Tag Archives: Ask A Builder

Thermal mass and passive solar design

In construction, thermal mass refers to heavy, dense building components with a high capacity to absorb, store and release heat, for example—logs, masonry, concrete and adobe.  These materials are used in the building envelope to provide structure, but their thermal properties mean that they can also provide other benefits. In this first article of a two-part series on thermal mass, we’ll address how thermal mass can be combined with passive solar design to reduce building heat and cooling load.  Next week we’ll examine the effect of thermal mass for more conventionally designed homes in three different locations.

Passive solar design uses a combination of building features along with the sun’s energy to provide heating in a home.  Typically, a home’s orientation combined with south-facing windows and a large thermal mass are designed to collect, store and distribute solar energy during the heating season. During the summer, features such as deciduous trees or awnings can block solar energy from entering a house and causing overheating. Many homes in Alaska use passive solar design to provide part of their heating needs during the year.

In passive solar design, there is no control system that dictates the movement of heat energy, as with a boiler or furnace.  To understand how this might work, picture a house with a concrete floor in a south-facing room on a sunny spring day in Fairbanks.  As sun’s radiation enters the room through the windows, it warms up the room and the thermal mass of the concrete floor absorbs this energy throughout the day.


At night, the situation reverses.  With no incoming solar radiation, the heating system will need to work to keep the temperature of the room at the set point. However, as the room’s ambient temperature drops below the temperature of the thermal mass, the stored heat energy in the massive floor radiates back into the room, stabilizing the temperature and delaying when the heating system needs to switch on. In effect, the thermal mass acts as a heat battery, storing solar radiation until the sun disappears and then releasing it back into the room. A properly designed passive solar system can result in energy savings for a home because the thermal mass can store excess heat during the day and allow it to offset nighttime heating loads.

Although thermal mass is often in the form of a concrete floor, there are other ways to incorporate it into a home—such as a wall that receives lots of sun or a masonry bench or shelves in the sun’s path.

As the days lengthen during the spring and summer, the large south-facing windows in the above example can allow too much solar radiation to enter a room and cause it to overheat. Some people install awnings or curtains, or plant deciduous trees to shade the windows. Thermal mass also helps prevent overheating, especially in early spring before deciduous trees have leafed out.  A room that might have become uncomfortably warm during the day instead experiences less rise in temperature as the solar radiation is absorbed by the thermal mass. This energy is released later in the evening when outdoor temperatures are cooler. Overall, the thermal mass acts to smooth out temperature swings in the room, enhancing indoor comfort.




What is a ground source heat pump?

A heat pump harvests energy from the ground to use for space heating for your home.

A ground source heat pump (GSHP) is a space heating appliance that uses electricity to convert geothermal heat to a temperature that can be used for indoor heating. They are common in the Lower 48 and quite popular in Europe. There are also several GSHP systems in Alaska, including commercial systems at the Juneau Airport, Weller Elementary School in Fairbanks, and the Alaska SeaLife Center in Seward. Fairbanks has more than 10 residential systems as well.

In the winter, ground source heat pumps work like refrigerators in reverse. Instead of taking heat from the inside of the refrigerator and rejecting it outside, a GSHP gathers heat from the ground, steps it up to a higher temperature using a compressor, and then “rejects” it inside a house. In the summer, some GSHPs can run in reverse mode to provide air conditioning – taking heat from a house and rejecting it to the ground. The main “fuel” for a GSHP is geothermal energy, but it uses electricity to run the compressor. Since the electricity is only acting to boost the geothermal heat (which is free), heat pumps are more efficient than electric heating appliances.

A GSHP consists of 3 parts: the ground loop, the heat pump and the distribution system. The ground loop gathers heat from the ground. It consists of loops of pipes buried in horizontal troughs or vertical boreholes. A pump moves a fluid through the pipes. As the fluid travels, it is warmed up by geothermal heat from the soil and returns to the heat pump with a higher temperature than when it left. Back at the heat pump, the fluid from the ground loop passes its heat to a refrigerant, causing it to evaporate into a gas. The refrigerant fluid then passes through a compressor and its heat can be “stepped up” before it transfers the heat to air or water for the distribution system. Heat pumps can work with a forced air distribution system or a radiant hydronic distribution system, however they generally are not capable of producing hot enough water for baseboard hydronic systems.

