Easy Question

[aeuzent]

For you guys this is gonna be a snap, personally I got no clue so I'm asking you.

We have an unfinished basement, meaning cinder block walls and an open ceiling with some insulation. It's all underground and a friend of mine told me that if I were to cut a vent into the A/C duct that the dirt surrounding the basement would keep it insulated.

I need to know what if anything this will do to my electric bill and is dirt really insulation?

[RyanHughes]

You definitely don't want to bring any more dirt into your ducts than what may already be in there. No, dirt isn't insulation. :)

[dash]

The earth arond the basement,won't get as cold or hot as the air surrounding the other floors.So it won't take much to cool it ,likely a little more to heat it.

Need a supply and return duct,best to call a Pro,to size them correctly.

[Jesus"]

I'm not sure if I am understanding the full question. Are you trying to insulate the duct internaly or externaly? what side of the fan do you what to cut the vent negitive or possitive pressure side? why do you think dirt is a method of insulation?

[jbarron4]

I don't know what you are after, but years ago I investigated using the crawlspace as a plenum. The idea is the seal and insulate the foundation walls and cover the dirt floor with an impermable barrier. Then the registers just go through the floor where ever you want one. The cold air return isducted from the center of the air conditioned spaces to the unit. I had planned to use a package unit feeding through the foundation walls.
Checking the heat losses/gains looked like a pretty good deal. The floors would remaing warm in the winter and cool in the summer. The crawlspace is conditioned just like the rest of the house and is kept dry enough to discourage most bugs etc.
Of course at the time North Carolina codes had a fit with this idea. Now they insist on sealing the crawlspace and introducing conditioned air to control moisture under the house. Of course for this to work the house must have proper drainage and the foundation has to have proper termite shields and sealing of the membranes.
I haven't given it too much thought since then, but given proper attention to detail it could work very well.

[muskyslayr]

Cheese guys... He's not asking if he should insulate his ductwork with dirt. That's crazy!! What he's asking is if the dirt is a good insulation for his basement. In a word no. It's better than the surrounding air on the upper floors, as stated in one of the previous posts that actually made sense, but it's not the best. That's why most of the time when they finish basements, at least in this part of the country, they build what they call energy walls inside the concrete block walls. Basically a stud framed wall with insulation and vapor barrier.

That being said, my basement is finished but when they did it they simply textured the block walls, like stucco. It looks good and I don't notice the heat loss/gain... I'm sure I lose some...

[RyanHughes]

Cheese guys... He's not asking if he should insulate his ductwork with dirt. That's crazy!! What he's asking is if the dirt is a good insulation for his basement.

You're right. After reading that twice I see what he was really asking. ;)

[RoBoTeq]

Dirt is indeed an excellent insulator and so your basement will not require much to heat it.

However; do not just cut a hole in the ductwork. The main ducting is designed to be at a higher velocity of air then the branch ducts that come off of it are. You should have someone who understands the ducting system to install a couple of branch line ducts, preferrably run to the outside walls with dampers at where they come off of the main ducting.

If you have an open stairwell or any openings to the floor above the basement, a return duct is not necesary. If you have the basement finished, a low wall return on an interior wall will make it more comfortable in your basement.

If you just cut an opening in the main duct and put a regester on it, you will be compromising the air pressures of the duct system and possibly create issues with your system air balance.

[gasser65]

An inline duct booster fan might be an option you could use here. It just depends on how the existing duct system is laid out.

[beenthere]

An inline duct booster fan might be an option you could use here. It just depends on how the existing duct system is laid out.
Maybe I misread the OP.

But, I don't think he has a supply in the basement to use a booster fan yet.

[sktn77a]

Is your HVAC in the basement? If so, and your supply and return ducts are not well sealed, you may be "conditioning" your basement already! However, I'm guessing that you would like the temperatures to be closer to the rest of the conditioned space in the house. Dirt is not a good insulator - it's a geothermal heat sink and heat source. As such, it keeps the basement cooler in the summer and warmer in the winter than above-ground building space. However, if you want to heat the space, you'll need to insulate it (dirt is a "heat sink").

[gasser65]

You are correct, he does not yet have a supply to the basement. What I meant was if he has to come off of his existing duct work at a less than ideal location a booster fan might be the way to go - provided he has a large enough system to handle the basement addition.

[RoBoTeq]

Is your HVAC in the basement? If so, and your supply and return ducts are not well sealed, you may be "conditioning" your basement already! However, I'm guessing that you would like the temperatures to be closer to the rest of the conditioned space in the house. Dirt is not a good insulator - it's a geothermal heat sink and heat source. As such, it keeps the basement cooler in the summer and warmer in the winter than above-ground building space. However, if you want to heat the space, you'll need to insulate it (dirt is a "heat sink").
I understand what you are saying about dirt being a heat sink, but not when there is block with a vapor barrier between it and the area to be conditioned. It takes very little heat to condition a basement completely underground.

[heaterman]

Play with the numbers in a manual J program. It's the above ground exposure that causes most of the loss / gain.

[TimmyA]

A totally enclosed basement that is fully underground won't have any heat load, but you have to factor in other heat sources from appliances like water heaters/tankless boilers/clothes dryers, etc.
If your basement is partially underground and you have windows, then you're going to have more heat load to factor in. You can estimate the heat load using this free online tool, http://www.acdirect.com/spacepak2/spacepakapp.html.
If you cut a vent in your duct you're going to need a return air path...if not, then you may be pumping cool air to the outside and sucking in hot air from the outside, increasing your electric bill even more.
Another factor to consider is that your basement (and maybe floor) is concrete, which could mean increased moisture and humidity distributed throughout your home.
Then there are the capacity and balance issues that could effect the entire system.