Checklist for preventing mold growth in tiny houses

Ultimately, the success of a mold prevention concept always depends on the weakest point. A predominantly good concept is not enough, which unfortunately makes things quite challenging.

This post is a very detailed – and naturally incomplete – checklist for the construction and planning of a mold-resistant tiny house.

Ventilation/ humidity control

• Demand controlled ventilation (almost) always necessary
  • Exhaust fan in the bathroom above the shower, temperature and humidity-controlled
  • Exhaust hood (may require opening for incoming air)
• Additional continuous ventilation (almost) always necessary
  • Ventilation system with/without heat recovery
  • Minimum solution: external wall vents or window reveal vents
• Washing machine and clothes dryer
  • Both release a lot of moisture during operation, strong ventilation is necessary, or place them directly outside
• Hygrometer
• If required: dehumidifier
• Window ventilation is generally not sufficient

Water installation

• Insulate cold water pipes + water tanks (condensation, especially in summer), insulate hot water pipes (energy-efficiency)
• Frost protection: Make complete draining possible
• Pressure test
• Bath/shower: constructive tightness where possible
• For safety: Water detectors, possibly with an automatic shut-off valve in the supply line
• (I like this:
  • All connecting pieces and connections accessible
  • Walls with heating installation/ washing machine waterproof, underneath tub with drainage)

Building materials

• Mold-resistant materials or paints can provide additional protection
  • However, the goal should be that regardless of the materials used, temperature and humidity make mold growth impossible.
• Open-pored building materials and surface treatments in the interior help to better balance out moisture peaks, but do not change the average humidity

Wall, floor and ceiling construction

• Clarify with a professional, possibly calculate insulation effectiveness & position of the dew point beforehand (e.g. with Ubakus.de). The models have weaknesses.
• Be cautious with recommendations from other climate zones: The direction of moisture transport may change (too with air conditioning in summer)
• Relative humidity depends on temperature and must therefore be measured/calculated directly at the component surface. For the interior and the insulation layer, a separate consideration is necessary due to the different absolute humidity
• As few continuous components in the external wall as possible: Thermally decouple where possible, continuous metal parts are a no-go (exception: supply lines)
• Common thermal bridges include:
  • Constructively conditioned all corners
  • Ventilation, extractor hood, chimney, incoming water/electricity/gas pipes, waste water, power cables to outdoor lighting/sockets,….
  • Windows and doors, door thresholds
  • Attaching of the tiny house to the trailer, e.g. with U-brackets
  • Isolation of the wheel arches (order trailer as a high loader if possible)
  • Everything in order? Plan concrete implementation in detail
• After construction, you can systematically search for cold spots with an IR thermometer or a thermal imaging camera

Inner shell/sealing level

• Airtight construction makes sense from an energy point of view
• Vapor retarders or barriers often useful for mold prevention
  • Use of vapor barriers/retarders (membrane or OSB), climate membrane
  • Use nail sealing tape or seal stapler points
  • Always use the right materials and adhesive tapes that match the system, prepare the substrate (e.g. dust-free)
  • Ensure perfect sealing: Risk with the smallest holes if damp room air can flow into the wall at one point
  • If necessary, surface-mount electrical and water installations or use appropriate boxes and sleeves
• Rule of thumb in North-European climate zone: the structure should become more open to diffusion from the inside to the outside
• With some diffusion-closed structures, there is zero tolerance for errors. A structure that is open to diffusion can compensate for slight leaks under certain circumstances
• Even a very diffusion-open structure can only remove little moisture and generally cannot replace (continuous) ventilation
• Airtightness can be objectified with a blower door test

Exterior shell

• self-explenatory: correct connections (gutter, window sills, window reveals, drip plates, exterior wall openings), tight even in driving rain, standing water, snow, etc.: This is the real challenge, unfortunately you will have to look elsewhere for information.
• Wherever possible, use structurally tight solutions with durable materials that are permanently tight without maintenance.
• If ventilation is required, dimension it sufficiently (size is not considered in the calculation at Ubakus.de).
• Wear parts (e.g. screw seals for trapezoidal sheets)
  • What happens if they are not maintained?
  • The tiny house will warp during transportation and silicone joints can invisibly tear (this may also affect the shower)
• Slight inclination for exterior wall penetrations so that any accumulating water drains outward

Roof, facade & window reveals

• Roof shape has a major influence on the possibility of ventilation, a shed roof is optimal
• Roof windows are weak points
• Rain gutter protects the facade (note width)
• In a ventilated outer wall, ventilate areas above/below the window, e.g., through cross lathing or ventilation slots below the windowsill

Underfloor

• Should the tiny house be moved even in wet conditions?

Pests, mice and insects

• Protect insulation, ventilation and all gaps against pests & nest building
  • Ants, for example, like to nest in rigid foam insulation boards, moths like to eat (untreated) sheep’s wool

Heating

• How/where is heating done? Are there cold corners/rooms?
• Is there sufficient air movement/convection?
• Does the heating affect the humidity?
• Frost protection or complete drainability for water-carrying heating systems

Furnishing

• Do not place the mattress on a slab, use slatted frames
• Ventilate furniture & installations below and behind
• Curtains can amplify existing weaknesses due to reduced air movement

Maintenance

• Check the roof, facade, shower (joints) and water installation at least twice a year
• Silicone joints age and must be replaced regularly

Construction

• Building defects are often only discovered after a long period of time: If you cannot assess the implementation yourself: Ask experts during planning/construction
• I found the considerations helpful:
  • What happens if water gets behind the first “layer” (outer facade/behind the window sill)? Over the years, this will happen
  • What will the tiny house/component look like after 30 years of use? For example: Will the aluminum vapor barrier in the floor possibly tear?
• A seamless construction documentation (photos & protocols) simplifies error detection, improvements, and repairs.

To be continued! Write me an email at mail@hauspilz.de if you are missing any points.