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I heard Tom Silva say that just having double hung windows provides enough ventilation for a house. He didn't say anything about having to open them or anything like that. Has anyone else ever heard this?
double hung windows can definitely ventilate an attic.
the idea is to open the top portion, lower the top window part way and open the bottom window the same amount. assuming that your attic floor is properly insulated the windows should be open 365 days a year. as the warm air rises in the attic during the day the warm air will escape through the top open window and cooler air will be drawn in through the lower open window. the idea is for your attic to remain the same temperature as it is outside.
i am also assuming that you have one window at each end of your attic and not two on the same wall.
Regardless of what Tom Silva may or may not have said, I think you'd be much better off ventilating the gable ends of your attic (the straight vertical attic walls present on most homes) with screened gable louvered vents---you could simply remove the present window sections & install properly sized square/rectangular gable vents.
What do you have now in your attic in regards to any soffit vents, ridge vents, etc.?
The obvious advantages would be that the louvers can be adjusted to prevent rain & snow from entering the attic, as well as the screens would provide protection against insects & pests (wasps, bees, bats, squirrels, etc.) from entering the attic; the flexibility of today's attic louvers combine a) adjustments in opening size from completely closed, to completely open & several adjustments in between; b) screened protection from insects, pests, c) attractive appearance d) low cost for vinyl, aluminum or stainless steel options.
Vinyl/aluminum vents are widely available at the home improvement stores, or on the internet & can often be done as a diy project, providing your house is not too high.
The site below contains numerous images of gable vents.
Double hung windows were invented to provide ventilation where natural cross ventilation is not available. The concept is that the cooler air enters the open bottom sash and exits through the lowered upper sash.
Where cross ventilation, such as gabel vents on opposit ends of an attic, or from eave vents to ridge vents, double hung windows would be unnecessary. As the other poster mentioned, an open double hung window is is vulnerable to rain intrusion.
It is interesting to note that true, closeable shutters have the louvers slanting outward when closed. This was done to provide protection from the rain while still allowing for good ventilation. In olden times, they would typically be closed at night.
In southern climes, shutters would often be hinged from the top to double as awnings to protect from both mid-day sun and rain. This method is often seen in Spain and Italy too.
There's no beating a modern ridge-venting system so I'm wondering why this isn't being considered here? As to the windows, many old homes here in the south were built that way. When warm weather arrived you opened the windows as already explained, and when cold weather came you closed them. You were also supposed to either close them of shutter them for rain. This took effort and someone remembering to do it which meant someone had to always be at home to do it. It's not a good idea because of that.
The houses that used these were also balloon-framed with no insulation and with plaster walls which acted as a thermal mass. In the winter there would be no convective airflow up the walls so interior heat was retained; in the summer there was convective airflow up the walls which assisted in keeping the plaster cooler through the day. Without open balloon-framed walls there's no advantage to this so today attic windows are mostly aesthetic in nature, and a proper attic ventilation system is used to prevent excessive heat build-up.
It's not that some older ideas have no merit, it's that we have better ways to do some things now. If these new methods were available when the old houses were built, you can be sure they would have been used because the old houses were generally built by craftsmen who cared and wanted to do the best that could possibly be done. Sadly that concept is rather rare these days.
I believe Tom Silva did do an episode on this exact question, and said those windows can cause ice dams by disrupting the ventilation from eaves to ridge.
Its a test of knowledge that bigger the windows the more the air passes in when you keep it open when you have a attic with double hung windows than single one's then it tries to create more air space and ventilation inside so having a double windows is always a better option.