We need to talk about moisture

Moisture causes all sorts of havoc when it accumulates where it’s not supposed to. These are the simple steps you can take to avoid that happening.

We’re all busy, right? We don’t all have time to read lengthy technical articles. But, if something goes wrong in the building fabric it creates major headaches for everyone.

So, we need three minutes of your time to make sure you’re building a moisture-safe construction.

Moisture in a building in the UK
Mould and staining caused by excess moisture in a roof. This looks like a very early failure.

What’s up with moisture?

Moisture causes all sorts of havoc when it accumulates where it’s not supposed to:

  • Increased heat loss; especially in fibrous & organic insulation
  • Rotting of organic material- maybe uncomfortable if its insulation, or downright dangerous if a structural timber rots
  • Growth of mould/fungus; leading to unsightly staining
  • Respiratory issues in residents from mould spores

While an increased heat loss due to moisture is uncomfortable and unnecessary, it is unlikely to be highlighted as a defect in itself. But things get messy, and even downright dangerous, once mould appears.

Three of the four issues listed are caused by mould growth. So, let’s get more acquainted with it.

What mould needs to survive

Of the conditions required for mould growth:

  • Warmth (ideally between 15-25⁰C)
  • Food (Construction materials; insulation, timbers, etc.)
  • Air, and
  • Moisture (greater than 50% Relative humidity)

All are unavoidably present in buildings. However, one we can control is the moisture content.

Moisture, humidity, steam, damp, condensation; Where does it come from?

  • Rain/Ground water (Outside)
  • Wind-driven moist air (Outside)
  • Moisture content of materials during installation (Within fabric)
  • Capillarity of material (Within fabric)
  • Convection; mass flow of humid air (Internal)
  • Diffusion; movement of moisture driven by vapour differences (Internal)

So, leaks from outside can be the cause of the damage. But equally, the moisture could be tracking from inside, or have been built into the fabric from the start.

“By preventing moisture getting into the building fabric, and by allowing any trapped moisture to escape, we can keep the humidity low & the chance of mould growth away.”

Mould in a building in Birmingham, England
Window reveals are often the first place to show mould, due to the thermal bridge here.

Rules for moisture safe construction

Follow these to ensure the best chance of a long, dry & happy life for your building:

  • Install a complete wind proof layer (above ground) or Damp Proof Membrane DPM (below)
  • Airtightness better than 1
  • Use dry construction materials; or allow them to dry before covering
  • No unventilated air voids on the cold side of the insulation- fill them with soft insulation
  • Use an appropriate vapour control layer; carefully installed
  • Design out thermal bridges across the insulation

In flat roofs

We recommend special attention in flat roofs, as we highlighted in the blog post about the vapour control layer SIGA Majrex. Rules for a flat roof:

  • Minimum of 3 degrees pitch
  • Use a dark external layer (black or dark grey)
  • Try to avoid completely shaded flat roofs
  • Ask SIGA for a moisture calculation
  • Or, don’t do it 🙂

Want to learn more?

Earn RIAI credits (if you’re in Ireland) or RIBA credits (if you’re in the UK) by attending one of our airtightness CPD workshops – in your very own conference room, tailored to your needs, your questions, your challenges.

Sounds interesting?

We look forward to discussing the details with you to make the best out of this CPD for you and your team.

Avatar for Bertie Dixon

Bertie Dixon

Bertie has practised building services engineering and sustainability consulting in the UK for 15 years. He is a certified PassivHaus planner. He is on a mission to help you deliver a high-quality building, one that does what it says on the tin.

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