How to inspect an attic for condensation, leaks, or moisture problems: this article describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems, in buildings. It describes proper roof ventilation placement, amounts, and other details.
This chapter "How to Inspect in the Attic or Roof Cavity for Signs of an Under-roof Condensation Problem, is part of our article series on "Attic Condensation". Also see CATHEDRAL CEILING INSULATION.
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Check the Entry & Exit Pathways for Attic Ventilation Air
After examining the eaves or lower roof edges, look up at the ridge for a continuous opening on either side of the ridge board, providing an exit vent path. On some buildings I've found that a ridge vent was installed but the installer just nailed it over the existing roof sheathing, forgetting to cut the necessary opening.
"Spot vents" on lower roof slopes, or power vent fans are not as good a substitute for the year-round passive venting provided by a ridge vent.
Our photo (left) illustrates a common construction practice on wood framed homes built in North America into the 1950's: at the house eaves blocking was nailed between or against the ceiling joist ends to minimize air flow into the attic at the house eaves or soffits.
You can see dark mold stains on the underside of areas of this roof. The presence of perforated panels covering the soffit undersides on this home amounted to faux-venting since there was actually no air pathway into the attic.
Check for Signs of Attic Condensation & Moisture
Inspecting attic spaces in winter weather in freezing climates you may find extensive frost over roof sheathing and framing, as shown in home inspector Arlene Puentes' photograph of a log home inspection (at left).
Even in warm or dry weather you often will see stains around roofing nails penetrating the roof sheathing, or condensate drip spots on the attic flooring or insulation (Left photo below).
This is caused by moisture condensing on the roofing nails and dropping to the surface below. This is sure evidence of a condensation problem. The pattern of drip marks may be remarkably just like the pattern of roof shingle nails which protrude through the roof sheathing.
Winter condensation on the nails, even forming frost in freezing weather, rusts the nails and stains the roof sheathing around them. Frost later melts off of these nails and drips onto the floor below. The pattern may occur more on the cooler or north slope of the roof.
If attic stains are from roof leaks, (right photo above) they will usually be much larger and will appear on the sides of rafters, on larger areas of the roof sheathing, and on larger areas of the attic floor.
If you see evidence of extensive roof leaks or condensation leaks, I'd also check the insulation and the attic side of the ceiling drywall for a hidden mold problem.
If you see condensation drip stains in the attic (left photo above), you'll be confirming what you suspected from outside -- inadequate ventilation.
Be careful and don't overlook viewing the roof eaves. There could be rotting fascia boards, decaying rafter ends, and delaminating plywood.
Look in that location for moisture-stained roof sheathing, and look carefully for ice dam leak stains into the eaves, such as the water marks shown in this photograph.
How to Inspect the Attic for Moisture - Stains Around Roofing Nails
Rust or stains around roofing nails that protrude through the attic sheathing (they're supposed to stick through) are a clear indication of high attic moisture.
Our photograph at above left shows roofing nails protruding through plywood roof sheathing visible in an attic where there is no under-roof condensation or moisture problem.
Our photograph at above right shows roofing nails which have rusted and stained the roof plywood in an attic where indoor moisture has risen through the building to condense on the under-side of the roof.
Our photo at left shows the typical staining pattern in an attic where there has been modest condensation at the roofing nails.
If you perform this inspection step in winter in a freezing climate you may find frost on these nails. In other seasons or in non-freezing climates you will see rust on the roofing nails and often stains on the roof sheathing surrounding the nail. On an older home that has been re-roofed, you may see these nail-stains but no nails - the old nails may have been removed during re-roofing, leaving just the rust-marked roof sheathing.
Excessive attic moisture condenses on the cool metal surface of the roofing nails, causing corrosion or rust on the nails and stains on the surrounding wood materials. These same condensation points - the roofing nails - will often map exactly the drip stains found on the attic insulation or attic floor below each nail.
Because moisture may enter the attic more at some locations than others, for example over a bathroom or kitchen, nail staining and drip marks will not be uniformly distributed throughout an attic.
How to Spot Attic Mold Due to High Attic Moisture or Roof Leaks
These photos show mold on attic surfaces due to high moisture in that area. While many inspectors notice dark mold or mold-suspect material on roof framing or roof sheathing, it is at least as important, and often more important to spot the light-colored molds that may also be present - often these are the more hazardous, particularly if building conditions cause air movement downwards out of the attic into the living space or into an attic-located HVAC duct system.
The brown/black attic mold on plywood in the left-hand photo is easy to spot and is often an allergen or problematic attic fungus like Cladosporium sp., Pithomyces chartarum, Ulocladium sp., or Aureobasidium pullulans but the light colored mold on the tongue-and-groove pine roof sheathing in the right photo was found to be Penicillium sp. which is more likely to be airborne and transmitted in the building.
How to Correct High Attic Moisture, Condensation, Leaks
A first step in fixing a wet or moist attic problem is the correct identification of the source of the moisture. Stains or even wet areas on the under-side of roof decking and on rafters can appear to be a roof leak but in fact moisture may be entering the attic not from above (outside and through the roof), but by rising through a building suffering from leaks, a prior fire extinguishment, or most common, a wet basement or crawl space.
Home inspector David Grudzinskiprovided the attic moisture photographs shown below.
From just the photographs, and without having inspected the building exterior, roof, nor other areas, the photos look like a roof that had numerous leaks, perhaps from worn out roofing, possibly even some rotting sheathing. The photos show areas of apparent mold on some rafters. But especially in the 2nd photo at above right, the very extensive condensation stains around the nail protrusions through the roof deck tell us that the whole attic interior has been soaking wet. Now for the big question: is this wet attic caused by roof leakage or is there a building water entry problem?
Mr. Grudzinski provided the additional, crucial diagnostic information about this wet attic:
In other words, an expert roof and attic inspection include an inspection of the entire building, basement to roof, in order to understand where moisture or condensation are originating and what may have been their effects on the building.
A Wet Attic Risks Hidden Mold Contamination of its Insulation
Also, we wonder if, in a soaked area like this where fiberglass insulation is present, because of the risk of hidden mold in the insulation (see FIBERGLASS INSULATION MOLD) it may be worth checking the attic side of the ceiling drywall below the areas of most-apparent-worst leakage into that space - looking for water stains there, mold, &c.
If you find significant levels or large areas of mold in your attic the mold should be cleaned - that is, removed. Do not rely on magic bullets like sprays alone. The spray approach does not remove the problem mold and it may spread it into otherwise uncontaminated materials like insulation.
Don't tear off the roof over a moldy attic: Unless the mold-causing conditions have also rotted framing or delaminated plywood roof sheathing, structural removal/replacement, such as a roof tear-off are unnecessary and inappropriate. But don't forget that if you see attic mold the insulation or ceiling drywall below may also be moldy.
See How to Find, Test For, & Remove Mold in Attics for details about where and how to look for attic mold and what to do about it.
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Frequently Asked Questions (FAQs) about attic or under-roof condensation, attic frost, soffit ice build-up, and related leaks or moisture in buildings
Question: diagnosing moisture high on building walls
I have a double wide manufactured home that I'm renovated. It has vinyl siding and perforated soffits common on manufactured homes. We've noticed moisture high in the walls all around the home and can't identify the cause. Any ideas? - T.B., Colorado
What T.B. found was a combination of accumulated snow or frost or ice in the home's eaves, possibly due to wind-washed insulation, moisture and condensation, or snow and ice building up in the eaves, combined with leaks into the house walls when weather warmed and the accumulated snow or ice melted. Details about this Q&A are at MOBILE HOMES, DOUBLEWIDES, TRAILERS.
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