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How to Treat Black Mould (Mildew)

The Problems with Mildew 

It is impossible to completely remove mould from your house as every building contains, within its fabric, mould spores. For the most part, in dry conditions, these remain dormant and consequently completely harmless.  However, in warm and damp conditions these spores will activate and form damaging mould.  Black mould, ‘Mildew’, is often viewed as black or dark green spots but can vary in coloration.  This is because mildew draws nutrients from surfaces and produces strong permanent dyes as a bi-product.  Advanced mildew may even develop a furry white appearance.

Mildew will grow on walls, ceilings and most decorative finishes; this not only looks unsightly but the mildew also generates unpleasant and musty odours. Household furnishings, carpets and clothes can also be damaged by the presence of mildew with fibres being weakened and permanent staining occurring.  In addition, and most important of all, since the mildew spores become airborne they pose a serious health risk and are positively linked to causing and aggravating allergies and respiratory problems, such as asthma; this is particularly the case in children. 

As mentioned, typically, mildew tends to develop on surfaces and furnishings in warm, damp and dark conditions were there is limited air movement.  Mildew will stop developing in dry conditions but it will not die and will reactivate if sufficient moisture returns.  Typically an aired house will not develop mildew during summer months but if poorly ventilated and centrally heated during winter the growth may be significant.  Mildew can be denatured ‘killed-off’ by various means including fungicides, bleaches, ultra violet light and ozone. Of these methods only fungicides with a residual effect will provide long term protection.  SkilledBuild offers a selected range of highly effective fungicidal washes, additives and surface maintainers which are easy to apply and of negligible toxicity to household occupants.

Who Can Remove Mildew?

Since mildew mould can pose a real health risk, it is crucial it is removed properly.

Although, typically any home owner can manage mildew as long as they understand the cause of the problem, denature any developed mildew and take preventative action to minimize its reoccurrence. 

However, if significant water damage from a flood has occurred or if the mildew is coming from an air-conditioning system it would be better to leave the assessment and eradication to a professional mould remediation company.


What Causes Mildew?

In order to fully deal with the problem of mildew and eliminate it completely you first need to understand what has created it. If you have a mildew problem in your house, even if you clean the mildew off affected areas, unless the cause is suitably addressed it will return.

There are essentially two major factors which when bought together encourage fungal growth (black mould) and these are poor ventilation and moisture. Without moisture mildew cannot develop.

1.    Condensation

If you have mildew, the likelihood is that it is due to condensation. This is good news as the problem can often be fixed fairly inexpensively.

Condensation occurs because airborne moisture from cooking and gas fires, bathrooms, and even household occupants themselves becomes trapped in our homes and condenses on cold surfaces, such as windows and external walls. Often paints blister and wallpaper lifts, providing ideal surfaces for mildew growth. This problem has become progressively worse in recent years as modern properties have been made virtually airtight and draught free in an effort to conserve heat losses.

Typical causes of condensation include:

  • Fitting of loft insulation.
  • Fitting Double-Glazed windows in older properties.
  • Fixed or mobile gas fires.
  • Blocking chimneys or existing air-vents.
  • Increasing the number of persons occupying a room or building.
  • Installing a Central Heating System.
  • Heating some but not all rooms in a property.
  • Teenagers overusing bathroom showers!
  • Dishwashers & clothes driers venting within the property.
  • Drying clothing on radiators.
  • Insufficient or broken extraction fans in kitchens and bathrooms.
  • Not ventilating the property generally on good weather days.
  • Unoccupied properties which are closed up for periods of time.
  • Allowing condensation or humidity to build-up.
  • Placing large furniture against external walls.
  • General building defects like a leaking roof, failed window frame sealants etc.


To see if condensation is likely to be the cause of your problem you should try using a ‘Humidity Sensor Tab’ to test the amount of humidity in problem areas.

Treating Condensation

If you do have a condensation problem there are a variety of simple solutions available. 

Essentially there are 3-stages :

1. Improve the ventilation of a room or the entire property.

This can be achieved by undertaking one or more of the following:

  • Unblock air-vents and consider additional vents in walls or trickle vents to windows.
  • Repair extractor fans or consider installing additional extractors in kitchens, bathrooms and any room where achieving airflow is problematic.
  • Install a positive pressure ventilator in the loft to ventilate the whole house.
  • Use a dehumidifier(s) for specific rooms or the whole house.
  • Vent clothes driers to external vents.
  • Manage the use of showers and bathrooms to prevent moisture build-up.
  • Mop up condensation moisture regularly.
  • Use reusable moisture absorbent packs to keep cupboards and small rooms dry.
  • Air the property at every available opportunity.
  • Ensure that there is not a significant temperature difference between nearby rooms, particularly were the warm rooms are the kitchen, bathroom or living room.


