MVHR insulation

I was in a meeting for the Existing Homes Alliance Cymru and we had a good presentation on the Project CALEBRE research from Loughborough University. One of the issues that they had in their work reminded me of another couple of cases I have seen with the installation of MVHR in homes. MVHR is Mechanical Ventilation Heat Recovery, so a system that ventilates your home with fresh air, but does so by using the warm and moist extracted air to pre-heat the incoming air.

One issue is that of insulation. In the Loughborough case, and in the two I have seen, the insulation was really poorly fitted around the ducting. It was there, but the corners were not covered properly and the pipe lengths were not wrapped up and sealed properly. This of course means that the warm air that you need to pre-heat the incoming air was not as warm as it could have been and also the then pre-warmed air was losing some of its heat in the loft space. So take care when having MVHR installed and make sure that you check the quality of the installation afterwards.

Two other issues emerged from the Loughborough study that are also really important.

1. It was found that the system was not calibrated correctly for the house as the installer did not check this until they were called back to sort the insulation problem. So it sounds as if this is not necessarily done as a matter of course. So ask your installer to check the settings once the system is up and running (with the insulation correctly fitted).

2. The cost savings that can accrue from using a MVHR do not kick in until you have a really airtight house. So unless you have an airtightness level of around 3 cubic metres per sq m per hour or better you will not get any carbon benefit. Having spoken to Ventilation companies they tend to state that MVHR can be installed in houses with an average 7 - 10 cubic metres per sq m per hour. Whilst this is true they can be fitted, it will not give you any cost or carbon benefit, so it is not worth while. Most new houses are built to a standard of 10 cubic metres per sq m per hour and so using MVHR in retrofit is really tricky to get right.

Walls adjoining houses - a source of damp

Many homes have garden boundary walls that butt into the main external wall of the house. The continuity of the materials can look really attractive, especially where it is an old stone wall. There can be complications though with the junction between the two structures.

We look to keep our houses dry and free from damp, but we do not expect the same from our walls. So we tend to turn a blind eye to the water that seeps into the garden wall, whilst desperately trying to keep water from entering the wall.

This has led us to applying waterproof render to our house walls and nothing on the boundary wall. The trouble is that if there is no capping on the garden wall, water will seep into it and this can then be transferred through the junction of the two walls and thence behind the water proof render.

This is normally 'cured' by injecting silicon vertically into the house wall, but it stands little real chance, especially in stone. So what to do?

Well, ideally your house walls are treated with lime render and so can breathe and let the water out, however this is making the lime do a lot of work. The best two scenarios is that you either reduce the amount of water entering the garden wall in the first place, via a simple capping system; or that you create a physical gap between the two walls.

If you choose the latter then be aware that the house wall would be giving the garden wall some inherent strength and stability and so this will probably have to be replaced by the use of a buttress type support in order to stop it all from falling over!!

Sagging roof?

One of the great joys of thinking about buildings is that you tend to remember to look up. So many of our high streets are regarded as being the same these days, but a glance above the signs that carry the all too familiar names and you will find a wealth of architectural interest. Keep going up and you will notice that many of our older houses have one of two main roof coverings: slate or tiles.

When the buildings were originally designed and built they would generally have been covered by slate. Most of this came from the quarries in North Wales of course and the history of this industry still covers large swathes of Snowdonia National Park.

However the modern day knock on from this is as follows. The roofs were originally designed and built to take the weight of this material. The industry, of course, does not stand still and soon cheaper alternatives became available in the form of cement and clay tiles. As with most things in the industry these cheaper alternatives were quickly taken up and applied to roofs everywhere. Unfortunately this has meant that older roofs, that were designed for slate, find themselves with the much greater task of supporting these heavy tiles. It is therefore quite common to see roofs bend and buckle under this weight. So if you are looking to replace a roof please think about the weight of the structure as well as the look. Note that if you do have a sagging roof, you will also need to check for rot in the timbers, damp ingress, movement in the wall plate, wall movement etc.

Thankfully, if your budget doesn't stretch as far as real slate there are a wide range of artificial slates that are available now that look like the real Welsh slate, but are in fact made from recycled materials.

Chemical damp proof course holes

As you may have gathered by now I have an issue with damp proof courses in older walls, however they do have their place and one of these is in brick walls. Here damp proof injections stand a chance of working as long as they are applied correctly. The silicon creams can flow into and along the mortar layers (in lime mortar) or into the bricks (in cement mortars).

The process for this is to drill out a hole for the injection gun and then to inject the cream or liquid into it. The treatment will then go into the structure and create a water proof layer (well in theory, there are some issues here with how totally effective it is, but that can wait).

All well and good, one might think. However I have seen time and time again a major flaw. Contractors, having created this water proof layer, are then meant to back fill the hole with a sealant (this might be cement, lime, plug or a silicone filler) but so many do not. This this effectively creates a system where rain water is allowed into the structure above the water proof layer! How ridiculous is that?

So if you do have a damp proof course injected into your wall, just check that the holes have been back filled. Also because most often they use cement to backfill this can often be pushed out of the hole as it is incompatible with the underlying mortar or brick.

Paths and damp

One of the most common forms of damp in buildings can be quite difficult to diagnose. It revolves around floor levels.

If your external floor level is higher than your internal floor levels it can bridge across the damp proof course (if you have one) or if you have a solid walled buildings then the damp soil / path can help to trap moisture in the wall.

This can be a result of historical additions to the outside of the house. Often the height of the external ground has been increased by things like:

Installation of pathways
Pavements being re-laid on top of existing surfaces at an increased height
Raised beds being placed against walls
...

Is there a way out of this? Well the only real way of stopping this transmission of water into a wall is to remove the problem. This means lowering the external ground level by the wall. Ideally you need a minimum of 150mm / 6 inches of difference in height between the internal floor level / damp proof course and the ground level outside.

This can be mean digging out a trench around the house to remove the excess soil / pavements etc. If this causes a major issue externally then you will need to install a drain around the walls. You will need to backfill this with large stones so that there is adequate drainage for water to be taken away from the wall and also for air to circulate around the base of the wall and help to keep it dry.