Attic Ventilation, Temperature and Moisture Problems Q&A

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Dear NH,

I just recently had the flashing around my chimney repaired. The roofer who repaired the chimney suggested that I have air vents installed at the peak of my house. My house is a Cape with a room on the second floor and walk-in storage space on one side of the room. He is recommending installing a ridge vent along the peak of the house. Will this lower the attic temperature enough to make it a worthwhile expense?



I am a big proponent of ridge venting as a supplement to gable vents. You really can't have too much attic ventilation, and the sad truth is few homes have enough.

The primary purpose of ventilating the attic, is not to lower attic temperatures, though admittedly a nice fringe benefit... at least in the summer months. The benefits of ventilation are twofold. The first is to reduce moisture levels all year round, which lengthens the life of roof decking by reducing mildew and rot. The second benefit is to equalize inside and outside temperatures in winter to reduce ice dam formation. See the ice dam article at .

You may or may not notice much of a temperature change, since other factors such as prevailing winds and the amount of shade on your house will affect the final results. One thing for sure... the hotter your attic is to start with, the greater the temperature effect of the ridge vent will be.

Dear NH,

This past weekend, I went into the attic to cover a whole house fan that I have, and to my surprise I had icicles hanging from the joists to the rafters! I know why the moisture was there… because of the heat loss going through the fan. But since I have covered up my fan the moisture is still in the attic, and is leaking onto my ceiling.

Can you help me solve this problem… first as to why there is so much moisture in my attic and second how do I get rid of it. By the way, my furnace has a humidifier attached to it, but it is set very low.



Though your problem is admittedly extreme (ice-stalactites growing in your attic... really!!) it is nothing more than a testimony to the importance of adequate attic ventilation. Moisture rises into the attic in all homes, finding a way even if your home is properly insulated. If too much moisture collects in a cold attic, it condenses first on metal such as the roofing nails exposed inside the attic ceiling. This ice can build up to massive proportions over time.

Of course, there is a point where even a well ventilated attic can become overwhelmed with moisture. This can occur when the homeowner over-humidifies the air via automatic humidifiers (such as yours) or room humidifiers. In some cases, the air in the home can be "naturally" overhumidified due to extreme weatherstripping. This is the so-called "sick house syndrome" where there are so few air changes that the air reeks of toxins… yuck!

A lesson to everyone with a whole house fan… you were wise to cover it for the winter. The louvers on these fans are like a gaping hole in your ceiling and allow not only moisture but valuable heat to escape! Other sneaky places for moisture to enter your attic are recessed light fixtures and bathroom ventilation fans. These can be "boxed" from above to reduce movement of moist air into the attic, but caution should be taken to allow air space around light fixtures UNLESS they are specifically designed to allow "zero clearance". Be sure your bathroom vent fans are vented to the outside and don't just blow into the attic. Finally, attic access holes and folding stairways are also possible weak points that should be insulated and weather-stripped.

You mention that you have gable vents installed in your attic. They must not be large enough to supply enough fresh air to clear out the accumulating moisture... hence the ice problem. The easiest thing to do NOW is to install a gable-mounted ventilation fan on either vent. Though most folks use these fans to cool the attic in the summer, they can also help cool the attic in the winter. The additional movement of cool, dry air should stop the growth of the icicles as well as cool the attic enough to stop the dripping. You should try to manually chip away any large icicles to limit further dripping. Of course, if the weather warms up you might get a quick meltdown; lay down plastic tarps with cloth towels or tarps on top of it. This will catch the water and prevent staining of the ceiling below.

When the warmer weather arrives, you can take the additional steps of installing larger gable vents or even installing a ridge vent along the roof peak. Though ridge vents are supposed to be used in conjunction with soffit vents (vents under the roof overhang), a ridge vent will supplement the existing gable vents and help to keep the attic temperature closer to the ideal temperature... which is the outside temperature!

I have more info and tips on attic ventilation online at the following link:

Dear NH,

Several years ago I owned a 3-bedroom rental unit. The tenant complained of water mysteriously appearing in her dishes in a kitchen cabinet. An inspection of the attic disclosed a furnace gas flue with holes in it. The gases had escaped into the attic, condensed on the sheathing, joists, rafters, etc. The warmth of the south-facing roof melted the frost during the day, it ran down the sheathing until it dripped onto the drywall, found a small hole and drained through the ceiling into the cupboard. Replacement of the flue solved the problem. I hope this might be assistance to someone.



Great story. I credit you for making the connection between the leaky gas exhaust flue and the moisture problem. Most folks don't realize that one of main components of natural gas exhaust is water vapor! A leaky vent pipe in the attic will release significant moisture, along with CO2 and CO… all undesirables! This is worse than venting a clothes dryer into the attic, since the heat runs when it is the coldest outside and the most likely time to condense on freezing attic surfaces!