Ground source heat pumps are considered a partially renewable technology, because the heat they take from the ground comes from solar and geothermal sources. They are entirely renewable if the electricity they use comes from a renewable source, such as solar or wind. Heat pumps are also very safe to operate, because there is no combustion. They can, however, be expensive to install because you have to excavate land or drill to establish the ground loop. Talk to an installer if you think a GSHP might be right for you.

For more information on ground source heat pumps, see a report by the Cold Climate Housing Research Center and the Alaska Center for Energy and Power: http://cchrc.org/docs/reports/Ground-Source-Heat-Pumps-in-Cold-Climates.pdf. It covers the performance, cost and payback of GSHPs in various regions of Alaska.

How does a rainwater catchment system work and can I install one myself?

A simple do-it-yourself rainwater catchment system.

Installing a rain barrel to collect rainwater for non- potable uses is an easy way to help the environment and save money. Water collection systems can be as simple as a rain gutter directed into a barrel or as sophisticated as a buried tank supplied by multiple sources with filtration and pump systems. The easiest way to collect rainwater is to catch it as it drops from your roof and eaves. Of course, this works best if your home has large roofs fitted with gutters, but even a small roof can collect significant amounts of rain.

For every square foot of roof, you can collect a little more than a half-gallon of water per inch of rainfall. Fairbanks has an average of 10 inches of rainfall a year (some years much more, others much less). This means a small cabin in Fairbanks with a 1,000-square-foot roof can collect about 5,000 gallons of water per year–more if you collect snowmelt in the spring.

One thing you need is a tank. Storage tanks can be fiberglass, wood, steel, concrete, plastic, or another material, though plastic tanks are by far the most prevalent in Alaska. If the system will store water during the cold seasons, then outdoor tanks and lines need to be insulated to protect from freezing. Buried tanks should be at least four feet below-grade and are often protected by a top layer of insulation to prevent freezing.

You can often find small above-ground storage vessels at feed stores, or companies that deal in fuel storage, which sell everything from 15 gallon-60 gallon plastic barrels, some with spigots and some without. You may be able to purchase a much larger used tank from local excavation companies (the professionals who install domestic water holding tanks and septic systems) as they replace underground storage systems from time to time. Large new tanks can be found at plumbing stores, excavation companies, or local tank manufacturers.

A basic cistern system involves a series of gutters and downspouts that converge at a centralized collection point that in turn leads into the tank. If the tank is above ground, it may be beneficial if it is fitted with a drain valve and an overflow diverter. Provided the tank is elevated above the demand source, you can use a gravity-fed system to move water. If the tank has access from above, you may be able to move water with a submersible pump attached to a hose. The pump will provide more pressure and a consistent flow rate. Over time, the tank will fill with sediment, which will require cleaning periodically. Also, it’s a good idea to empty and clean the tank each year. This will help control algae growth, but also prevent damage due to freezing in winter. Be sure you support the tank adequately – just one gallon of water weighs around 8 pounds.

In general, the natural process by which rainwater is formed causes it to run slightly higher in acidity. In addition, the characteristics of your particular rainwater can be affected by sulfur and other pollutants in the air (if present), your roofing material, and any debris that may collect in the catchment system such as leaves, pollen, and bird droppings, for example. Downspouts, gutters, or the tank’s opening can be fitted with screens to keep large debris out of the system. More advanced systems include a trap to minimize unwanted matter from getting into your main tank. A trap is basically a smaller tank containing baffles. Water enters this smaller tank first and filters out sediment and other materials. You may want a system for diverting water from the collection system until a good rain has had chance to wash your roof of heavy pollen or other accumulations.

Consider what your roof is made of and ask the manufacturer to make sure your roofing materials are not toxic. It’s possible that old roofs may use asbestos shingles or other toxic materials.

If you’re considering a permanent catchment water distribution system, the acidity of the rainwater may need to be adjusted to reduce the long-term corrosive effects of the water on metal plumbing components.

