Hi, I'm looking for some advice on IWI options and the potential for extension works to cause rising damp issues for a house built in c1902. The walls are typically 9" brick.

The front and the back are on different levels, so that we currently effectively have basement (c 1.5m head height), ground and first at the front and ground, 1st and 2nd floors at the rear. Raised timber floors with air vents at ground level, from what I can see at basement level there is no existing insulation under the floor and just 100mm or so of fibreglass in the loft.

We are in London and the front of the building is south facing, no signs of existing damp. The house in in a conservation area and adjacent to a locally listed building so EWI is unlikely to be an option.

It is in a terrace but has an an access arch through and a rear outrigger, so most of the ground floor also has an external ground floor wall to the side and even the small suspended timber floor over the top of the access arch has 4 large air vents underneath.

We are planning to dig down and underpin the 4m x 4m basement so this has sufficient headroom to be usable and install a delta membrane or similar system and c50% of the rear elevation will also be extended, for which I assume a new concrete slab/dpc and c80mm of insulated PIR would be the usual spec and perhaps a new concrete slab may be suggested for the existing rear ground floor. We may also potentially add a dormer to the loft.

They are not all in great condition, but all the existing windows are double glazed UPVC.

It is also currently split into flats, so we will need to do significant work to the electric/gas supplies and plumbing, so given all the work that we plan to do in any event to extend and covert back to a single dwelling, moving sockets and possibly radiators off the external walls shouldn't be a major issue if this is recommended.

My queries are:

1) how do we mitigate the extension changes to the basement and ground floor leading to rising damp in the existing structure?;

2) would any IWI options be recommended, again bearing in mind the milder weather in London and the south facing front external wall?; and

3) what are the things to consider for chimney breasts and the chimney with regard to insulation/ventilation?

4) should we also be thinking about benchmarking most of the works against a likely need to achieve EPC C in the future when selling the property? I would also add that I expect to want to sell the property in 10 years to move back to the area of London that we live in now with regard to EPC and the cost of IWI vs payback in heating cost savings.

Ideally the IWI would be a basic insulated plasterboard DIY option that I could just mechanically fix, without the added work of needing to add a coat of anything to the rear.

Having had real issues with finding a plasterer that could competently use lime plaster in the past I would much rather use an alternative plaster if possible, accepting that some breathability may be lost.


Sorry for the long post, any advice will be gratefully received!

Nick

For me 400mm of quilt in the loft
Prefer EWI to IWI in both cases thicker the better, I like 200mm
I am unconvinced by rising damp,
Chimney breasts on wholly internal walls don’t need to be insulated but the will such heated air out of a building. On external walls, I fill to level,of the top of the loft insulation with eps beads and trove a brick for ventilation and cap the pot.

I would aim high, go for A or B , C is very poor and it is easy and cheap to double the insulation while you are doing it compared to doing it again at some future point.

Hi Nick, it might be worth looking at https://www.leti.london/retrofit if you haven't done so already.

Thanks Dave, I wasn't aware of this resource.

Nick

EWI has significant disadvantages in a terraced house (as well as the visual ones) that you end up with cold bridges all around the front and rear wall insulation, where it meets the foundations, both neighbours, and the roof. Whereas IWI can be joined easily onto the underfloor and loft insulation and can be returned down the inside of party walls, so fixing those cold bridges. If there are internal partition walls that meet the front/rear wall, IWI can be continued through them if they are lath/plaster, but needs to be returned down them if they are masonry.

Set against that, IWI reduces internal space, in my experience this isn't noticeable unless in (say) a narrow stairwell. IWI will be messy and disruptive, especially if there are finishes you want to keep (cornices etc).

So it's a judgement call which one is appropriate for each building, one size doesn't fit all. We went for IWI in our previous old house and are tending to EWI in our current one.

On the plus side, terraces intrinsically lose less heat through external walls than detached houses do, and so floor and roof insulation and airtightness are relatively more important.

Search for the Historic Scotland Refurbishment Case Study series, they found a lot of insulation was possible in old buildings without compromise on damp etc.

Also consider the embodied carbon/resources in the materials you use - timber and mineral wool are better than cement, lime, polystyrene, etc.

Finally you didn't mention how you will heat the building and provide hot water - a U=0.3 building with a heatpump uses less primary energy and carbon than a U=0.1 building with electric or gas heating, so that needs to be factored in your design.

Good luck!

It's about the proportion of cost assigned to energy efficiency and the need for a whole house strategy.

The latter is a problem on here in that GBF has failed to move on from well meaning individuals as the basis for knowledge of green building. If we had developed our thinking we'd be experts in published whole house methodologies, instead of being experts in individual aspects of green building.

If that had happened, we would probably point you to low-cost whole house refurb design methodologies which might enable you to incorporate far more and save more energy than might be thought through 'common sense'.