The colder it gets outside, the lower the safe level of moisture in the air on the inside - that is, the level at which condensation problems could develop.

Too much humidity indoors when it is very cold outdoors can cause condensation to form on windows (Even with a storm sash) and could cause condensation problems inside the walls if the house does not have vapor barriers to keep moisture from penetating through to the insulation.

One way to tell whether the indoor relative humidlty is too high for the current outside temperature is when condensation starts to form on the inside of the windows, especially if the windows have storm sash on the outside.

Once it has been determined that extra moisture is needed on the inside (there are instruments one can purchase that will read inside humidity much in the way that temperatures are read) then the next step is to decide what kind of humidifier is required to solve the problem. There are various types.

Those houses that have hot air heat will find it simplest and most efficient to install a unit that works with the furnace so that moisture is automatically added to the air when the furnace is on. Those that do not have hot air heat can either buy portable, console-type units that some in attractive cabinets which can be set up anywhere or, better yet, they can have central units permanently installed which will treat the whole house but which are independent of the heating system.

Regardless of type, the humidifier selected should be equipped with a humidistat - an automatic control that will maintain the predetermined and present level of humidity in the house (much in the way that a thermostat maintains the present temperature level). Also, it is important that the unit selected have the capacity required to do the job - that is, the ability to disperse the amount of water needed per day. This will vary with the size of the house, the tightness of construction and the dryness of the air.

As an example, an average size house with about 1,500 square feet of living area might need anywhere from five to 10 gallons of water added to the air inside each day, depending on whether storm windows were loose or snug fitting, and depending on the general tightness of construction.

A home humidifier can be either a fully automatic, permanently built-in unit, or it can be a portable, console type that resembles a niece of furniture and so can be installed in any room - or even moved about if desired. In homes with forced hot air heating systems the most common, and most efficient system is to install a permanent unit next to the furnace (on either the main supply duct or the return duct) that will automatically disperse the needed amount of moisture into the air each time the furnace comes on.

The simplest and least expensive models are the plate types. These have a pan of water at the bottom and a series of absorbent plates or pads which soak up water from the pan, the expose it to the flow of heated air from the furnace so that it evaporates into the air stream. They are actually the least efficient type of humidifier in that they deliver the least amount of water to the air, and they also are generally more prone to liming or getting clogged with sediment. This necessitates fairly frequent cleaning or de-liming of the plates, or replacing them with a new set.

A more efficient type that will also deliver more water per hour is the air-driven, powered humidifier. In these there is a rotating drum or clylinder of absordent foam or similar material which soaks up water from the pan. The drum is rotated or driven by the stream of air flowing past it, and thus disperses moisture more rapidly. For still greater capacity (more gallons per hour) there are also electrically powered humidifiers which have small motors and fans mounted on the inside to force air more rapidly past the evaporation media drums (the plates or plans).

All of these models are installed in one of the main ducts near the furance (some go across the two ducts - supply and return), and all are hooked up permanently to the water supply with a small diameter tube so water is automatically supplied as needed. Some models, designed for use in hard water areas, also have a feature which provides for a washing action to help keep the evaportative media clean. This may be in the form of a continuous flow of extra water that washes the unit; or it may be designed to flush automatically at predetermined intervals. These "self-cleaning" models all use more water, and must be hooked up to a drain. Those that are electrically powered will, of course, also require running wires to the motor to supply the needed current.

In addition to greater capacity, powered models have one other advantage over nonpowered models such as the simple plate types: They can be hooked up to a centrally located humidistat that will automatically control the humidity level in the air. In the same way that a thermostat controls a furnace and helps keep an even temperature, the humidistat is set to control the relative humidity inside the house so that it stays at or near the ideal 30 to 40 per cent range.