If you are looking into building an advanced water catchment system, consider going just a few steps farther. Fitted with additional filters and plumbing, a cistern can provide grey water for indoor use and in some cases, drinking water.

For information on designing a system, check these resources:
Rain Barrel Construction by Cold Climate Housing Research Center, GW Scientific, City of Fairbanks, and Fairbanks Soil & Water Conservation District: http://cchrc.org/docs/green_inf/Rain_Barrel.pdf
Water Cistern Construction for Small Houses by UAF Cooperative Extensive Service: http://www.uaf.edu/files/ces/publications-db/catalog/eeh/HCM-01557.pdf
Information on Best Management Practices for rainwater catchment in Alaska: http://cchrc.org/docs/best_practices/BMPRWcatchment.pdf
Other ways to reduce rainwater and pollutant runoff on our website at http://cchrc.org/green-infrastructure.

What to do about moisture levels in your wood fuel


By CCHRC Staff

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Q: Where can I get information on the moisture content of wood for burning in my stove?

Freshly cut wood can be very wet and can contain up to 80 percent moisture.

In terms of moisture for wood burning, 20 percent or less is ideal.

Fortunately, Fairbanks is a fairly dry climate, and if wood is cut in the spring, split, stacked and covered, it should be dry enough to burn by the fall.

However, do not cover a woodpile with tarps that drape over the sides.

Wood should be stored in a woodshed, or covered with a spare piece of plywood, roofing tin, or anything that will allow air to flow through the pile.

Also, stack the wood on pallets to avoid exposure to ground moisture.

The bottom line is, the drier the wood, the cleaner and more efficiently it will burn.

The more moisture in the wood, the less energy will come out of it as heat because the moisture has to be burned off as steam first.

Excessive moisture also creates problems with creosote.

Burning wood with a moisture content of 25 percent or higher the amount of pollution increases in the form of harmful particles exiting the chimney.

A moisture meter can identify how wet wood is.

Moisture meters are handheld devices with prongs that stick into the wood. They can be purchased locally or online and consumer models are relatively inexpensive.

Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, e-mail us at akhomewise@cchrc.org.You can also call the CCHRC at (907) 457-3454.

Air exchangers work but study up on them


By CCHRC StaffQ: I understand it is important to get fresh air into my house, but exchanging air in my home means the warm air is going out and cold air is coming in. I pay quite a bit to heat my home and reheat all that air coming in. Can air exchangers help to solve this problem?There are several types of air exchangers on the market, but not all of them capture heat from the outgoing stale air.Q: When should I start plugging in my vehicle?

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Commercially available exterior wall vents combined with a fan designed to operated all the time will provide fresh air for a home.

These devices are the least expensive, but provide no heat recovery feature.

A heat recovery ventilator (HRV) is a more expensive device that has a heat exchanger inside, where the air flowing out of the home passes by the air flowing into the home, without mixing the two. As the warm air moves out, it transfers some of its heat to the cold air moving in.

The heat recovered by this process is in the 60 to 75 percent range, which is significant because any amount of heat that is recovered represents air that the homeowner does not have to pay to reheat.

As the cost of fuel increases, this savings will be more significant.

An energy recovery ventilator recovers heat and moisture as well. Unfortunately, these systems cannot be used in the Fairbanks area because extremely cold air will freeze the device.

Many Interior Alaska residents are retrofitting their homes now.

Adding insulation and tightening a house makes ensuring you have good indoor air quality more important than ever. Insulating a home will conserve heat and adding an air-exchanging device will clean the air.

But only an air exchanger with a heat recovery option will do both.

Be sure to consult with a licensed professional to help design and or install any ventilation system.

Many of us will start plugging in our vehicle right away when it gets cold but plugging in will have an unfortunate affect on our electric bill.

The Alaska Department of Environmental Conservation provides the rule of thumb: plug in for at least a couple hours before starting the vehicle when it is 20°F or colder.

At that temperature, you can get by plugging in for less time, and as it gets colder you need to plug in for progressively longer.

If you find you need to leave your car plugged in substantially longer than these guidelines before it starts smoothly, then you car may need maintenance.

Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, e-mail us at akhomewise@cchrc.org.You can also call the CCHRC at (907) 457-3454.