Stirley Farm EnerPHit: Dealing with moisture

Dealing with moisture

Stirley Community Farm - Yorkshire Wildlife TrustDealing with moisture penetration through the external masonry walls and the prevention of damage to the internal building structure at the Stirley Farm project has been a major concern.

The barn is really exposed to the elements due to the site’s elevation above Huddersfield facing west looking at Saddleworth Moor. I have spent more years than I care to remember as a builder struggling to cope with wind driven rain penetrating the rugged looking – but rather porous –  locally quarried sandstone that has been used in this area since the industrial revolution. (The somewhat hardier gritstone of the area had already been exhaustively over quarried by the time Victoria came to the throne).

Sandstone masonry walls at Stirley Community Farm

Sandstone masonry walls at Stirley Community Farm

We need to design the project to minimise the effects of wind driven rain penetrating the masonry wall. There are a number of measures that are standard good practice which help  reduce the impact of driving rain in the first place, such as good drainage (e.g. French drains), keeping rainwater guttering in good order and ensuring lime pointing is to a good standard etc.

We also have to deal with the possiblility of ‘solar driven moisture’. This is a well-documented issue for unventilated cavities where, in the summer months, a sodden wall can dry on the outside and water vapour can be driven inwards through the masonry wall. This could lead to the build up of moisture within our inner timber structure, causing potential catastrophic failure.

There have been recent problems in a Belgian Passivhaus, where there was too much moisture build up in the building due to construction with a poorly ventilated cavity.

http://www.greenbuildingadvisor.com/blogs/dept/musings/belgian-passivhaus-rendered-uninhabitable-bad-indoor-air

WUFI analysis for Stirley Farm project

WUFI analysis of ventilated cavity at Stirley Farm

WUFI analysis of ventilated cavity at Stirley Farm

In our case, at Stirley Farm we are going to ensure that the cavity between the masonry wall and internal structure is well ventilated. We are very grateful to Niall Crosson of Ecological Building Systems who has given us some expert WUFI analysis on our Stirley Farm project. His analysis shows that moisture levels would rise rapidly if the cavity between the old structure and new timber cavity was unventilated. However, with the ventilated cavity that we are planning, the WUFI graphs show that moisture levels are within acceptable parameters.

Stirley Farm unventilated cavity  WUFI analysis – download pdf

Stirley Farm ventilated cavity WUFI analysis – download pdf

I’m very happy about this as the WUFI analysis backs up our design approach at Stirley Farm and gives me confidence that we will avoid moisture problems at the project.

On site

A lot has been going on onsite, including substantial work on the foundations etc. I will write another blog soon including a CAD section of detailing around junctions etc. On Friday there was a very enjoyable launch event at the farm which showcased the project to various local dignitaries, including Barry Sheerman MP for Huddersfield and representatives from Kirklees Council. Yorkshire Wildlife Trust’s approach to both low carbon buildings and farming was very well received.

Bill Butcher, Director, Green Building Store www.greenbuildingstore.co.uk

Free resources on the project are available at: www.greenbuildingstore.co.uk/enerphit

5 responses to “Stirley Farm EnerPHit: Dealing with moisture”

  1. Neil Allen says:

    Hi Bill,

    Following the progress on Stirley with interest, and have reached similar results re the ventilated cavity on one of our own projects. The 64,000 dollar question is however, what constitutes adequate cavity ventilation, from our research, that stated in UK regs is wholly inadequate, and the swiss regs should be looked to. These are substantially greater. Be good to hear your latest thoughts on this aspect of the solution.

    Regards

    Neil Allen

  2. Bill Butcher says:

    Neil,

    To be honest I do not know. Can anyone else help? I’ll need to sort it out soon!

    Bill

  3. Will says:

    Really interesting work and great blog.

    Suppose you could work out a rough worse case with how much air flow is needed to carry the max WUFI calculated moisture away at 80% odd external Relative Humidity. Would be big over estimate though I’d have thought.

    Following this with interest as Brighton seafront has similar driving rain issues to exposed moor near Huddersfield – if with different materials.

    Will

  4. Junko Suetake says:

    Hello Bill,
    While we wait to hear from Swiss, I looked up some Japanese research. The vapour pressure (in Pa) in the cavity was stabilised in 10 minutes to 40 minutes, depending on the width and height of the cavity and the opening ratio of the vents. But the smaller vent results in higher vapour pressure as stabilised.
    They found a bigger difference in the vapour pressure between 40mm and 30mm than between 30mm and 20mm of the cavity widths. Not sure why.

    Assuming you may have got a good 100mm cavity, and narrow or small vents at the bottom, but a wider opening at the top???
    Best wishes,
    Junko

  5. Simon McGuinness says:

    I have specified air bricks at low level to achieve ventilation of the cavity and stabilise the moisture balance of the masonry wall. However, I am also persuaded of the need to close the capillaries of the outer face of the wall in order to prevent excessive rain wetting and reduce the potential for frost heave. I specify a vapour open sealant where there is no render, dash or limewash finish. Porous brick, particularly on northern elevations, is especially at risk.

    Frost heave is also a problem if the external wall has inadequate capping or roof overhang or if ground moisture causes rising damp.

    Ventilating the cavity, whilst addressing moisture balance, tends to reduce the temperature of the wall making it more likely to suffer frost heave. The wall becomes essentially a garden wall exposed to freezing air on both sides.

    Farm out buildings tend to be occupied by animals in the worst winter weather, effectively adding an internal heat source. This may have been sufficient to raise the temperature of the wall by a few degrees, sufficient to prevent it freezing. Passive standard internal walls and ventilated cavities may conspire to prevent the outer wall temperature being raised above the critical 4deg C.

    For historically significant construction it may be wise not to insulate to Passive standards, if at all, and to provide continuous low level heating from renewable sources in order to minimise the risk of damage to the fabric. The discredited electrostatic damp proof course may actually be a good way to introduce low-level (in both senses) heating to the most vulnerable part of the wall, although I have not employed this solution myself as yet. It would be reasonably easy to power this resistance cable from a suitably sized PV array located some discrete distance away from the historic structure.

    Another point to be considered is the need to subdivide the ventilated cavity with suitable cavity barriers to prevent fire spread within the cavity. The codes on this vary from country to country.

    – SMcG.

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