A domestic insulation project. Prelude – why am I doing this?

Hopefully most of the blog posts about this project will be more pictures than words, but below is some of the background to the decisions made and the theories behind them.

The house is in Machynlleth, West Wales, and is an end terrace house over 3 storeys, with its end wall facing north. It was built around 1900, out of slate with lime mortar. The outside is cement rendered, the windows are all double glazed, and there is some limited internal wall insulation (foam board) in certain rooms. The orientation means that there is relatively little solar gain. Heating is from a woodpellet boiler which supplies a wet central heating system. The woodpellet boiler is also a room heater, and there is a standalone wood stove in the study.

The motivation for the current project is thermal comfort. Due to a combination of budgetary constraints, unwillingness to move out during the works and amount of risk I was willing to shoulder, I had no intention of doing the entire house in one go. Instead, the works were focussed on three of the coldest rooms in the house; a downstairs study, a bedroom, and the bathroom. There were small isolated patches of damp in the downstairs study (a couple of patches around 4cm2), more significant damp in the bedroom (a window reveal, and a corner wall), and the bathroom ceiling had a definite tendency to grow black mould in the winter. The aim of the project is simple – to make the three rooms warmer and more comfortable. However, choosing a wall insulation system is not simple, and how I ended up making a decision is described below.

Decision 1 – Internal versus external wall insulation

Solid walls can be insulated on the inside, or the outside. Both have advantages and disadvantages. In my case, the house is an end terrace with a pavement along the end wall, with part of the wall being below ground level. External wall insulation would be complicated by the need for the wall profile to match that of the neighbours, the need to not take up pavement space outside, the need to wrap it around various roof details and extensions to the building, and the issue of some of the wall being below ground. Internal wall insulation brings with it different problems; it is disruptive to the occupants, will reduce the size of rooms and it will often also include thermal bridges – cold areas which are uninsulated (e.g. at the junction between a ceiling and the room above), unless a deep retrofit is undertaken (for example by removing a floorboard in order to be able to insulate some of the potential thermal bridges). Both internal and external approaches can run into problems with being unsympathetic to the architectural heritage of a building; covering vernacular detail such as mouldings, exposed arches and timberwork.

In the current project, internal wall insulation seemed like the most realistic option.  

Decision 2 – Vapour permeability and breathing walls

There are two potential philosophies when building a wall. We can build it such that no moisture penetrates it at all and attempt to maintain this state for the entire lifespan of the building, or we can accept that water may migrate into the wall, and we therefore need to ensure that it can find its way out again. We also need to consider water vapour in the air, as well as the obvious sources of liquid water, such as the ground, and wind-driven rain. The term ‘breathing wall’ refers to the movement of water vapour across the wall, and not to air movement. So it is more correct to talk of ‘vapour open’ (breathing) or ‘vapour closed’ than to refer to walls as breathing. Older solid wall properties are typically designed to be vapour open.

When retrofitting insulation to a wall, in theory we can pick either vapour open or vapour closed. Most modern insulations (e.g. polystyrene, polyurethane) are vapour closed, or are vapour open but installed with membranes to prevent moisture movement (e.g. mineral wool). They have excellent thermal properties. If you are completely confident that no moisture could get into the wall from any source, then you could insulate a solid wall with materials of this type. However, every point where there is a gap in the insulation is a potential source of moisture ingress. Additionally, junction details around eaves, and floors, where electric sockets are mounted, and the joins between the boards would all need to be perfectly sealed in order to avoid trapping moisture in the wall. Even with perfect installation, some walls are always going to have a risk of moisture penetration, e.g. those that are below ground level, or walls where wind-driven rain is inevitable.

But how bad is it to have damp in a wall? A damp wall has a worse thermal performance than a dry wall, but in practice, the main problem with moisture in walls is the increase in degradation risk, principally to timber (e.g. joists, wall plates). Additionally, damp walls create mould risk, and mould is extremely bad for health, particularly for people with pre-existing respiratory conditions. The issue of moisture in walls is also complicated by temperature; air holds much less water vapour at low temperatures, so the temperature gradient across a wall (i.e. warm indoors, cold outdoors) can cause water vapour to condense inside the wall. If this happens in the vicinity of timber, then serious problems can result.  

Given the fact that the house has solid slate walls and is relatively exposed, a vapour-open insulation seemed like the best option.

Decision 3 – Is airtightness important?

