Moisture inside wall cavity will crack plaster, peel paint

Hey there, time traveller!
This article was published 10/11/2002 (8452 days ago), so information in it may no longer be current.

Question — My problem is in my north-facing bedroom. The plaster inside, under the windows, is cracking and the outside boards, in the same area, are peeling. I had my windows replaced this summer and had the house painted outside. Do you think I should break the plaster inside and put more insulation in where it’s cracking? –Janis Gordien, e-mail

Answer — The cracking plaster around the windows in the home may be partially due to age, as the older plaster dries out and shrinks, or from mechanical damage when the old windows were removed. But, a more likely cause is from air infiltration and moisture buildup around the frames or condensation on the old windows. The peeling on the newly painted siding suggests moisture is building up inside the wall cavity. It is trying to escape to the exterior and is causing the paint to peel, as it moves to the exterior. This moisture may be due to a lack of insulation around the window, allowing warm, damp house air to move into the wall, or from precipitation from poorly caulked or sealed window frames and brick moulding.

The one thing that is not clear from the question is whether the complete windows were replaced, including frame and brick moulding, or if newer “box units” were installed. Box units are retrofit windows, made to measure, to fit within the existing wooden window frames. These are most often PVC units and are much easier and less costly to install that full frame windows. They are normally less expensive to buy, because the brick moulding, jamb extensions, and inside casing on the older windows are already in place. These windows work well, but only if the existing window frames and trim are not moisture damaged.

If box units have been installed, then the original frames would not have been altered, making damage to the plaster walls from installation unlikely. In this case, the insulation around the window, or lack of insulation, is unchanged. The new windows will have much better seals and glass, minimizing air and heat loss, but the space around the old frames may still be drafty. In this case, adding insulation around the old frame should improve the situation.

If the entire windows were changed, from brick mould on the outside to casing on the inside, then the walls themselves may have minimal insulation or openings. In this case, the caulking on the new windows may have been poorly installed, or have shrunk and dried up, leaving a gap around the exterior.

In either style of installation, the first place to look is around the exterior of the windows. Check the brick moulding for gaps or missing caulking. If the caulking is not in good condition, it should be replaced with a good exterior grade of caulking, to fill any gaps. Metal window flashing should be installed over the top of the window and may be damaged or missing, which will lead to leakage. Rotten outside trim or deteriorated paint on the outside may also lead to moisture and cold air intrusion. If any of these defects are found, they should be repaired before any inside work is attempted.

If the exterior components are in good condition and no obvious source of the problems can be seen, then inside remedial action should be evaluated. The most likely area for air or moisture leakage is around the window frame, itself. Often, in older homes, a large gap between the wall framing and the window frames was left mostly unfilled. There may be a small amount of wood shavings or old deteriorated insulation stuffed into this cavity, leaving an air space around the window. This can be investigated by carefully removing the inside window casing and chipping out the small amount of old plaster over the inside of this gap. If this space is largely unfilled, insulation may be added to this area before replacement of the casing.

Most window installers now use a minimal expansion, blow-in foam insulation to fill gaps like these around windows and doors. They are easy to use, moderately priced and dry to the touch quickly. Many are polyurethane-based compounds that provide a good air-vapour barrier and are moisture resistant. They can be trimmed with a simple utility knife, once partially cured, to allow complete filling of the cavity and easy removal of excess material.

If all these avenues are explored and repaired and the problems persist, a lack of insulation in the wall around the window is the obvious culprit. Old insulation or wood shavings originally installed may have settled or deteriorated, allowing air and moisture movement through the wall. Removal of the inside plaster and re-insulating or filling the entire cavities with blow-in foam insulation may be the ultimate solution, but should be the last remedy to consider.

Ari Marantz is owner/inspector of Trained Eye Home Inspection Ltd. and is the P. R. Rep. for the Canadian Association of Home Inspectors- Manitoba (www.cahi.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