Cold fasteners may be cause of spots on ceiling

Hey there, time traveller!
This article was published 28/05/2006 (7357 days ago), so information in it may no longer be current.

QUESTION — Our 1,450- square-foot house is approximately 35 years old. A few months ago the nails on the ceiling and interior walls of the living room started to show up as black dots. We can now see a dark spot for every nail or drywall screw in that room. I’m concerned it could be mould building but I’m clueless as to why it would happen all of a sudden.

I have verified the attic to see if there was frost build-up, but everything is dry up there. We did have a considerable amount of snow this winter and it could be that one of the roof vents may have been covered for quite a while. We’ve had just as much snow in previous years but never had this problem with the nails.

The only thing I can think of is the fact that I installed new aluminum soffits over the old wooden ones and made six-inch holes between each rafter, in order to permit greater airflow. Was it this increase in airflow that would have caused the stains and how do I now correct the problem? I don’t want to go to the expense of covering it with a good plastic primer and a coat of paint and have to do it all over again next spring. I also thought of increasing the number of roof vents since, I’ve increased the airflow in the attic but don’t want to do that and create another problem or make the problem worse.

–Omer Fontaine, e-mail

ANSWER — While it may be very difficult to pinpoint the reason that you are experiencing a problem that has never before emerged, the recent soffit upgrade is a strong possibility. Offering advice for repairs to stop the problem will be quite speculative, but there may be little required to stop the problem, anyway.

The likely cause of the dark spots showing up on the ceiling where the drywall nails or screws are installed is thermal bridging. Simply put, these metal fasteners are colder than the surrounding drywall due to their physical connection to the attic through the ceiling joists. The small stains are likely from dirt, dust, soot, smoke, grease, or a combination of all these airborne particles that are concentrating on the cooler fasteners embedded in the ceiling drywall. Also, the fasteners may be creating gaps in the air-vapour barrier, and air leakage can occur in small quantities around the screws, carrying this dust and dirt along with it. This common occurrence often appears as shadows, in straight lines, which follow the ceiling joists or wall studs in exterior walls. It is quite often seen in older homes with wood-burning fireplaces. The good news is that other than being a cosmetic eyesore, rarely does this cause any major issues.

As far as your recent soffit renovations causing the problem, there may be some correlation. In that case, it may be an indication of a positive rather than a negative change in the home as an overall system. While this seems contradictory, the evidence of thermal bridging may show an improvement in the performance of the attic. My rationale behind this statement is that the colder fasteners are an indication that you have significantly cooled your attic temperature by opening up more soffit ventilation. A cold, well-vented attic is something to strive for in minimizing roofing deterioration and moisture damage in the roof framing and sheathing. The fact that you saw little frost in the attic this past winter indicates that air movement is good in this area.

The one item that you should address is the level of insulation in the attic. If the fasteners are being affected by the colder attic, the solution may be to simply add more insulation. This is normally done in reverse, more ventilation to accommodate increased insulation, but either way will work. If the insulation is minimal in the attic, or is not adequately covering the ceiling joists, the screws may be quite cold because they are attached to the cool wood joists. Adding another layer of insulation above the ceiling and joists should help to prevent heat loss and warm up the joists and fasteners. In this case, the problem may simply disappear, or stop getting any worse. You are correct that adding more roof vents may be required when the insulation level is bumped up. However, care must be taken to prevent blocking the soffits when adding a greater thickness of insulation. This can be accomplished by installation of air chutes at the perimeter of the attic.

The other issue to address is your concern over mould growth. The black spots are not likely mould, but if left unchecked for many years, some mould could form on the screw heads if they are cold enough to form condensation. While this should not be a major concern, the increased insulation recommended will help prevent this, as well. Once the insulation and roof ventilation is upgraded, there should be less air loss from the home and less chance of mould growth.

Once the attic upgrades are done, you should deal with the stains. Some of the stains may be cleaned, if they are dust or soot, before repainting. When painting, it is important to seal the fastener areas with a proper primer to prevent further bleeding through of the stains.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com