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Thursday, 13 December 2012

Mould at ceiling edge

I have seen a lot of mould at the edges of ceilings, especially where the wall is an external one. So what is the cause of this?

There are a number of potential suspects:
  1. Lack of ventilation
  2. Water ingress from leaky roof / blocked gutters
  3. Lack of insulation
1. It is unlikely to be lack of ventilation, although poor ventilation in higher humidity rooms (bathrooms, bedrooms, kitchens) can make the situation worse.

2. Water ingress can be another source of damp and hence mould, but here the mould tends to follow the water, so will head down the walls as well. It is worth noting that gutters should be checked regularly to ensure that they are working properly.

3. Lack of insulation is the probable culprit here. Cold spots caused by a lack of insulation makes the surface cold enough for the mould to grow nicely. Insulation, especially in the eaves, is notoriously poorly fitted. It is already the area in the loft that has the lowest level of insulation due to the junction of roof and ceiling, but it is also tricky to fit, so many people find that it has not been done properly.

So we would recommend having a look in the loft space and checking that the ceiling is uniformly covering the it. I would also ensure that it is as well insulated as possible in the eaves (whilst remembering not to block them as this could cause other ventilation issues for the loft space).

Once the insulation is correctly installed, treat the mould with Auro Anti-Mould Treatment and then you should have a permanent solution.

As an aside, I once found a mould patch in the middle of a bedroom ceiling. No leaks etc, just a odd spot of nasty mould. A quick trip up into the loft found that the insulation had been installed really badly and there was a bald patch that correlated exactly with the mould in the room. Thankfully there can be easy solutions to what can appear to be major problems with houses!

Thermal Mass or Lightweight Build?

Timber framed houses are regarded as being low in thermal mass

Stone houses are thought of as high in thermal mass
Thermal mass is a simple idea but is actually complex in nature. The general idea runs thus:

Lots of high density materials (like stone, brick, concrete) can even out the temperature in buildings as they are able to absorb high daytime temperatures and then slowly release this stored heat back into the building during the cooler evenings. So buildings, made of these types of materials, will naturally and automatically create more constant diurnal temperatures for the occupants during the summer. In the winter these same materials will require heating from an external source (central heating for example). It will take them longer to heat up, but once warm they will remain warm and again help to regulate the heat through a 24 hour cycle.

Lots of low density materials will not have the capacity to 'store' excess heat and so light weight buildings tend to heat up and cool down quicker. So in the summer they cannot absorb the suns heat as well and so they are not so warm in the night time. In winter they will heat up quicker, but again cool quicker.

This all sounds quite straight forward. The larger the amount of heavy heat absorbing material the more constant the temperature in a house.

So in theory if you are planning to stay in a house for most of the time (working from home, retired, etc) then building or buying a thermal mass based house could be a good thing. If however you are out of the house for most of the day and only use the house in the evenings, then a light weight house seems more appropriate.

However there are some complicating factors, most notably:

Where is the thermal mass? - Does it get the passive solar gain from the sun, or is it shaded
Is the habitable area in touch with the thermal mass? - New buildings might be made of brick, but this might be separated from the internal environment by layers of insulation, so effectively the only internal mass might be some plasterboard.
How airtight is the building? A low thermal mass house might be very airtight and so able to keep some heat in, whereas a high thermal mass house might be very draughty and so lose much of its gained heat because of this.
How well insulated is the building? Again high thermal mass that is made from a good conductor of heat can take more heating in the winter due to a lack of insulation.

The picture is therefore more complicated. Life, eh!

When buying a house you don't have a choice over what it is made from. But it is worth bearing in mind that older pre-1919 solid walled homes will have a higher thermal mass than a modern house. This is borne out by experience of many where older houses are cool in the summer and require constant trickle heat in the winter, whilst modern homes can be too hot in the summer and only require short periods of heating in the winter.

There are therefore a series of advantages and disadvantages to both systems. But could we create a house that has just the advantages?

