It's a truth universally acknowledged that the importance of insulating our homes is not to be underestimated, but when you have two-foot thick stone walls instead of cavities that can be filled with insulation what do you do?
My partner and I live in a 1700s Somerset cottage, and we knew when we bought it that the house had been completely covered in a cement render, leaving it uninsulated and damp. As a result, it's been very chilly throughout the few autumns and winters we've had in the house, and after braving sub-zero temperatures, we decided it was time to resolve the issue once and for all.
Insulated lime render can be used externally, but we’re using it on the internal walls of our cottage to improve its energy performance, make it warmer and restore the proper mechanisms of stone walls to prevent damp. Not only that, but we’re doing the work ourselves…
What is insulating lime?
Unfortunately with the old single glazed windows and no insulation, the stone section of the house can get chilly, draughty and damp. So we're choosing insulated lime render to solve the problem
Without going into too much of the nitty gritty of hydraulic and non-hydraulic limes and their benefits, lime plaster is made from sand, water and lime (from limestone, not the citrus fruit) and is a traditional method of protecting and decorating older homes. In short, it’s what gives historic homes that wibbly wobbly look.
Lime is also breathable so, when applied to the walls of stone buildings, it allows moisture to wick in and out, keeping the house healthy and preventing the build up of water that can lead to damp.
In the not-too-distant past, there was a bit of a trend to rip out or cover up the original lime render with cement, thinking it was the best way forward to prevent damp and give a cleaner, more polished finish. Unfortunately, this usually lead to houses not ‘breathing’, thus trapping moisture inside and making them even colder and more damp. This is the case with our house.
Insulated lime is a product that utilises fibres, such as cork, wood or recycled glass alongside the lime to improve the energy efficiency of the lime plaster. In my opinion, it’s best applied during a renovation project, like the one my partner and I are undertaking, as it is a notoriously messy job.
There are quite a few different lime render products on the market and we’ve chosen Cornerstone’s insulating render, which claims that that with a 50mm two-coat application, it can improve the U-value (a measure heat transmittance) of a solid wall by 50%. It offers better insulation than standard lime render, which makes it particularly appealing. There is a SuperTherm version that claims a 75% improvement in U-values, but that was out of our price range.
‘There are various different types of insulative plaster, ours work by using lightweight aggregates that trap air within the plaster,’ explains Justine Stephens from Cornish Lime. ‘These aggregates are significantly lighter than standard plasters. The air slows the rate at which heat moves through the wall surface and reduces heat loss.’
Our choice was primarily because we’re not too far away from suppliers Cornish Lime and we attended an excellent one-day rendering and plastering course, which gave us the knowledge to be able to tackle this job ourselves.
Will insulated lime save us money?
Applying insulated lime to our cottage will almost certainly save us money in the long-run, even if it feels like an expensive investment right now.
The cottage section of our house currently only has roof insulation, so any additionally insulating measures will be positive in our eyes! We know from a heat loss assessment done on our home we lose around 45% of our heat through the uninsulated walls, and we've worked out that to insulate with the lime will cost us around £1,600.
In theory then, insulating by 50mm on the external walls will save us approximately 20-22% in heat loss per year, which adds up to roughly £200-220 annually. In just over 5 years, we will break even.
‘There are multiple options for insulation, but when looking at solid stone walls, lime is a much better option for a number of reasons; reduced heat loss, internal surface temperature increases helping reduce condensation, but most importantly you have high vapour permeability and moisture buffering,’ says Justine Stevens. 'Because lime plasters are vapour-open, moisture can move through the plaster rather than becoming trapped (solid walls will always have some level of moisture present). The plaster can absorb and release moisture in response to internal humidity, helping to manage damp rather than exacerbate it.’
How we're applying the insulating lime render
Chipping the cement plaster took so long, but the end result will be worth it
Most insulating limes can be applied directly to existing lime plaster if it's in a sound and good enough condition, but that wasn’t an option for us.
The first satisfying, but exhausting, task for us then was to remove the plaster. This stage took us far longer than we expected as the cement layer was incredibly thick (up to 10cm) in some places. We also uncovered a cast-concrete and breeze block layer which took a ridiculous amount of time to remove. Finally we were left with a bare shell of a room.
If you didn't want to attempt this yourself, then any conservation or lime-specific plasterer would definitely be able to apply the insulating lime render. But if you know a plasterer that tends to work with gypsum, you may need to have a chat first as the insulated lime process can take much longer and needs some know-how regarding application and safety. If you’re aiming for a pristine finish that you would see from gypsum plaster with neat corners and smooth edges, then getting a professional in is most certainly the way you should go. Without a careful hand of experience, the end result might be far from what you’re after.
However, our farmers’ cottage is extremely rough-and-ready — the walls aren’t straight either vertically or horizontally and my partner and I both love the look of our wobbly walls. Also, we’re at the tail-end of our renovation project with still a lot to do, so taking on this job ourselves will save us thousands.
But we also wanted to make sure that we knew what we were doing. To understand what we would need and to get to grips with lime rendering, we attended a course with Cornish Lime that took us through everything from the history and science of lime to the proper application and health and safety. Lime is an alkaline and when it touches your skin or gets in your eye it can cause serious damage. If you’re tempted to DIY like us, I would really recommend a similar course and always wear proper PPE.
Safety goggles are a must if you are dealing with lime, and these anti-fog goggles will ensure you have clear vision while undertaking the rendering work.
Wearing long sleeved tops and full trousers, or overalls is a good idea when working with lime, but you could face exposure on your hands and forearms so these long latex gloves offer good protection.
Dust can get everywhere, but you definitely don't want it getting into your respiratory system. This reusable N95 respirator comes with replacement filters to see you through the project safely.
Between removing the plaster and applying it, we had the cottage rewired and replumbed to accommodate the new bathroom upstairs and better positions of radiators.
Unlike regular lime renders, where you need a scratch coat, float coat and skim coat with drying times in-between each step to ensure longevity, this insulating plaster can be applied in two stages with just a day between. It can then be finished with a fine plaster ‘skim’ coat to create a more polished-looking wall.
We’ve only rendered one room so far, which will act as my home office and a spare room when guests come. Applying the render was super easy, but it took a bit of practice to get a nice smoothness to each wall. Neither of us had plastered before so it was all a bit of an experiment that I’m sure will get better as we progress room by room. It took us around five hours per wall, so about two days to do the entire spare room. I have really weak wrists from a long-standing injury so I had to take frequent breaks as we went along and it was definitely easier having two people as one could be mixing up the next batch while the other applied the coat, swapping buckets as we went.
We can already feel the difference, even without the radiators installed, which is so gratifying and I’m so pleased with the finish we’ve been able to achieve. Only time will tell the actual difference it's made to our bills but, regardless, it’s all better than when we found it, which is something I often need to remind myself when renovating.
This will be a significant upgrade the efficiency of our home, but there's plenty of other less labour-intensive and expensive ways to save energy at home that we're also incorporating into our daily life.