Fabric Retrofit in a 1930s Semi-Detached House

Fabric Retrofit in a 1930s Semi-Detached House

In this article I want to walk through the individual fabric retrofit projects we have carried out over the years and how they have affected the heat loss of our house.

These improvements sit alongside our wider renewable journey with solar, batteries, an EV and a Vaillant heat pump. But it was the solar installation back in 2018 that really kicked things off. The gateway drug, if you like.

You can track the performance of our solar PV array right back to 2018 here: My Solar Generation History

None of this happened in one go. It has been incremental. Small improvements made when a room was being refreshed or work was already planned. The renewables followed the same pattern, added at different points as our understanding and funds allowed.

Table of Contents

Use the Table of Contents below to jump straight to the section that matters most or scroll through at your own pace.

Property Background

We have lived in this house since 2005. It is a 1930s semi-detached property in Sheffield. When originally built it was two storeys with roughly 65 square metres of floor area.

Construction is traditional brick, inner and outer leaf with a 60mm cavity. The bricks themselves are incredibly hard. Any tradesperson who has tried to drill into them has commented on it.

I suspect it was originally a two bedroom house. Over the last 25 years it has become a three bedroom property, first by reconfiguring internal stud walls and later with a loft conversion. A single storey dining room extension was added to the rear around the year 2000.

Total floor area today is about 90 square metres.

For context, we now run a 5kW Vaillant Arotherm heat pump. Measured peak heat demand is around 4kW at minus 3 degrees outside, based on our OpenEnergyMonitor data.

Introducing Spruce

Spruce is an online heat pump design and modelling tool that allows you to calculate room by room heat loss, size emitters, and build a whole house view of how a property performs. It also allows installers to combine enquiry handling, heat loss surveys, system design, estimates and paperwork into a single workflow.

I used the design section of the app to model my entire house. I inputted the construction details, fabric elements and their U values (walls/windows etc), along with the existing radiators in each room. That provided a full house heat loss calculation and an emitter survey that I could use as a consistent reference point.

One of the things I found most useful is how easy it is to test changes. You can adjust a single element, such as upgrading a wall or changing window performance, and immediately see how that affects the heat loss of an individual room and the property as a whole.

For example, if you are considering something like triple glazing, a few clicks will show you how much difference it would actually make by changing that single window. That makes it far easier to sense check retrofit decisions and avoid assuming a change will have more impact than it really does.

For this article, Spruce has been particularly useful in understanding how much the fabric improvements made over the years have reduced heat loss, and just as importantly, where the biggest gains actually came from.

Windows and draughts

The house already had standard double glazing throughout when we bought it. I have replaced a few windows since, but always like for like.

I think the bigger gains have come from dealing with draughts.

A thermal camera is excellent for this. It quickly showed cold air movement around skirting boards, floors and window frames. You can also do a lot with nothing more than your hand on a windy day.

Where I could, I sealed these entry points. Nothing over the top, just stopping uncontrolled air movement.

The dining room as built

The dining room extension was already in place when we moved in. It is a single storey rear extension with a suspended floor, pitched roof, two side windows and large patio doors to the rear. It sits on the north side of the house and gets very little sun.

The walls have insulated cavities, likely rockwool. The ceiling and floor insulation were to early 2000s building regulations standards, which by today’s measures are poor.

You can see how much recommended U values have changed over time in this article:
https://www.ecomerchant.co.uk/news/a-brief-history-of-building-regulation-u-values-with-examples

We will come back to the improvements made to this room later.

Loft conversion frustration

The loft conversion was added around 2010. We lost the small front bedroom to make space for the stairs but gained a much larger bedroom in the loft.

With what I know now, I get frustrated seeing what the architect asked for and what the builders actually delivered.

The specification clearly called for insulation levels that would have been decent even today.

For example, this is what the architect specified:

• 100mm PIR between rafters plus 52.5mm insulated plasterboard below to achieve 0.18 W/m²K

• 70mm PIR in dwarf walls and dormer cheeks plus a further 20mm over the framing, with taped joints

None of this work was completed properly. The secondary layers were missing, nothing was taped and no real care taken with the insulation.

Despite this, everything was signed off by building control.

