Energy conservation has altered the control of moisture in buildings. It’s a long often misunderstood story that is still being written.

The goal of a HVAC system is to provide a constant humidity and temperature interior. When the interior weather is different than the exterior, you can have issues with moisture, because moisture and temperature always want to equalize across the barrier of your wall. Just like a glass of iced tea in the summer, warm humid air condenses against a cooler surface. That condensation dew point has to be managed. You have to keep it outside your walls.

Old houses never had conditioned air. They were the same temp and humidity, inside and outside. They didn’t have to worry about moisture condensing inside their walls and rotting them out.

When you confuse old and new practices, you create issues.

If you have exterior humidity above 50%, it doesn’t matter what the temperature is, your HVAC system should be dealing with your indoor weather. You should have a dehumidification mode on your HVAC to deal with low temperature deltas and high humidity situations. Use that. Don’t open your windows and let that moisture into your house.

The biggest mistakes are made in attics and crawl spaces. The old way was to ventilate them. The new way is to include them in the conditioned envelope. You will find differing opinions and code requirements at different locations and from different designers. It can get confusing on an Internet forum.

Opaone,

These kinds of problems are very subjective. Meaning quality of work and how it was done can sometimes influence what happens after the home is built.

After looking at the thread you pointed to it is my conclusion that this particular home suffered expansion of what appears to be tongue and groove wood type floor used for the cathedral ceiling.

Any wood product, no matter what it is used for (flooring, ceiling, wall paneling etc.) must be given room to expand and contract. Typical it doesn't need much room if the prep work is done properly. In this case properly means acclimated to the structure in which it is being installed.

The reasoning behind acclimation is that if you take wood from one structure and immediately begin using it in another without acclimation it could cause this sort of problem that you link to in your first post to this thread.

If it was cold in the structure the wood product was stored at the wood contracts to it's smallest size. Then the builders come in and install the product 'tight' with no room to expand.

This is likely what occurred. The builder was under a dead line to finish the home most likely and didn't acclimate the product to the structure. Then to make matters worse put the product in tight with no room for expansion. That bowing you see is the end result.

If the product (wood type flooring) or whatever it is for that matter would not do this just due to humidity problems alone. This points to improper installation in my opinion.

New homes tend to leak less air than older homes. An experienced HVAC company can help you improve humidity and indoor air quality.

Thanks all. My primary question is about what happens when windows are opened (big giant air leaks) and so there is no HVAC controlling things. Radical thought but people actually did this for thousands of years. :-)

For example, it is currently 79°f and 66% humidity with a slight breeze. It is marginally comfortable to have the windows open and I'm guessing that were we in our new house that we'd have had the windows open (and whole house fan on) through the morning until about now when we'd close them and turn the HVAC on.

So, what potential problems with the 66% humidity and then with the humidity dropping when we turn the HVAC on? Older houses were built to withstand temp/humidity swings. Are new?

Be very careful installing a whole house fan. Those penetrations between conditioned space and unconditioned space are weak points and trouble points.

I had a cousin who thought he had a leak in the AC condensate overflow line, and the primary was backed up. Water was dripping down the hatch of the whole home fan, and onto the floor of the hall below. Quite a bit of water.

It turns out that the hatch was not sealing properly, and hot moist attic air was condensing against the cold indoor air. Created quite the rivulet.

This is why home Energy Raters hate recessed lights. And any other hole between the zones. Because they are always problem areas.

66% humidity would void your warranty on floors, cabinets, moldings, and anything else made with wood. As would 15%. Humidity control is part of how you take care of your home.

I don't see a problem opening the windows of a new construction house when the temperature and humidity are in the mild range. The problems with new construction are when windows are tightly sealed and moisture can't escape. That condition did not exist in old houses.

Therefore as stated above new homes are not susceptible to humidity problems when the windows are open.

So, what potential problems with the 66% humidity and then with the humidity dropping when we turn the HVAC on? Older houses were built to withstand temp/humidity swings. Are new?

HVAC is for comfort of the home dweller. It's not required for any structure. It won't really do much to cause problems if shut off in 'most' cases. More problems occur by actually running it. (improperly)

Housing crisis 101: Shadow Inventory (homes) sat vacant in areas of high humidity for many years after crisis Circa 2007-2008. (Florida)

You think a bank that owns a vacant home is going to run the air conditioner with no one living there? Uh, no.

With that said, furnishings of the home can be effected due to high humidity problems but usually this relegated to a small insignificant part and any damage related to such is nearly always due to improper installation. That is my opinion.

Proper installation of any wood product takes into account that the wood product will expand and contract. If it's not taken into account, damage will occur weather you run the HVAC system or not.

There are other items besides wood products that will suffer damage due to high humidity content, but those concerns extend beyond the discussion of this thread.

"66% humidity would void your warranty on floors, cabinets, moldings, and anything else made with wood. As would 15%. Humidity control is part of how you take care of your home."

Based on?