Tips for French Drains

Drying out your home starts at the foundations

Moisture is always on the move in solid walled buildings. We are constantly asking our houses to cope with water from cooking, breathing, rainfall, soil etc. Using nature to help us cope with this is the best way of managing these pressures.

One of the ways of reducing the pressure on walls, especially around the ground, is to give the walls an opportunity to release the water before it starts to cause internal damp issues. Ensuring that you have at least 150mm (6 inches) between the internal floor level and external ground level is really important. If your house doesn't have this then you would be well advised to provide a well drained area around the wall and foundations.

Putting large stones / gravels around the base of the wall (especially when combined with a perforated drainage pipe) has two effects: 

1. It allows rainwater to drain away more easily into the surrounding soil / drains

2. It allows air to circulate next to the wall and hence help dry it

So it is a good idea to have a drain around a solid walled building (as long as it doesn't affect the foundations). However, digging this out is a bit of a job and the last thing you want is to see it get compromised by an influx of small particles (dust, soil etc). This will just slowly clog up the system and eventually reduce the effectiveness of the system. So is there a way of stopping this from happening?

Thankfully the use of commonly available geo-membranes can help to stop dirt from getting into the system. So after you have dug out the trench (and potentially installed a perforated drainpipe) line it with some garden geomembrane (we would recommend a heavy guage system) so that the channel is lined along the bottom and up the earth side of the channel (allowing enough over to cover the channel and up the wall 10cm once filled). Then fill up the channel with large stones / old bricks etc almost to the level that you want. Once done then fold the geomembrane over the stones. The membrane is then held in place with smaller gravel that is spread over the top.

By having a slight lip on the geomembrane next to the wall means that if you need to do something to the drain then you can pull this back and not risk dropping the chippings into the channel itself.

Happy digging!

From grey wood to great wood

From Waltons Shed blog - thanks

It is at this time of year that we are out enjoying our gardens, but many of us are sitting on tired looking furniture. UV light degrades wood slowly but surely and as part of this process it creates greyed surfaces on our tables, chairs etc.

Getting the furniture back to looking great can be important, both for looks and for its longevity. However many of these types of products are not very eco-friendly as they tend to use acids to strip back the top layer of the wood to reveal the less effected wood underneath - therefore refreshing the look.

Osmo PowerGel is one product though that is eco-friendly (it uses oxalic acid that is found in rhubarb leaves). This is a fully biodegradable product and has also been designed to be easy and safe to use. A video from Osmo can be found here showing the Powergel in full flow!

Eco Home Centre sells the Powergel and also the various Osmo wood oils that are suitable for furniture. This range of products will help keep the furniture looking great but also has the great advantage that when reapplying the oils in future years you don't need to sand the surface back. Osmo works with the wood rather than sitting on top like lacquers and varnishes. This makes it much easier to live with and so will help reduce maintenance of your now great looking garden furniture!

Internal Wall Insulation on Solid Walls

This is a typical industry standard way of insulating solid walls internally (IWI). Use a high performance insulation board (or between stud) with a vapour control layer. This appears to give a great new U value (better insulation) for the wall and also deal with damp.

The implication is also that the more insulation that you put on the better the wall performs thermally. This is a quick and easy solution for cutting carbon emissions, surely?!

Well, if you have read my other posts on U value calculators and damp predictors then you should know that there are several alarm bells ringing.

1. The existing wall is probably performing around 70% better than you expect.
2. The accreditated insulant and system has only been assessed for water vapour movement, NOT liquid movement

Work by Dr Caroline Rye and Jo Atkinson (nee Hopper) also highlight some more issues.

Dr Rye's in-situ testing (i.e. measuring what is really happening) shows that the movement of water in the wall can be seriously affected, so much so that by applying non-breathing insulations with vapour barriers etc can actually pull water into the wall. The 'dew point' is also brought further in the wall and the chances of it happening are increased. This means that it is more likely that water will be sitting around delicate areas of the wall like joist ends. This is a major concern.

Jo's work (and some new work that is currently on-going) also shows that poor workmanship of insulation can lead to thermal bridging. This thermal bridging was mean that levels of insulation beyond a certain level become fairly meaningless. Evidence seems to be suggesting that any more than 60mm of conventional insulation on internal walls is fairly pointless as the heat will just escape through the thermal bridges. So more is not always best!

So IWI has it's issues. Is there an easy answer then?

