Hello - Architect turned self builder, building a new house in Hampshire

[AppleDown]

My GreenRaft quote was est. of £30-35k for 181m2 at 0.12W/m2K. AFT are £8,900 for just the kit, with minimum 0.1W/m2K.

[SteamyTea]

1 hour ago, Kelvin said:

Pick a value you’re happy to spend up to

Or better still, pick a thermal performance, then find a way to build to that price.

 

Easy to save £60k by getting sensible kitchen and bathrooms, rather than showroom fancy ones.

[saveasteading]

10 minutes ago, IanR said:

My own, similar to many on here, has a single layer of A193 Mesh,

That's plenty. I must only have been noticing the heavy ones.

@Kelvin what mesh  did you have?

 

Our project, ground bearing on solid ground, had 2 layers of heavy mesh specified. That was until I objected and we used 1 layer for crack control.

Lazy / cautious SE spending our money.

If we as the BH community can get slabs efficiently designed, then we will be making a difference.

To be fair to SE's, when clients choose the very cheapest quotes, they aren't getting many hours of input.

Edited June 7 by saveasteading

[SteamyTea]

1 hour ago, Kelvin said:

Pick a value you’re happy to spend up to

Or better still, pick a thermal performance, then find a way to build to that price.

 

Easy to save £60k by getting sensible kitchen and bathrooms, rather than showroom fancy ones.

[JohnMo]

8 minutes ago, AppleDown said:

My GreenRaft quote was est. of £30-35k for 181m2 at 0.12W/m2

Same ballpark our problem site - 195m2, 0.09 U value, strip foundation with reinforced concrete slab. £32k

 

Strip foundations reinforced at the front 2m wide (due to being at edge of a hill), insulated stub walls, 200mm with 2x rebar reinforced concrete, 200mm PIR insulation (U value 0.09) 70mm upstands and 100mm fibre reinforced finished slab. Included materials, time and concrete pump lorry twice.

[Iceverge]

3 hours ago, SarahHutch said:

However the sites orientation and a few design quirks mean that trying to get it passivehaus certified may prove complicated and costly

 

Almost any design can be made into a passivhaus, it just depends how much you want to spend. 

 

Have you ran your design through PHPP? 

 

3 hours ago, SarahHutch said:

natural materials

 

Caution this approach. Some natural materials have worse embodied energy than synthetic ones. 

 

 

3 hours ago, SarahHutch said:

regards to the insulated raft foundation, this was suggested by our structural engineer as the best solution to achieve a high level of insulation with very minimal thermal bridging.

 

Again it's a thing of many variables. Much depends on your wall construction, ground conditions etc. in our case strip foundations were cheaper. Properly designed they can be excellent thermally too. Maybe have a shot of THERM. it's a bit of a pig to get used to but can give an excellent insight into thermal bridging. 

 

[Alan Ambrose]

An easy way to estimate the u-value fairly accurately is to use the thickness of insulation only and ignore the rest of the stack-up. That’s one number. The other bits often won’t contribute much and will mostly net out with the inevitable thermal bridges.

 

By all means when you’re getting close to knowing the full wall/roof build up put in ubakhus or similar. Until then I wouldn’t waste my time.

[Kelvin]

1 hour ago, saveasteading said:

That's plenty. I must only have been noticing the heavy ones.

@Kelvin what mesh  did you have?

 

Our project, ground bearing on solid ground, had 2 layers of heavy mesh specified. That was until I objected and we used 1 layer for crack control.

Lazy / cautious SE spending our money.

If we as the BH community can get slabs efficiently designed, then we will be making a difference.

To be fair to SE's, when clients choose the very cheapest quotes, they aren't getting many hours of input.

Same for the Greenraft slab but we never did it that way and stuck with the trench fill etc. We used A252 for the concrete slab. 

[G and J]

This may be a daft question, but is a PH slab simply a foundation with lots of insulation and no thermal bridging?  If so what u value is it required to hit?

[IanR]

21 minutes ago, G and J said:

This may be a daft question, but is a PH slab simply a foundation with lots of insulation and no thermal bridging?  If so what u value is it required to hit?

Insulated rafts, that are often used for PH type builds, are typically sitting on 300mm of EPS insulation, plus a perimeter insulation that may be around 150mm thick, isolating the floor and foundation from any contact with the ground. Detailing of the floor-to-wall joint is still required to achieve a cold bridge free design. U value would typically be around 0.1

The video at the attached link gives a good explanation of AFT's Eco Raft. Not their highest performing foundation, but further gains start to become marginal.

https://www.advancedfoundationtechnologylimited.co.uk/our-products/timber-steel-icf-framed-building-foundation/

Edited June 7 by IanR

[ToughButterCup]

Like this (146 sq m)

The black bits are Schaumglass on top of the piles