insulating the old bit of the house

[dr j j jenkins]

we are rebuilding and extending much of the house and are interested in acheiving high standards for this area. However, one third/quarter of the external wall will remain old solid wall which cannot have external insulation required.

My questions are

1. How best to insulate the old walls. Presumably internal, breathable proprietary insulation.

2. Given the old bit can only acheive a certain standard, is it still worth pusuing very high standards in the new bit.

thanks

[Mark Siddall]

JJJ,
External rather than internal is the best option for minimising moisture related problems. Appropriately considered vapour permiable design is best.

[Alan Clarke]

Assuming you do have to insulate internally, "breathable" is too simplistic - this means "vapour open", and you risk interstitial condensation on the now cold solid wall. The options are to use a vapour control layer (VCL) or an insulation that wicks moisture back into the room. In a retrofit the VCL is hard to seal at joists etc, and if not sealed then moisture can get round your barrier, so the alternative is worth exporing - try Ecological Building systems for products and advice. Other issue is rain absorbtion by the solid wall - this may want treating with a sealant - once you insulate the wall it isn't so warm and doesn't dry out so easily.

As to insulation levels, you will probably be reaching U-values of 0.3-0.4 on the solid wall, can be risky to try lower, but this doesn't mean you shouldn't aim for half the heat loss on the newbuild, ie U 0.15 -0.2. It should be easier and cheaper to achieve a given U value in the newbuild, compared with retrofit, though given the area of old wall you need to do a reasonable job there too.