Articles - Condensation Is All Relative: Humidity That Is

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Systems Inc.
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Condensation Is All Relative: Humidity That Is:

It frosts up your windows on little cat's feet and mists your walls with a delicate fine sheet. Outside your house the moisture is trying to go, but cold surfaces condense it giving mildew places to grow.
With unusually cold weather, the moisture on the ground and high winds, we are seeing more window frostings and much more mold and mildew in houses than usual. The reasons are straightforward enough; the increased soil moisture contributes extra moisture into houses and the colder air and strong winds create a lot of very cold surfaces on which the extra moisture can condense.

Side Effects: In extreme cases moisture can be seen running down windows and walls with mildew and fungus rapidly covering entire walls or attic roof sheathing. It ruins paint, warps wood trim and nearby furniture, and damages carpeting and flooring. Clothing in closets becomes covered with fungus, and often must be discarded.

The mildew and mold smell foul, and the huge concentration of spores in the air trigger allergic reactions. Some houses can become impossible to live in.

First Some Basic Facts: The warmer air is, the more moisture it can hold. If air at a certain temperature is holding all the moisture it possibly can, it is said to be at 100% relative humidity. If it is holding half of what it possibly could, it is at 50% relative humidity. Air at 100% relative humidity would have to contact something only slightly cooler to condense some moisture while air at 50 % relative humidity would have to contact something a lot cooler to condense any moisture.

When the weather isn't too cold, the ideal indoor humidity of 35% can't contact anything cold enough to condense any vapor; but let the thermometer drop enough and the glass of window panes (even sealed insulated glass ones), metal frames on sliding glass doors, and some cold spots on outside wall corners will condense vapor. For some houses with particularly cold window frames like the steel casement type, routine moisture condensation changes to icing when the biter cold weather strikes. If you have a power humidifier attached to your forced air heating system, be sure to lower the humidistat setting when the weather gets really cold. A temperature/setting scale is usually included right beside the humidistat.

The Solution Is Simple: You can either lower the interior humidity level or warm up the surfaces on which the moisture condenses. Since the latter is often difficult in the case of sliding doors or quite impractical in the case of un-insulated exterior walls, it is usually best or easiest to work on lessening the indoor relative humidity. An inexpensive combination hygrometer/thermometer can be purchased for less than $10 and will let you know when high relative humidity is the cause of your problems.

Simple Cases: Throw out excess moisture. If your kitchen or bath windows or walls sweat a little after cooking or showering, that is to be expected. Use your exhaust fans (re-circulating ones in kitchens won't help) if you have them and consider installing them if you don't.
Warm up windows and frames. Install exterior or interior storm windows and consider applying wood trim over metal trimmed windows. Bed the trim in caulking to ensure continuous contact and prevent condensation between the metal and the wood.
If your windows already have storms over them and the interior sash frosts up, the exterior storm window is too loose. Check the caulking around the storm window frame (be sure to leave the two weep holes along the lower edge open) and the felt along the sash edges. If the storm sash gets frosted, the interior window parts fit together too loosely. Use 3M's "V" strip or rope caulk or felt around the edges. Be sure the individual sashes are pulled tightly together when the lock is secured.

Tougher Cases: If you find frost on the underside of your roof sheathing, it means that there is insufficient ventilation in the attic to discharge the moisture from the living space below. (This is common on older houses which have had siding applied. The installer frequently covers the gable louvers over with perforated siding pieces.) The general rule of thumb is that you need one square foot of free (screening reduces air flow and effectively cuts the rough opening size in half) ventilation for every 300 sq. ft. of attic floor space if there is no vapor barrier under the insulation and 150 sq. ft. if there is. Increase ventilation by recapturing your gable vents, adding soffit vents, or installing a ventaridge system along the peak of the roof.

Condensation high up on wall ceiling intersections usually results from outside air currents through the framing and against the wall surface. This is usually due to sloppy framing or caulking or insulation that has been skipped or moved out of position, or some combination of all of the above. Caulk any outside cracks and survey the ceiling from the attic to see if any insulation has been missed or blown back. Go behind knee walls or walls that back to inaccessible attic spaces or garages since wall insulation here is notoriously poorly installed and frequently spottily done. Always inspect behind the gutters in attics with loose fill insulation since wind currents will frequently push the insulation toward the center of the house. Lay short sections of blanket insulation in the voids.

Darkened wall or ceiling areas are usually dust collections due to moisture condensed over colder (poorly insulated) areas. Occasionally darkened outlines of the wall studs or attic truss chords will show as darkened vertical lines running up the walls or across the ceilings. The wall lines are typically 16 inches apart (reflecting the stud spacing) while the ceiling lines are usually 24 inches apart (reflecting the truss placement) The cause is usually a dirtier surface due to dust collecting in the somewhat moisture surface areas. Confirm by washing or erasing the dirt with a pencil eraser.

Impossible Cases: Occasionally a downdraft furnace with slab ducts will get water in the ducts and become one giant humidifier. The water must be drained from the ducts (usually with a small sump pump under the furnace) and kept from returning and usually with proper landscaping outside. Occasionally the slab duct system must be abandoned for a duct system in the attic or along the walls.

Solid masonry buildings with plaster walls that are retrofitted with tight fitting windows sometimes get condensation and mildew problems. The new windows being tighter than the old ones, cut down on air infiltration and build up moisture condensation. The plaster walls and ceilings are good vapor barriers, so the moisture stays inside to condense on the un-insulated walls. These buildings usually respond well to air-to-air heat exchangers, a device that promotes ventilation without losing much heat energy.

Here are some common sources of increased humidity that often go undetected. Crawl spaces under houses without plastic over the dirt. The earth's moisture rises right up through the floor.

Downspouts that empty close to the foundation and poor grading away from foundation walls. Basements with sump pumps usually have water in the sump pit and back under the slab itself. Moisture can rise through the slab and into the house. A piece of plastic left on the floor overnight will usually show vapor on the underside in the morning if there is a problem.

Uncontrolled humidifiers without humidistat controls.

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