What Are These Black Stains On the Walls?

Eliminate Ghosting in 4 Steps


Understanding Ghosting

Dylan Chalk photo of ghosting, dark shadows on wall and ceiling

These dark shadows on the wall and ceiling are “ghosting”

“I have strange black stains on my walls and ceilings and I’m afraid I have a mold problem, can you help?” This request came from a nice older woman who was inquiring if she could hire me to evaluate a mold problem recently developing in her home. When I asked for more details she described the mold patterns as black stains on the walls and ceilings; the stains seemed to be following the framing of the house. Even over the phone I recognized the symptoms.

The good news for my caller is that the stains are not mold and they are unlikely to cause an indoor air quality problem for the occupants or a structural hazard to the building. The problem she was describing is called ghosting, or thermal bridging, and though innocuous it can be a nuisance. It is helpful to understand why this happens and what can be done to correct it. If you are out looking at houses to buy, you may very well see this.

Dylan Chalk photo shows ghosting - or black stains - on ceiling indicating heat loss

Black stains – or ghosting -on ceiling indicate heat loss and a poorly insulated ceiling

Understanding the problem

To visualize thermal bridging and the resulting ghosting patterns that emerge, let’s think about a house in the winter time. You are inside your cozy warm home; you are cooking and cleaning and breathing and showering and this creates humidity inside the home. You might also be having a wood fire or burning candles or incense; the stage is set for ghosting. Now let’s look at what happens.

In the winter, the insulation in your house, if it is doing its job, will help keep the cold out and the warm in. Insulation is installed in the stud bays inside the walls and on the floor of the attic. The limiting factor for insulation in traditional code-built applications is that the insulation does not cover the wood framing of the building, just the empty spaces between the framing. In today’s houses the attic insulation is sufficient to cover the framing in the ceiling but often not in older homes and not in some vaulted ceilings. The result is that roughly 30% of your walls and ceilings are not really insulated other than the marginal insulating value provided by the wood framing.

Ghosting happens when this mildly damp, warm and possibly sooty interior air hits the mildly cold sections of walls and ceiling that are un-insulated because they are where the wood bits hold up the house. The result: black sooty stains on your walls and ceilings which follow the framing of the house; sometimes you can even see the nails in the sheetrock which, because they are metal, are even colder and more prone to becoming a condensing surface. Typically, the amount of condensation is not enough to cause a mold problem or a structural problem but it does make these irritating lines on your walls and ceilings. So….. How do you fix it?

Understanding the repair

1. Eliminate indoor air particulate:
  • Stop burning candles and having wood fires or burning incense.
2. Monitor your relative humidity inside the house:
  • Buy an inexpensive temperature / relative humidity gauge and try and keep your indoor relative humidity below 55% during the cold winter months. You can typically do this by turning on bath fans – this will have a drying effect on your building as you vent moist warm air to the outside and replace it with dry cold air.
3. Re-paint effected surfaces
  • Use a stain-killing paint to seal up the stains and then re-paint the walls and ceilings. This is the expensive and inconvenient part.
4. Improve insulation where you can
  • If the ghosting is happening on a ceiling or a place where you have the ability to access an attic space and insulate the ceiling joists, this would insulate the cold wood bits and help prevent this from happening again.

Important note

Occasionally, you will see this problem inside the stud bay of a wall or ceiling. If you see this you know that the bay here is not correctly insulated. The repair in this case is, unfortunately, to open up the wall or ceiling and insulate it. The difference between these two issues is where you see the black stain marks:

  • Black stains on the framing and you are dealing with thermal bridging which can be difficult to correct with insulation.
  • Black stains inside the bays and insulation is missing – generally more of a construction defect.

The attached photos show you some examples of ghosting.

I hope this helps you better understand what to look for as you explore potential homes. Enjoy your house hunting! Remember, knowledge makes it fun.

Dylan Chalk photo of ghosting

Photo showing ghosting due to missing insulation in vaulted ceiling

Dylan Chalk photo of ghosting on wall

Ghosting indicates possibility of no insulation in vaulted ceiling

Infrared photo shows missing insulation

Infrared photo shows missing insulation in vaulted ceiling


Dylan Chalk

Dylan Chalk is the owner of Seattle-based Orca Inspection Services LLC – www.orcainspect.com. He is the founder of ScribeWare inspection report software offering innovative and simple report-writing solutions – www.getscribeware.com. He is also the author of The Confident House Hunter – a book to teach home buyers how to look at and understand houses: Cedar Fort Press Due out July 2016 – www.dylanchalk.com

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