Hi all,

A couple of years ago, we bought a two-storey 4-bedroom 1890 detached Victorian house in north-west England (108 sqm footprint, 350 sqm wall area) in need of substantial rennovation. We have replastered, replumbed and rewired the interior and it is now heated by a modern gas boiler and radiators, with wet underfloor heating for the kitchen-diner area. The exterior was rendered long ago, and the rendering is in extremely poor condition and needs to be replaced. At the same time as replacing the rendering, we are trying to decide whether to also add external wall insulation (EWI), as the walls are currently uninsulated. We would use breathable external wall insulation, and prefer a natural product such as wood fibre. We have quotes for (uninsulated) Ksystem TC15 Silicone Solo for the render (base, mesh, primer, silicone), which is marketed as "breathable".

The house has "early cavity walls", so the cavity is only a few cm, and not wide enough for cavity wall insulation. We decided not to insulate internally due to worries about condensation forming in the walls. There is a loft, and a cellar under the front of the house.

I have used the SapJS calculator at https://openenergymonitor.org/sapjs/ and entered our building's properties, as best as I can estimate them. The modelled annual heating energy usage is about 48,000 kWh, which matches our actual annual usage (measured from gas bills) to within about 8%. I am somewhat skeptical that this is not a fluke, due to all the uncertainties involved, but for the moment let's assume it is roughly right. The model predicts 42% of total heat loss going through the walls.

I used a current U-value for the walls of 1 W/m2K, based on the different layers (inner surface, plasterboard, timber battens/gap, inner leaf, air gap, outer leaf, render, outer surface). Bringing this down to 0.3 W/m2K with EWI to meet building regulations would reduce total heating energy usage to 33,000 kWh, and save about £1000 per year.

Our motivations are not purely financial; we would also like to reduce our energy expenditure for environmental reasons, so even if it took a while to pay back, we would probably still do it, so long as the upfront cost (over and above the existing project cost of rerendering) was not prohibitive.

Ideally, we would eventually like to install a heat pump and solar panels, but our understanding is that the heat pump would only be powerful enough if the house is well-insulated.

My concerns are:
- My modelling with SapJS is a lay-person's calculation, and I would like to get this professionally assessed. The important question is "How effective will the EWI be?", because if it does not save significant energy, it is not worth doing.
- The house has many other heat losses, e.g. from draughts and uninsulated suspended floors in the front of the house. Does this reduce the effectiveness of the EWI, or is this an entirely decoupled problem?
- The wall cavities open at the top into the (cold) loft, and there are air bricks in the outer skin for ventilation to reduce moisture in the cavity. If the walls are left like this, then hot air will easily convect out of the top, presumably causing massive heat loss. We could seal the cavities at the top and block the air bricks, but will this cause problems with moisture/condensation? Could the cavity instead be vented internally? Since the cavity will now be warm, as it will be inside the insulation, maybe this is not a problem? Alternatively, could we fill the cavities, since now that they will be warm, interstitial condensation shouldn't be a problem?

What sort of professional could I hire to give me the best advice, taking into account the particular nature of this old building, and all the "edge cases" that might be relevant to the decision? I have found companies who will do an energy assessment, but will this be careful and customised enough? I've heard that EPC assessments are often very rudimentary, and I need more than a basic tick-box exercise. Do I need an architect? Some sort of building engineer?

How can I know whether the EWI installer will take proper care of all the junctions and edges, which I understand are vital for avoiding cold bridges? Do they need to worry about interstitial condensation?

I would appreciate any advice that people here might have.

I did a very similar project for a client last year, same floor area, but less wall area (your wall area seems very large, being 3 times the footprint, usually about double).

I'd say your numbers look about right, with a quick comparison to the heat loss calcs I did, though what window and floor Uvalues you used I don't know, which would alter the 42% number.

Consider insulating under the floors if they are suspended timber, though perhaps you've refurbed internally already, so won't want to start lifting floors etc.

Get a quote for the EWI, and compare it versus the render only quote. Then you'll know if it's worth considering financially. Your GAS heating makes the financial return case more tricky.

