Insulation on sandstone

[Herefordhouse]

Hi all,

Thank you in advance for the time on this thread. I'm currently looking into insulation solutions for my property. Its an 1860s sandstone home. 2 of the external walls are pebbledash render with masonry underneath and 2 are exposed sandstone. The current plan would be to do EWI for the two rendered walls - probably some form of EPS/Kingspan insulation with a rendered layer on-top. Then for the two sandstone walls a breathable IWI solution on the respective internal walls. I've seen there are several breathable solutions available such as insulated lime, wood fibre boards, diathonite etc.

My thinking is the walls that need to retain breathability have the correct methods applied, with a form of insulation, and then the other two walls have modern efficient insulation. But is it a sensible decision to do a mixed approach to the insulation (especially on a property of this age). Furthermore, i've seen that you can get an improvement in the insulative u values of your property using the older breathable methods. However, to gain decent u values, do you actually need to cake on the stuff. Is there a best practice method for cost vs efficiency vs space required for such walls.

One last question, on the internal walls, we will need to replaster a lot of these walls as well. Is it possible to just plaster these walls in the normal way, since there is no breathability requirement. Theoretically if this is the case then it would only be the external walls on two sides of the house where we would need to use lime plaster with x insulation solution.

Many thanks in advance, and any tips of any form would be greatly appreciated.

Many thanks,

Josh

 

[jamest]

Why not insulate the sandstone walls externally? If you can insulate externally, it's a better option for several reasons. One is that the external wall will be colder if you insulate internally and will be more prone to frost damage in winter as a result - water will soak into the sandstone, freeze and break the stone surface. If you insulate externally, this won't happen. If you want a breathable external insulation, there are plenty of options. The other good thing about breathable insulation is that it can be close to carbon neutral and sustainable. Cork board insulation is sustainable, for example, and carbon neutral (apart from transport costs), lime render is also carbon neutral (near enough, the cement industry is a huge net carbon producer). If it were me, I would do the whole thing in breathable insulation regardless of whether the wall is sandstone or brick - bit more expensive, of course, but not much.

 

[Herefordhouse]

thing is the sandstone is beautiful on the outside, so we want to retain that aspect.

 

[jamest]

I see, fair enough. In that case, IWI it is. There are some recommendations for the thickness of IWI to reduce the possibility of the external wall getting too cold - I've heard that no more than 6 cm thick is recommended and that 4 cm is the point of 'diminishing' returns (i.e., you get good - presumably roughly linear - improvements up to 4 cm of insulation and from then improvements per extra centimetre start to decline). If your sandstone is pointed in soft lime mortar, that could help reduce some of the freeze-thaw weathering action. Also, it's recommended that internal walls that meet external walls are insulated 50 cm or so inwards from the meeting point to avoid cold bridging.

 

[a twig]

You might find this thread useful:

Plastering question

Hello all, After nearly a year of serious unanticipated structural work we've now finished and can get on with fitting windows, plastering and everything else that we thought we'd be finished by now. I have a question regarding use of wood wool boards on an external facing brick wall. This...

www.periodproperty.co.uk

Covers the use of wood wool and wood fibre as IWI

 

[aockenden]

I think having 2 walls externally insulated and 2 walls internally insulated is not ideal as you wont be able to form a contiguous thermal envelope, aka you’ll have some cold bridging somewhere for sure.