hempcrete for dry lining mass concrete house

[LeenToo]

Hi Just wondering has anyone any experience of this either using hempcrete blocks or the hemp panels as an internal skin up against an existing mass concrete wall? I have a theory that being breathable they would make for a nice living environment and enhance heat retention with EWI of wood fibre or something else breathable on the outside. I used a hemp and lime plaster on internal walls of a stone house in a previous project and was impressed with the result

[Mike]

I've only used hemp batts against brick and stone walls, but provided the concrete is dry and resistant to rain penetration from outside and the thickness isn't great (rule-of thumb - no more than ½ of the EWI thickness), then I wouldn't expect a problem.

 

However if you can install enough EWI, that would normally be the preferred option, rather than insulating both sides. The wall is then more closely coupled to the inside temperature, the possible build-up of humidity in the IWI isn't a concern, and it's easy to hang things on the wall.

 

Consider also how you'll link the wall insulation to the ceiling / roof insulation. And how you'll tackle airtightness.

[Jilly]

Does breathable matter if the wall is (presumably) solid? 

[nod]

8 hours ago, Jilly said:

Does breathable matter if the wall is (presumably) solid? 

Exactly 

They don’t 

[LeenToo]

9 hours ago, Jilly said:

Does breathable matter if the wall is (presumably) solid? 

 

1 hour ago, nod said:

Exactly 

They don’t 

Yes because it is old concrete made from beach sand probably including large stones I have been told by a conservation architect who works on these buildings to treat it as a breathing wall.

[sgt_woulds]

Even if breathability didn't matter, internal humidity regulation would, and natural materials like directly, (clay or lime) plastered wood fibre or hemp help with this in properties where effective internal ventilation is hard to achieve.

 

Will you be fitting MVHR?   

[nod]

2 hours ago, LeenToo said:

 

Yes because it is old concrete made from beach sand probably including large stones I have been told by a conservation architect who works on these buildings to treat it as a breathing wall.

I’ve been using lime and Clay works for forty years 

Trust me your wasting your money if you are expecting concrete to be breathable 

[LeenToo]

45 minutes ago, sgt_woulds said:

Even if breathability didn't matter, internal humidity regulation would, and natural materials like directly, (clay or lime) plastered wood fibre or hemp help with this in properties where effective internal ventilation is hard to achieve.

 

Will you be fitting MVHR?   

yes I think I will use MVHR

[LeenToo]

6 minutes ago, nod said:

I’ve been using lime and Clay works for forty years 

Trust me your wasting your money if you are expecting concrete to be breathable 

So no point in using a breathable IWI?

[sgt_woulds]

That would be my conclusion too, other than the benefits to the environment in not using oil-based unnatural insulation.

 

IWI requires careful consideration and should be carefully assessed before specifying the insulation solution.

 

When using natural, breathable insulation for some impermeable walls, (e.g. concrete rendered/pointed or granite, flint faced) it may be more appropriate to incorporate a framed system which allows the addition of a moisture vapour variable membrane, (e.g. STEICOmulti renova or SIGA Majrex® or PRO CLIMA intello plus).  These ‘smart’ membranes will limit the amount of water vapour entering the fabric but still allow the wall to ‘breathe’ during warmer periods.  Correct installation of the membrane, and sealing connections to all surrounding elements is the critical factor with this approach.  Ideally, this would be fitted behind an additional service void to guard against accidental penetrations later, for example, when hanging pictures.  Plus your follow-on trades will love you.

 

The best advice is to have the external walls assessed via hygrothermal software -  such as WUFI - which will take all of the site variables into account.  Internal wall insulation is more complicated than external due to the way it moves the dew point within the construction.  In addition, standard U-value calculations will not correctly account for the sorption properties of wood fibres nor their ability to pass on liquid water through capillary action.  WUFI purely considers moisture issues and how the various elements of the building fabric will deal with the volumes based on site-specific conditions. 

 

Back to Earth offers WUFI assessments.  Chris is a great font of knowledge regarding the renovation and upgrading of older properties and is a good first point of contact when specifying IWI.

 

Siga & Pro climba also provide free WUFI assessments if their V-VCL membranes are specified and used in a batten and board approach.