Insulation and vapor barrier under the floor with wall insulation and you won't need to supply much capacity to the area. So your solution will be be good design.

@dan1888 not sure you understand? I'm trying to avoid heating basement when not in use.

Do you know the humidity of the basement? At the cabin, I have to heat the basement to keep the humidity low.

Where is the project located?

If you insulate to a specification you can calculate based on your location, you won't need much more than air exchange circulation. And you won't have much humidity in the summer.

For forced air just close the return vents in the basement.

@Mark Bischak, Architect project is located about two hours east of Toronto, Canada (Prince Edward County)... @dan1888 closing off vents was what we intended to do. However, I've read that doing this means:

• Your HVAC system must work harder.
• Your HVAC system may sustain damage.
• Your energy usage will increase.

That's why I was asking if there are any alternatives.

Here's a fuller explanation of those bullet points I posted above:

"Closing Air Vents: Why It’s a Bad Idea in Winter

When you close air vents in rooms in your home, those rooms become colder and bring the overall temperature of your home down. Unfortunately, most home heating systems are not designed to interpret these differences in temperature, meaning that despite your choice to close vents your heating system will continue to try to heat the whole house. Making your heating system work harder to bring the temperature up in all rooms of the house.

Further, reducing airflow to one room increases air pressure directed elsewhere throughout the duct system. Because your home’s HVAC system is designed to regulate the temperature of the entire house, when ducts are closed, this pressure can build up and may eventually cause damage to your ducts or heating system."

I googled the humidity in your area. Your basement may require heating to prevent mold.

@User thanks for researching that! Wouldn't want to turn heat off completely, and even closing vents make it completely unheated - but keeping temp at 60 degrees, say, would be fine to prevent mold, no?

Typically, spaces below grade do not require a large amount of energy to regulate the temperature. The effort to reduce the energy used to heat the below grade space may not be worth it. Contacting a local HVAC person would be a good investment in your specific situation.

What is the area of the rest of the house? Has the HVAC contractor done a load calculation? Is the basement below grade?

If the basement is below grade and the walls have been insulated then you don't need many vents to keep 1800 sq. feet of space warm. You cold close off the vents and not cause harm to your HVAC systems. If you want the better control on heating then you could spend the extra money and make the basement a separate zone. A bypass could be installed to balance the air flow. If you are going to install a separate zone a 2-stage variable speed furnace would be a good option. That is a lot of extra costs for something that which may not be used very often.

Another option is to install 2-3 supply vents and supplement with electric heaters. It does not take much to warm up a basement below grade.

I also find it a little hard to believe that you will have a mold problem during the winter in such a cold climate. In cold climates the humidity tends to be low. You should not have a mold problem if the humidity is below 50%.

The OP may have 70% plus humidity based on google.

@mike_home Thanks for this info - will review with contractor and their HVAC person.

Found this re humidity for Toronto:

I live a bit farther west of the OP. There is no indoor humidity problem in the winter. With a forced air system you'll need to add humidity during the winter just to reach 30%.

Relative humidity is a function of temperature. It may be 80% outside at 20F degrees. The indoor humidity will be much lower.

Re 80% @ 20 degrees, not where I live.

The OP has no experience, and I doubt if anyone here knows what is going on in a home that has yet to be built.

Where I live the outdoor humidity after a wet snowfall can be very high. But indoors it is a struggle to keep it above 40%.

re 40% humIdity , I thought the OP has anxiety with high humidity in a home under construction. The OP has 70% humidity.

mike, How does your comments help?

I googled the humidity in your area. Your basement may require heating to prevent mold.

Jim,

Perhaps I misunderstood your comment. But I thought you were saying if the basement is not heated then there might be mold problem. Are we talking about during construction or long term?

mike, I do not live in the OP’s area, but my basement has high humidity.

mike, I think you do not know what you are posting about. The OP wants a finished basement in a high humidity location.

I have a basement and I know the humidity in summer and winter. If you have high humidity in the winter it could be water infiltration from the surrounding ground. A simple test can confirm this.

The OP does not know the humidity. A simple test can not be performed because the building has not been constructed.

I am waiting for the evening news, and Mike is someone to do. Mike, have you read any of the posts?

Yes.

Jim. The OP may not know the exact winter humidity in his unbuilt basement. It will be quite similar to the humidity in homes in the area. It may be similar to the home he's living in now. In no case will it get anywhere near 70%.

If the basement was constructed with a vapor barrier under the basement slab the small possibility of excess humidity in the winter if heat and air flow is reduced will be even less. So laying down 10mil plastic is the first step. He can also put down foam board insulation.

"With a forced air system you'll need to add humidity during the winter just to reach 30%"

As someone mentioned, indoor relative humidity drops during cold weather because that's what happens when you raise the temperature of cold, dry air. If the house and the forced air system are properly sealed, the need to add moisture to the air should be no different with a forced air system than it would be with radiant heat.

The best solution for high humidity in a basement is to be sure it has a proper vapor barrier (prevent it to begin with) and, if not enough, a dehumidifier.

Just to piggyback onto Elmer, forced air causes humidity issues when there are leaks. Most old houses are very leaky and the duct systems are leaky. Even relatively recently built houses.

My last house built in 2010 had a humidifier. It was Energy Star of the time tight.

My 2019 house has no humidifier and the humidity runs a bit higher in the winter (never below 43% or so). It is beyond current ES standards. Both are forced air.

I suspect, Ontario would not let the OP build a house that doesn't have basement walls (at least) properly insulated and with a proper vapor barrier.

In this setting, whether the basement is 60 degrees unconditioned or 70 conditioned will be pennies and not worth worrying about.

Everyone wants to worry about closing off unused spaces and not conditioning them. Maybe it is from guilt about building such a big house? There usually isn't really any logical reason for doing so. Worry about your vehicle that is way bigger than you need 99% of the time. Use a heat pump in hydro dominated Ontario rather than NG. But don't worry about your well insulated basement.

Basement heating and cooling loads tend to be much smaller than the heating and cooling loads of the conditioned living areas above them. For this reason, it's difficult to make a basement a zone utilizing the HVAC system conditioning the first (or more) floor(s) of a home.

Basements--especially if they have finished living space-- typically need dehumidification and fresh air make up to ensure the comfort and health of the occupants. I recommend you speak with your HVAC system designer about an integrated dehumidifier and options for dealing with smaller heating loads such as the use of electric resistance heat.