A new way forward maybe!

kboc

I have just finished my house and already planning my next.

I love the idea of insulated rafter and a block wall off this with ext insulation. I now appreciate the thermal mass of concrete in my current house and insulation/cold bridging is an obvious issue.

My investigations have shown that EWI is expensive. The insulation and plaster are comparatively expensive with a cavity wall.

I have come up with a suggestion. If I use an insulated foundation rafter and dig a foundation around the perimeter of the rafter. This foundation will allow a cavity of 300mm which can be full filled with beads or another appropriate product. The internal leaf will come from the insulated rafter and outside leaf from the foundation and further detailing of the block work so there is minmal coldbridging (this can be easily done) will this give me a house doing (more of less) the same job as EWI at far better value for money?

My disclaimer, this is a loose idea. I do appreciate wall ties are an issue and an engineer will have to detail this, but in the current climate these extra tasks could be done equally at value for money.

What do you think?

Muchas gracias

Poor Uncle Tom

kboc wrote: »

I have just finished my house and already planning my next.

I love the idea of insulated rafter and a block wall off this with ext insulation. I now appreciate the thermal mass of concrete in my current house and insulation/cold bridging is an obvious issue.

Could you expand on this a wee bit please, I don't understand what you are describing here.

kboc

Poor Uncle Tom wrote: »

Could you expand on this a wee bit please, I don't understand what you are describing here.

A standard insulated raft with the block work starting on the raft (instead of an alternative like TF) so there is no cold bridging with block work and ground. Maybe i am wrong but is this not what slight leaf building method of EWI is?

sydthebeat

kboc wrote: »

A standard insulated raft with the block work starting on the raft (instead of an alternative like TF) so there is no cold bridging with block work and ground. Maybe i am wrong but is this not what slight leaf building method of EWI is?

"single leaf"... yes it is.

Carlow52

I am having difficulty with the Op's ideas here so goggled bridging and got this
http://www.macrete.com/mainarticle.jsp?c=null&newsID=591&p=fpr

Plenty bridging here, thermal or not:)

sas

What kboc is describing appears to be an insulated raft (per the usual detail) with a seperate strip foundation around the insulated raft to support a block outer leaf.

This is not a new idea, our scandivanian builder that drops in and out every so often has details describing this on his website for both TF and full fill cavity.

It was something I asked my engineer about along the way. He had concerns about differential settlement due to the 2 foundations systems being independent of eachother. It wasn't something I was terribly serious about at the time so that opinion was based on his initiall off the cuff thoughts.

To be honest, if over the long term the EWI I am applying to my house develops issues, this is in the back of my mind as one of the potential fall back positions.

Poor Uncle Tom

I think I see what you mean, something like:

An overall strip foundation (slightly wider than normal) to take the insulated formwork for the super structure support and the 'cladding leaf' would be supported from the overall strip as well. This would ensure uniform settlement but it would make a costly foundation as you are virtually doubleing the concrete usage.

beyondpassive

Like This?

sas

The issue I see with this is that surely all supporting walls need foundations i.e. you may (depending on the house design) need strip foundations beneath the raft foundation inside the house footprint to support internal structural walls.

It all adds to the cost basically.

beyondpassive

Here's how the structural engineer's interprets this detail for a passivhaus build now nearing completion, with a high surface area to volume ratio on a job I have no connection to (by OLS Architects).

As I interpret it. The Inner leaf transfers the loading through the EPS which deforms universally under loading down to the hardcore. The outer leaf has only self weight and lateral loadings and is merely a veneer. However it is tied to the inner leaf through wall ties of the Teplo or Vartry eng variety. The Strip sits under the ringbeam and so both will settle at the same rate. Of course point loads need to be considered carefully. Natural engineer one size fits all solutions cant be used to assess this solution, they have to do numerical calc's.

This is an Irish solution to the Passivhaus requirements. While I'm a timber frame man myself, this is what punters want. Block inside and outside. Who are we to argue. This solution isnt appropriate to cut and fill sites. For the majority of wide cavity builds usually around 20kWh/m2.a PHPP, I use standard strip foundation with thermally broken inner leaf using Perinsul or quinlite to the inside.

sas

beyondpassive wrote: »

This is an Irish solution to the Passivhaus requirements. While I'm a timber frame man myself, this is what punters want. Block inside and outside. Who are we to argue. This solution isnt appropriate to cut and fill sites. For the majority of wide cavity builds usually around 20kWh/m2.a PHPP, I use standard strip foundation with thermally broken inner leaf using Perinsul or quinlite to the inside.

This makes alot more sense to me and would be considerably cheaper.