It makes little sense installing insulation and then leaving a window open. When expressed like this, it seems obvious that heat loss through direct movement of air (e.g. the open window) is more significant than heat loss through a solid surface. However, insulation without attention to airtightness is extremely common. When added up, all the tiny gaps in the building fabric can easily be equivalent to leaving a window open. It is only because the maths is a bit involved that we don’t calculate this very often.

However, attending to airtightness during insulation projects requires the installation to be pretty intensive (i.e. destructive) in order to get as close as possible to the points that are actually causing the problem, and for work in these areas to be done to a high standard. Unfortunately, it is rare to find properly installed internal wall insulation, and consequently enormous differences exist between the projected improvements in thermal comfort/performance, and those actually achieved.  

Key to airtightness is the junctions between elements (e.g. joist ends, abutting walls, wall-ceiling junctions). In this project, the walls will be stripped back to allow these junctions to be made airtight wherever possible, and I will blog about how these junctions are treated as the project progresses.

Moisture in buildings – why do walls get damp and why does it matter?

It is commonly thought that damp walls are due to moisture penetrating from the outside. There are some instances where this is true; wind driven rain, overflowing gutters causing water to pour down walls, or vegetation growing up walls and increasing local humidity. However, the idea of ‘rising damp’ and the fact that moisture in walls comes from the ground is generally overstated. Much has been written about the dubious practices of surveyors using inadequate tools to diagnose damp (e.g. moisture meters that are based on electrical conductivity) and how this results in unnecessary installation of ineffective injectable damp proof courses. See here for an example.

In practice, many walls are damp because of the water vapour that comes from normal household activities (cooking, showering, breathing). Breathing and perspiration alone results in around 1.5kg of water per person per day. Cooking might add an addition 2kg, and showering another 750g/shower. Behaviour can therefore have a huge influence on how damp walls get; if you have an open plan kitchen without a cooker hood, or you leave the bathroom door open after a shower, or dry laundry indoors, you are likely to add several kg of water vapour to the indoor air. Simple behavioural changes, combined with installation of kitchen and bathroom extract fans can solve many damp problems.

In an old house, we will typically start by installing double glazed windows. This is undoubtedly a good idea, and significantly reduces heat loss, and also improves comfort (more on comfort later). However, water vapour that used to be obvious as condensation on the single glazed window, is now less obvious, and it will now condense on whatever other cold surfaces are in the building, namely the walls. The problem is particularly acute on walls with little air circulation (hence that damp patch behind a cupboard in the spare bedroom, or in window reveals). This means that a damp patch in one room can easily be due to water vapour generated somewhere else in the house; migration of water vapour is unseen. If you’re not convinced about this, consider how frequently delicious wafts from cooking prevail in rooms other than your kitchen – if this happens, then it is obviously true that water vapour can migrate in a similar way.  

There is an excellent animation explaining buildings and moisture in simple terms here.

So what did I end up insulating my walls with and why?

For the reasons described above, I decided to go for a vapour open insulation option, and to insulate internally. From this point, the choice was between rigid or semi-rigid board insulations (e.g. cork board, woodfibre) and cast hemp-lime.

I first saw lightweight hemp-lime installed in Bristol by Neighbourhood Construction. Their approach to hemp-lime differs from other installers in a number of key ways. The feature of most interest to me was that because they used a brand of lime with some cement in it, as opposed to a conventional NHL, the material could be installed at a much lower density than is used by other installers (around 200kg/m3, compared to around 400kg/m3), thereby significantly improving its insulation value. The material sets a bit quicker, and is rigid enough for instant removal of the formwork, allowing a more rapid casting approach than standard hemp-lime installations. As part of my day job with the BEACON project, I worked with Steve Cole of Addasu, Neighbourhood Construction and Hawkland Ecological Construction to test this insulation system, as described here. Part of this involved trying different plants in the mix other than hemp. To cut a long story short, the end point of this is that we are going to use miscanthus instead of hemp in this installation. I will inevitably blog about this particular decision at some point.

Project management and installation: Steve Cole, Addasu.

4 thoughts on “A domestic insulation project. Prelude – why am I doing this?”

    1. In short, no. Much as I would love to, I’m not expecting to get the whole house airtight enough to do this. But I have recently bought a CO2 meter, which is part of thinking in a bit more detail about ventilation (controlled and otherwise!).

      Liked by 1 person

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