The project I mentioned above were on electric storage heaters, so we installed an ASHP. The EWI brought the heat loss down by about 60%, and so the ASHP could be a large single phase, rather than a huge 3 phase. Gas cuts across all those considerations. That said, you mention possible future ASHP. Get your heatloss at 21oC delta T down to 7-8kW, and you're in ASHP teritory easily.

Maybe consider polystyrene (EPS) EWI (get a quote at least, as well as the "natural" product). I suspect EPS will be substantially cheaper, partly due to material costs, but more so due to availablity of installers.

Close off the wall vents, but maintain the floor vents (for suspended floors). Close off the cavity at the wall heads.

Posted By: jam295Bringing this down to 0.3 W/m2K with EWI to meet building regulations

I wouldn't stop there, at that quite abitrary 'target'. Go all the way, in insulation thickness, to the optimum point of diminishing returns (by my rule of thumb, 200mm EPS or wood fibre), or alternatively to the calculated thickness reqd to achieve the Passive House target in the case of your particular house's configuration and climatic region. Thickness of the insulation boards makes up a small part of the total cost of a EWI project; making it a bit thicker or thinner has a quite affordable impact on the total cost.

Posted By: jam295Hi all,

At the same time as replacing the rendering, we are trying to decide whether to also add external wall insulation (EWI), as the walls are currently uninsulated. We would use breathable external wall insulation, and prefer a natural product such as wood fibre. We have quotes for (uninsulated) Ksystem TC15 Silicone Solo for the render (base, mesh, primer, silicone), which is marketed as "breathable".
We decided not to insulate internally due to worries about condensation forming in the walls.

This is a good plan. I did the same thing (woodfibre+silicone render) on the front on my house. It's great.
There is a video about the project (and retrofit in general) here: https://wookware.org/house/retrofit/ (also on youtube).

I have used the SapJS calculator.
Bringing this down to 0.3 W/m2K with EWI to meet building regulations

Don't make your EWI that thin. Most of the cost is in the work. If you are doing it, do it properly. I had 160mm on the front, and 200mm on the side. These were dictated by the step-side of tiles (had to add one at the gable end), and the soffit overhang, and the requirement to get to EnerPhit or close. (which typically means U=0.15ish for walls)

On calculators, SAPjs will give you some ballpark figures, but SAP is fundamentally a bit crap, because it is designed as a comparison tool, not a house model.

PHPP is generally reckoned to be the best house model, but it is very detailed so either takes a long time or costs a lot (£300?) if paying someone else.

The new Zebra model from Bath uni is a spreadsheet model like PHPP, but much simplified, whilst also remaining reasonably accurate. https://www.zebra-model.org/

I've put my house though all three tools. Zebra and PHPP give results within 17% of each other. I don't have the SAPjs numbers to hand, but can dig them out.


There is also HEMS (UK govt proposed replacement for SAP at some point - a dynamic, rather than static, model), espr (Strathclyde Uni) and the US energyplus. I have not used these 3 seriously.

Ideally, we would eventually like to install a heat pump and solar panels, but our understanding is that the heat pump would only be powerful enough if the house is well-insulated.

Any house can be heated with a heatpump (or more), but if you do this EWI first (and maybe some other fabric improvements like airtightness) you will more than halve the size of HP needed. My house will have less than 10% of its original heating load when I'm finisihed with it - i.e. a 90+% drop.

My concerns are:
- My modelling with SapJS is a lay-person's calculation, and I would like to get this professionally assessed. The important question is "How effective will the EWI be?", because if it does not save significant energy, it is not worth doing.

EWI is generally excellent. Combined with decent windows, airtightness and loft/roof insulation it will make your house amazingly nicer to live in. A 90% reduction in heat load is a perfectly reasonable target. The EWI alone will not do all of that, but a decent chunk. the 42% number you got is plausible, especially if the glazed area is not too high.