Well there certainly appears to be an easier way of maintaining a much healthier wall. This is to use a breathable wall insulation (without a vapour control layer) with a breathable finish over the top (a lime or clay plaster finished with a breathable paint for example). This advice translates into something like using 50-60mm wood fibre boards and ensuring that reveals are also insulated as much as possible (this tends to be a 10-20mm board as a maximum due to space constraints).

By maintaining the flow of water vapour through the walls from inside to out allows the wall to operate as it was designed. It also allows liquid water to show up, so if there is a problem with a leak, rising damp, etc then at least you know about and can fix the problem before it starts to affect the wall adversely.

By having a slightly 'less efficient' system for the wall allows a little more heat into the structure and this means that the potential for any dew point is reduced to within very safe levels and also kept closer to the outer elements of the wall and hence away from joist ends etc.

So even though on paper this type of solution appears to be the worse option for carbon savings, actually in the real world it is the best option. It will give you thermal comfort, a healthy internal environment, natural humidity control, a safe structure, visible contact with the wall and also have less embodied energy in the products.

CADW's Heritage Cottage

This is great find for Wales - an almost untouched period terrace from 1854. CADW have purchased the house and are doing a lot of research on the property that will help inform how we should all look after our terraces in Wales.


CADW are looking at how the house functions now and then investigating how to make it more sustainable for the future. So things like insulation, glazing, draught proofing, damp, low energy heating and lighting etc.


The great thing is that the figures that they will be using will be the correct one! It is not being assessed by the blunt instruments like EPCs, they are taking real readings like in-situ U values, thermagraphic images etc. The lessons that will be learnt from this will be really important as they could and should inform us all about what interventions are better for this type of solid stone walled house in the climate / weather that Wales enjoys.

As a starter for ten CADW have produced a short You Tube video about the project. Well worth a few minutes of your time if you own a Welsh terrace!

What are breathable walls?

From http://www.ewht.org.uk

I have come across a few people who when talking about breathable house structures state that they keep their windows open a bit and the chimney helps. The term breathability / breathable does conjour up images of lungs and hence the passage of air through a wall. However, what we are talking about is moisture transfer.

The mainstream industry narrows this down to the transfer of water vapour. However, when looking at older solid walled buildings we actually need to think of breathability as the transfer of moisture. This means water as a liquid as well as a vapour. Anyone with an old house will recognise this as being a major difference and one that can cause a host of problems: After all rising damp, driven rainwater ingress, etc are liquid water issues.

So in order to make good decisions we must accept that for older houses we must think of breathable walls as one that allow the movement of water through their structure.

The general rule of thumb (even for modern timber framed buildings) is that the outer elements of a wall should be between 4 and 5 times more breathable than the inner elements. This means that a gradient of moisture transfer is maintained in the structure where water is encouraged to move from the inner elements to the outer ones. By allowing water to pass through to the outside keeps the inner elements free of damp. The outer elements are allowed to get wet, but the thickness of the structure means that any external water should remain in the outer elements (issues like poor maintenance can still affect the workings of breathable walls).

Materials have different porosity's and hence understanding how 'breathable' they are is really important. Other factors like surface area, orientation, shading, climate are also important, but the breathability of building materials is key. I shall not go into detail here, but understanding the principles of breathability is vital to making informed decisions over renovation / refurbishment work.

If we close off the breathability of a wall by using materials like: insulated plaster board, cement renders (especially those with water-proofing in), conventional paints, we are starting to interfere with how it works. These modern materials are designed to be waterproof (even if they have a degree of water vapour breathability). So if we apply them to the outer elements of buildings we are asking for trouble. The way out for water from the inner elements is blocked and hence the concentration of water will start to increase in the wall. This is made even worse and potentially dangerous when liquid water is not stopped from entering the wall (e.g. rising damp, wind driven rain, leaks from guttering, poor seals around windows / doors etc). This water just sits in the wall and can start to rot joist ends, transmit heat more effectively, cause mould, ...

Even making the inner leaf less breathable by using conventional paints, modern insulations etc can cause problems. Breathable walls mainly dry to the outside, but water is also given off to the internal environment. By stopping this you will find that paint can start to peel off, that condensation  issues are made worse and ultimately that the moisture gradient changes from internal to external to being external to internal!

Messing with a solid walls' natural breathable state is fraught with hazards. Maintaining the wall's breathability is therefore really important. So don't get fooled by conventional materials that state that they are breathable and suitable for solid walls, they are only using measurements for water vapour. You need porous materials for porous walls.