes, the brick/block walls originally designed had a 100mm cavity filled with Dritherm32 batts so wall thickness > 300mm.  I am prepared to go for 102.5mm brick skin, 50mm cavity, 9mm OSB, 140mm stud + further insulation & service cavity/plasterboard which would be >300mm but <400mm.  What concerns me most is the insulation under the floor being only 100mm and the cold bridge at the wall/floor join.  Would appreciate advice on how to do better.

[Bitpipe]

Did you pay a premium for the founds being in situ?

 

What would it cost a man and a machine to grub that up & dispose? 

 

I think you will struggle to get a passive standard house to work on those foundations - as you say, not enough depth to get decent insulation in especially if you're considering a low temp UFH solution which usually works well with the low heating requirement.

 

 

[ProDave]

Unless there is a really really tight planning restriction on the ridge height, I would simply raise ground floor FFL 100mm to add extra insulation.

 

Extra wall insulation will mean slightly smaller rooms. Can you live with that?

 

The BEST thing you can do to make this energy efficient is pay very careful attention to ALL the detailing, particularly air tightness.

[Russell griffiths]

If it’s on strip footings, I would look at knocking all internal walls out and form an insulated raft inside the face brickwork 

the only part I would keep would be the footings and face brickwork. 

But it does depend on what you paid for it and if you have any height restrictions. 

 

Your neighbours version is woefully inadequate. 

[Mr Punter]

Not sure where your FFL is related to the photo but I assume it is top of brickwork.  Maybe look at removing the top course of blocks and filling cavity down to the foundation with concrete to get to your 140mm bearing.  You could then run some coursing bricks and a course of Marmox Thermoblock as an insulated base to the timber frame.

[nod]

1 hour ago, Russell griffiths said:

If it’s on strip footings, I would look at knocking all internal walls out and form an insulated raft inside the face brickwork 

the only part I would keep would be the footings and face brickwork. 

But it does depend on what you paid for it and if you have any height restrictions. 

 

Your neighbours version is woefully inadequate. 

Plus one 

In the grand scheme of things It would be nothing to start a fresh 

and save you a load of hassle further down the line 

[Phaedrus]

Many thanks for your replies and suggestions.

 

1 hour ago, Bitpipe said:

Did you pay a premium for the founds being in situ?

It's hard to say.  Considerable groundworks have been completed on all three plots and my neighbour has house and garage slab laid and timber frame erected.

 

1 hour ago, ProDave said:

Unless there is a really really tight planning restriction on the ridge height, I would simply raise ground floor FFL 100mm to add extra insulation.

This could work.  If I put the UFH in the slab I could gain another 75mm and if I dug out a bit deeper then perhaps I could approach the 300mm EPS I've seen on Passiv slabs.

 

1 hour ago, ProDave said:

Extra wall insulation will mean slightly smaller rooms. Can you live with that?

Yes, I'm adding >30m2 with an infill at the rear so losing some floor area with a better wall insulation is ok.  Going to a 300mm double wall + 50 cavity and brick skin is probably too much though.

 

1 hour ago, Russell griffiths said:

If it’s on strip footings, I would look at knocking all internal walls out and form an insulated raft inside the face brickwork 

the only part I would keep would be the footings and face brickwork. 

This seems more palatable then demolishing the lot!.

 

1 hour ago, Mr Punter said:

Maybe look at removing the top course of blocks and filling cavity down to the foundation with concrete to get to your 140mm bearing.  You could then run some coursing bricks and a course of Marmox Thermoblock as an insulated base to the timber frame.

Thanks, do you have a detail of what that would look like?