In the UK we almost make our new homes inside out with regards to thermal mass. We tend to put the thermal mass on the outside (bricks) then insulate it from the living space and then line out our homes with a thin thermal mass product like plasterboard. We are also generally very poor at making airtight homes and also at managing ventilation. So imagine a house that has a light weight external wall, lots of insulation and solid brick or block internal walls. High thermal mass floors located in sun areas can also act as thermal collectors of heat, whilst those that are not could be lighter weight, well insulated and airtight. Large windows to the south will allow the sun in to heat the thermal mass and small windows to the north (that only allow heat to escape) can be smaller and better insulated (triple glazed).

Passiv Haus houses are good examples of where these types of ideas are used. They are also very airtight and use heat exchangers to minimise heat loss through ventilation. All very good. There are a few issues with them though that require a change in living practices in the UK - all rooms the same temperature, using the ventilation system for fresh air in the winter rather than opening windows etc.

So if designing a new home, it is worth thinking about what materials you use, where you use them, why you are using them and how to use them.

If you refurbishing an older house then be mindful to keep the thermal mass working for you. Higher thermal mass insulation like woodwool can be useful, but be careful not to isolate the thermal mass behind structures like insulated dry lining as can lead to a host of other problems. Never easy is it!

Monday, 3 December 2012

Damp internal walls

How is this possible? Damp on an internal wall? - So no water ingress from blown render or poor pointing, no leaking gutters, no cracked tiles, no raised pavements above damp proof course, so what is going on?

Internal walls in older buildings were not built with very good foundations or very good damp proof courses. Some had slates installed, others bitumen, but many have nothing at all. The basic idea for these old houses was that they either had suspended wooden floors, or solid packed earth floors with quarry tiles. These coped with damp is different ways.

The suspended wooden floors were well ventilated so that the moisture from the soil would be 'wicked' away by the draughts. The solid floors were covered with a breathable covering thus allowing moisture to slowly evaporate into the house itself.

Both of these systems was helped by having breathable walls - the bricks were covered with lime plasters and breathable lime paints, so again any moisture in the walls could be dispersed into the internal atmosphere with little fuss.

So why are we seeing problems now?

There are a few reasons:

1. We have replaced many suspended floors with solid concrete floors. This process involves laying down a damp proof membrane under the floor and wrapping this up against the internal walls and filling the floor area with insulation and concrete. This means that any moisture in the ground can no longer come up through the floor. Sounds good. Unfortunately, this only puts extra pressure onto the internal wall. Any moisture is effectively concentrated into the internal wall foundations and hence the walls become wet through capillary action.

2. We have blocked up the vents under existing suspended wooden floors. This means that the moist air above the soil is not wicked away. This allows moisture to build up under the house and hence the walls can get wet.

3. The internal walls in our homes have often been replastered over time. Old lime mortar has been removed and more modern gypsum plasters used. These gypsum based products are not designed to survive in a moisture rich environment. So any sustained moisture in a wall will cause the plaster to fail. This in turn means that the plaster starts to become hydroscopic - attract more moisture to it. A self fulfilling failure system is hence in place.

So are there any solutions?

1. For replacement solid walls it is a bit late really as the pressure on the walls is so great. However, you can inject damp proofing silicone creams (Eco Home Centre sells DryZone) or install a new physical damp proof course. This will help to reduce the amount of water that is able to reach the living area. Care also needs to be taken to ensure that the plaster used is breathable and also that there is a gap between the floor and the plaster (normally hidden behind the skirting boards). I would also ensure that the skirting boards are treated against moisture so that they do not rot from the back.

2. Unblocking the vents and ensuring that there is a good draught under the floor should allow the walls to dry out.

3. Using breathable renders and plasters internally (predominately lime based) will allow moisture to travel through it without harming the finish (note that paint finishes need to be breathable as well - lime wash, claypaint and natural emulsions are all possible).

Choosing the right solutions can be difficult, so please contact the Eco Home Centre for more guidance if required.