- The house has many other heat losses, e.g. from draughts and uninsulated suspended floors in the front of the house. Does this reduce the effectiveness of the EWI, or is this an entirely decoupled problem?
- The wall cavities open at the top into the (cold) loft, and there are air bricks in the outer skin for ventilation to reduce moisture in the cavity. If the walls are left like this, then hot air will easily convect out of the top, presumably causing massive heat loss. We could seal the cavities at the top and block the air bricks, but will this cause problems with moisture/condensation? Could the cavity instead be vented internally? Since the cavity will now be warm, as it will be inside the insulation, maybe this is not a problem? Alternatively, could we fill the cavities, since now that they will be warm, interstitial condensation shouldn't be a problem?

As you have realised, the one significant risk is if the cavity has a gale blowing through it as that will 'bypass' the EWI and greatly reduce it effectiveness. So you do want to look at fixing holes into the cavity, and checking that the top edge is sealed. This report is excellent for understanding that issue: https://aecb.net/thermal-bypass-risks-a-passivhaus-report-by-mark-siddall/

I sealed the tops of my cavities all round (not all at once - over several years). And my cavity was already filled (quite badly, big gaps, with fibreglass bits). The EWI worked very well indeed when only half the cavity-top was sealed, but I don't have air-bricks to let air in the bottom.

Yes, once the house has a nice 'waterproof tea-cosy' on the outside, there is no reason not to fill cavity. Normally people put insulation in and up the U-value a bit, but it could be filled with something else to add to the internal thermal mass (which would make a very stable-temped building as you would have both leaves operating as internal thermal mass).

The air-bricks need to stay so long as you have a suspended timber floor that is not being dried any other way, but make just sure that they only vent into the underfloor space, not into the cavity (just a plastic pipe through is all that is needed).

What sort of professional could I hire to give me the best advice, taking into account the particular nature of this old building, and all the "edge cases" that might be relevant to the decision? I have found companies who will do an energy assessment, but will this be careful and customised enough? I've heard that EPC assessments are often very rudimentary, and I need more than a basic tick-box exercise. Do I need an architect? Some sort of building engineer?

You need someone who understands building physics. EPCs are indeed useless.
Try and find local retrofit professionals who are members of the AECB. Go along to your local group if it holds meetings and talk to them. There are job-titles, and qualifications, for this stuff e.g. 'retrofit designer'. And companies that specialise in it, e.g. '21 degrees' who are yorkshire-based and very competent.

I would advise working out a whole-house plan, before embarking on any one stage of this. Once you have done a Zebra or a PHPP model for the house it gets much clearer how much the EWI, the windows, the suspendedn floor, the airtightness, the roof account for your heat losses, so you can start to make sensible plans about what you ought to do in the long term. I ended up doing new 3G windows and EWI at the same time because it's way better to do both and in that case it makes sense to do the windows first.

The other thing you need to think about is what level of retrofit are you trying to achieve? AECB carbonlite (50kW/m2.a)? EnerPHit (25kWh/m2.a)? Something else? I always advise anyone to aim for at least carbonlite as a good basic standard for 'reasonable building'. If you current building is really using 48,000kWh/yr for a 108m2 footprint then that is a truly appalling 480kWh/m2.a. I think of 300 as 'really bad' so anything you do will be a huge improvement. But presuming you intend to stay for a few years, you don't want to be doing any of this twice, so get a handle on it to start with and make a plan.

I started off aiming for carbonlite, but switched to EnerPHit part-way through when I realised it was possible, which is not ideal. I would have done a quicker and better job, and saved myself quite a lot of faff (it was mostly DIY) if I had realised this up-front.

How can I know whether the EWI installer will take proper care of all the junctions and edges, which I understand are vital for avoiding cold bridges? Do they need to worry about interstitial condensation?

If you employ a good retrofit company they will get this stuff right, but random EWI installers will not. The only way to be sure you get good results is learn enough to tell when you are being given bad advice/a bad job. You have clearly understood the basics pretty well. Now you just need some numbers and some details specific to your building. This forum will generally get you pretty good advice, but at some point you need to get a building-specific assessment, which you can do yourself, but will be a lot quicker and easier (but more expensive) if you get in some professionals who have done it before.