Creating a passive system that will not cause you to spend money on a utilities to help with the energy efficiency of your home.
One of my pet peeves has been over active versus passive systems when it comes to developing green options for the home. Active systems are great, but they still require energy to operate them; whereas passive systems are design options, which function without any energy input. Attic spaces are particularly troubling in Houston, because for much of the year their temperature can be well over 120F, which reduces the efficiency of the air conditioning system. I was looking into different options for dealing with my own attic, when I simple solution popped into my head.
Let us go over some attic basics first. Case studies are showing that Houston homes would be more energy efficient without attics. They trap the heat and the cold which in turn effects out air conditioning units. (The temperature in the attic causes air passing through the system to rise or fall before and after it is conditioned). Radiant barriers and insulation do help regulate the temperature. We also need air flow to remove moisture and also to help with temperature regulation. The formula to calculate how much ventilation is needed is square footage of the attic (typically the same square footage as your foundation/first floor) divided by 150. The number produced is the amount of vent space needed in the home. A passive system relies on natural air flow. An active system will include a fan.
My first step was to add a radiant barrier under the sheathing for my roof, and to add more insulation to my ducts. My attic has already been well insulated. Sealing and insulating the ducts helps keep the air travelling through them at a constant temperature. However, I have the unit in my attic, where conditioned air is still exposed to a higher or lower temperature than I want, depending on the time of year. The plenums (the big boxes for the return duct and register ducts) do have some insulation in a typical construction. Since I have a gas furnace, I am faced with an additional problem: fiber insulation can easily burn. I decided to give my unit a covering made from a radiant barrier. I cut pieces of the radiant barrier to fit over the housing, keeping it away from the openings for the burner compartment and the duct for that compartment. It might not be a conditioned space, but it does regulate the temperature better.
My second step involved adding additional vent space. Vents are not always placed on the front portions of homes, because they may not look good. I painted some vents from my builders supply to match my house color, and I placed them over holes that I cut into the soffit. Most people never notice them. I did have enough vent square footage, but I did not have air flow through all of the attic. What would be nice here is to have a system to open vents in the summer to force heat out, and to close them in the winter to keep heat in. I met a home inspector once who said he did this with gable vents, but I think this may be harder with my soffit vents.
My third step invovled planning what kind of mechanical attic fan I should add. I looked for solar powered in keeping with my green goal. I have two choices at this point: a fan that sits above a hole in the roof drawing air out; or a fan that sits in the attic pushing air through existing vents.My home’s footprint is U-shaped, so I would need at least two units. For an in the roof model, I was looking at roughly $300. For the inside the attic unit, I was looking at roughly $250. I would install them myself, so I would save installation fees.The prices were not unreasonable; however, I did start thinking about a problem after I had completed an inspection: mechanical units break down eventually. I thought about this fact, and I realized that a good number of attic fans that I encountered were not functioning for one reason or another. Although I thought that the units that I was looking at were good, I could not escape my nagging feeling that they may be a waste of money.
Problem: how can I create additional air flow in my attic to help regulate the temperature? Moreover, could that air be at a constant temperature to help the attic maintain a constant temperture year long? I decided that this system had to be passive- no parts to fix, no energy needed, and it would continue to operate no matter what happens. I first considered different venting options. Air flow would be improved, and most of my criteria would be met, except for the constant temperature bit. Where could I get air at a constant temperature year long? Well, everyone probably knows that once you go below the frost line (two feet is standard in Texas, but it is not really that deep in Houston), you will have the earth at a constant temperature. This idea helps with the concept of earth thermal storage. If I could take outside air down into the ground to pick up that constant temperature, then bring that out to the attic, I would have my criterias met. I did this drawing when considering what to do.
Design considerations for this system:
1) I have to prevent rain from coming into the air inlet. Meaning that some type of covering should be in place.
2) I do not want this to be an access point for pests into my attic. Window screening over the opening would suffice. I may also want a screen at the outlet in the attic.
3) I do not want flooding to be an issue. Inlet should be a foot above ground to cover any possible issues around the parts of my home.
4) I do not want the tube going up to be too unsightly. For other service conduits, I have painted them a color that matches the home color. I would also need to pick strategic spots.
5) I need this constant temperature air delivered to different parts of the attic. This means several underground tubes at various locations around the home.
6) I need to dig deep enough, but I may run into other undergound services. We consider our frost depth to be two feet in Texas based upon the Pan Handle’s temperatures. I might get away with the top of the tube below six inches for Houston, but still, deeper would be better. Pipes for gas, water, or even underground electrical cables become an issue. Call 811 to find out where the gas pipes are. For water supply, look where the meter is to where the water enters your home. For waste water, this will be harder, but draw a line from your athrooms to the street to find the possible location. Electrical can be harder, but this should be handled when they come out to check for the gas pipes. Harder I say, because they may be checking for the service entrance cables, but there may be a cable to the garage or shed that was not considered.
I have not undertaken this project yet, but when I researched my idea, I was happy to see that a similar idea had been implemented in homes without power for the living spaces, so it convinced me that I might be on to something. I think that my cost may be in the area of $300, but my labor time will be more. I do not mind that. I am thinking that I could use a plastic pipe, so tools needed would be a saw, tape measure, shovel, and a drill (to fasten the pipe with clamps to the home, I would need holes for anchors). PVC pipe is easy to join with the pvc cement.
Of course, passive systems do not always work as well as active systems. Could I heat and cool my home in Houston with only a passive system? Not my current home. However, I think my passive system for the attic has more advantages over the mechanical for the reasons stated in the post. For that reason, I will start working on the system to see how it performs.