These photos show what I found when I took the plasterboard off years later.  What a bloody mess.

If you are having work done, I’d highly recommend you try and understand the architect’s specification and make sure the builder follows it.

Loft mini improvements

The photos above came from a small eaves cupboard / store room at the side of the loft bedroom. Because it was hidden, it became my first low-risk experiment with insulation, foaming and taping. It was messy and involved a lot of expanding foam. LOL.

I added insulation where there was none, replaced damaged pieces, foamed gaps and taped all joints before reinstalling plasterboard.

It was a small area so the heat loss saving is hard to quantify, but it taught me some techniques that became very useful later.

Cavity wall insulation

Between about 2005 and 2008 there was generous government funding for cavity wall insulation. I took this up through my energy supplier at the time.

Mineral wool was blown into the cavities via drilled holes. I think bonded beads tend to be preferred these days?

Only the original external walls of the main house were filled as the extension already had insulation and this was pre-loft conversion.

I cannot verify coverage quality, but nearly 20 years later we have had no damp or moisture issues.

If I remove cavity insulation in Spruce, total house heat loss increases by over 1300 watts. Roughly a 25 percent increase.

Cavity wall insulation made one of the biggest single differences to the heat loss calculations of the house.

Front bedroom bay window

My wife always said her side of the bed was colder (she is on the window side).

A thermal camera confirmed large cold areas around and under the bay window.

When we dismantled it, we found nothing inside. No insulation at all. Just plasterboard, air, some timber and the external tiles.  No wonder it was cold.

We rebuilt the inside of the bay window with insulation between internal timbers (leaving an air gap on the outside), taped all joints and added an additional insulated layer on the inside.

We also lifted the floor and added wool insulation between the floor joists to reduce draughts.

Work was finished with plasterboard and plaster along with a new larger radiator (K2) than the one that was previously there.

Spruce suggests this work saved over 100 watts of fabric losses, which is about 25 percent of the rooms total loss.  Which is a decent improvement.

On top of the fabric losses we also have to consider the savings via reductions in ventilation / air movement.

The bottom line is that my wife no longer complains about draughts at night.  Happy days!

Bathroom retrofit

The bathroom was taken back to brick when we decided to install a brand new bathroom. This allowed us to add  internal insulation on the two external facing walls.

Despite the room being small, the space loss in the room was negligible from adding the insulation.

Spruce estimates a 70 watt reduction in fabric losses. Nearly 25 percent of the room’s total loss was removed.

This reduction was enough for the new designer style radiator (Eastbrook Deddington) to run at lower flow temperatures via the heat pump.

Dining room improvements

Even before we had the heat pump installed, the dining room was always the coldest room in the house.

Which makes sense as it is a single storey extension with 3 walls and a pitched roof to the outside world.

When it came to thinking about improvements, external wall insulation was never a practical option and we had already missed the chance for internal insulation when we fitted the kitchen a long time ago.

That only left the dining room ceiling and floor as options.

Ceiling

As per the 2000 era building regs, there was around 100mm of mineral wool in the ceiling.

To make things better we first had to make things worse, so we pulled the entire ceiling down.

We installed 75mm PIR between rafters, foamed and taped all joints. The mineral wool was reinstated above, maintaining ventilation at the eaves. A further 25mm layer of PIR was added below the rafters and again, fully taped.

The work was completed using new plasterboard, plaster and cornice coving.

Spruce suggests only about 70 watts saved in fabric heat loss, roughly 10 percent. But ventilation losses have also been reduced which comes with added comfort.

Floor

When lifting the suspended floor we found polystyrene packing-style insulation that had collapsed over time.  So in many cases this was not helping at all as it was now laying on the ground.

To improve matters we installed 100mm PIR between joists, foamed and taped everything.  Every joint and all the perimeters were sealed.

We also added timber battens underneath to stop the insulation ever dropping.

The work was completed by putting the flooring boards back down before a new flooring was laid.

Spruce estimates around 80 watts saved, another 10 perfect. But again, air tightness improvements likely saved far more as well as added comfort.

Pulling it all together

On paper, fabric savings for all the work total around 350 watts.  Around 10-15% of the house heat loss.