 2. Remove the Existing Mildew Problem.  

  • Clothes and textiles and curtains can be laundered, preferably with a suitable additive sold for treating mildews (Note: Unfortunately it is likely that some evidence of the black/blue/green mould staining will remain). 
  • Other building surfaces should have damaged fabric and decorative materials, like wallpaper and flacking paints, removed.  Treat the remaining surfaces and substrates using the ‘SkilledBuild Mould Steriliser and Killer’.  This solution will penetrate porous surfaces and denature ‘kill’ the mould and spores.
  • Wallpapers should be hung using the best quality paste with mould inhibitor
  • Re-painting should be undertaken with either a water or solvent based paint blended with an Antifungal Paint Additive. This additive provides long term protection against mould developing on the painted surfaces.

Please remember that if you do not adequately tackle the cause the mildrew then it will eventually return.

 3. Maintain areas to prevent re-occurrences

  • Now you have eradicated the mildew the final step is to manage any re-occurrence.  There is no substitute for tackling the causes of moisture build-up as described above, but certain rooms and surfaces are prone to mildews, particularly bathrooms.  It is best to use a non-toxic fungo-biocidal ‘anti-mildew’ spray which kills mould/spores whilst leaving a residual treatment with lasting effect.

Taking decisive action will save money by protecting your property and possessions from damage, but most significantly will almost certainly provide an improved living environment to enhance and protect family health.

For more information on condensation, see Skilled Build's How To Identify and Remedy Condensation guide.

2.     Rising Damp

This is when water is drawn up from the ground in a capillary action, similar to how a sponge works.  It is often found in older properties where a damp proof course (dpc) has failed or is non-existent.  Typically the damp rises only to a maximum of 1 metre (3 feet) above the floor or outside ground level (whichever is the higher).  Rising damp mould and condensation mould are often confused.  Long term rising damp problems will cause considerable damage to a property as well as creating health risk issues for occupants, similar to condensation mould. Rising damp is also a significant contributor to instances of dry-rot.

If you believe that rising damp may be the cause of your mould or damp problem then you should consider installing a new chemical damp proof course system.  SkilledBuild DampOut+ Damp Proof Creams offer an effective solution for most requirements.  Please refer to SkilledBuild's guides on How To Identify and Treat Rising Damp.

After putting a rising damp solution into action you will still need to address the problem of the mildew removal by following the steps detailed below in this document.

3.     Penetrating Damp

An alternative cause of mildew may be penetrating damp – this is where water penetrates walls from the side.

Fixing this problem can be fairly easy. Reasons for penetrating damp include:

  • Leaking gutters & downpipes.
  • Porous bricks in an exposed location.
  • Decay of pointing mortar.
  • Cracks in bricks or pointing.
  • Failure of seals to window and door frames.
  • Cracks or de-lamination of external renders.
  • Old & flaking external wall paints.
  • Frost damage & spalling brick faces.
  • Overflow pipe discharge down a wall.

Make sure that these leaks and defects are rectified and that all associated repair works are correctly undertaken to prevent further water/moisture penetration into the walls. 

Failed pointing may also be an issue. Pointing is the mortar used to fix bricks/stone when building walls. It is vulnerable over time because of weathering and the freeze/thaw cycles. This leads to cracks occurring which will allow the ingress of water or breakdown of the mortar to become weak and ‘sand like’.  You will need to rake out old decayed pointing and apply fresh durable mortar. 

You may consider it to be a good idea to treat the wall with a water resistant seal such as SkilledBuild DampOut+ Water Seal.  This clear liquid provides long term invisible protection for the wall and prevents penetrating damp whilst allowing a degree of micro-porosity to enable moisture trapped in the brickwork to escape.  Remember that a wet wall loses more heat than a dry wall so using this treatment will reduce heating bills! 

All works to both internal & external surfaces after rectifying penetrating damp problems will benefit from a pre sterilization treatment and the addition of a fungicidal paint additive also retards future mildew growth. 

Actually Removing The Mildew

Once you have determined and fixed the cause of the mildew you need to remove it.  If you are using a SkilledBuild product, always follow the product label, How To Use Sheets and Health & Safety Sheets found on the relevant product pages.  Also wear suitable protective equipment.

As a general guide, the treatment requires:

  1. Firstly, scrub the mould off surfaces with water and detergents.

  2. If porous materials have been highly contaminated with mould it should be removed and replaced. This may include soft plaster and plasterboard, drywall, timber, carpet and ceiling tile etc.It is important to not just paint over the mould as this is not removing it and most likely it will simply return and the paint is likely to peel.

  3. Sterilise the surface using SkilledBuild Steriliser & Mould Killer.  Allow to dry.  Re treat as recommended.

  4. If painting, add SkilledBuild Fungal Paint Additive directly to the desired paint and blend thoroughly.  Paint as normal.

  5. It is essential to ensure that all surfaces are fully dried before painting.  The use of a dehumidifier or a fan may speed up the drying process.

  6. In addition, try to increase ventilation in frequently moist areas such as bathrooms. If ventilation is not entirely achieved, the use of a domestic dehumidifier will help reduce humidity and control future mould growth.