That sounds small for the effort involved, but it ignores ventilation losses. Reducing uncontrolled air movement likely saved a similar amount again.

Remember, Total Heat Loss = Fabric Heat Loss + Ventilation Heat Loss

More importantly, the interventions have transformed comfort. Fewer draughts and more stable temperatures means the house feels better to live in.

Combined with weather compensation heating controls on the heat pump, it has made a noticeable difference day to day.

Fabric Fifth, in practice

Over the last few years I’ve found myself agreeing more and more with the argument Nigel Banks makes around Fabric Fifth, mainly because it lines up with what has actually happened in this house.

Very little of what we’ve done would count as a deep fabric retrofit. There’s no external wall insulation, no triple glazing, and no attempt to make the house perfect in one go. Instead, most of the gains came from fairly ordinary, targeted work. Cavity wall insulation. Sorting draughts. Adding insulation where walls, floors and ceiling weren’t at a great level. Taking a bit more care with air tightness when rooms were renovated. All of this together made a noticeable difference.

In some ways it feels like the basic advice was right all along. Cavity wall insulation, decent double glazing, and a sensible amount of insulation in the loft. That has been the recommendation for years, and it holds true regardless of how you heat your home. These standard measures help whether you are still on gas or running a heat pump, and they form a solid starting point if you are thinking about switching to low-temperature heating.

Crucially, all of my work happened alongside and after moving away from gas, not before it. The heat pump works just fine in a house that isn’t perfect, as long as it’s designed properly and the radiators can deliver the heat at lower flow temperatures. In some ways, having the heat pump in place made it easier to see where the real problems were, which then shaped what fabric work was actually worth doing.

My experience fits well with the idea behind Fabric Fifth. Get off fossil fuels first. Run the system more intelligently, using things like time of use tariffs, solar and batteries. Pick off the low-cost, low-disruption fabric measures that give you a decent return. Only then start thinking about deeper fabric work, and only if it genuinely makes sense for the house, the people living in it, and the money available.

The dining room is a good example. Built around 2000, it always performed worse than much of the original 1930s house. External wall insulation would have been expensive and awkward. Instead, improving the ceiling and floor insulation and tightening up draughts delivered real comfort gains. The fabric heat loss numbers look low on paper, but the room stopped feeling cold and uncomfortable, which matters far more day to day.

Deep fabric retrofit clearly has its place, especially for homes with serious comfort or health issues. But our house shows you don’t have to chase perfection to get meaningful results. Incremental fabric improvements, done at the right time, alongside smart tariffs, on-site generation, storage and low-temperature heating, can get you most of the way there without huge disruption.

You can see how that combination has worked in practice by looking at the long-term heat pump performance data here: https://energy-stats.uk/heat-pump-performance-history/

Summary and take aways

If there’s one big takeaway from all of this, it’s that the basic advice has been right for a long time. Cavity wall insulation, decent double glazing, and a sensible amount of insulation in the loft still form the backbone of a good-performing home.

Those measures help regardless of whether you heat with gas or a heat pump, and they make a solid starting point if you’re thinking about switching to low-temperature heating.

• Make improvements when opportunities arise. Decorating and renovations are good times to think about it.

• Room renovations are always a great time to install larger and upgraded radiators.  These could help you lower the flow temperature of your boiler or heat pump to increase efficiency.

• “Some” insulation is always better than no insulation.  The biggest gains come where there was nothing before.

• Cavity wall insulation is worth serious consideration if your house is suitable.

• Air tightness matters, but balance it with ventilation.  Stopping draughts improves comfort, but homes still need to breathe.

• If you are having work done, make sure builders follow the specification. Drawings and building control sign-off do not guarantee insulation has been installed properly.

• Simple tools like U value calculators are useful for quick checks.  I find this little tool fantastic for showing U values from construction layers.

• Whole house modelling tools like Spruce help you see the bigger picture.

• A good local retrofit assessor is worth their weight in gold. Understanding how your house is actually built can save a lot of time, money and frustration.

• Also seek out local retrofit communities like Carbon Co-op and SY Ecofit.

Ours has been a long, incremental journey. Looking back, every small improvement added up to a house that is easier to heat, cheaper to run, and far more comfortable to live in.