Keep Cool

May 2000

How do you know how much air flow is needed to maintain a reasonable temperature in a sunroom? Using a fan for cooling relies on the temperature difference between the inside and the outside of the room.  If it is only 40° outside, almost any fan will cool the room down fast! But if it is 90° inside and 80° outside, it’s going to take a lot longer even at 10,000 cubic feet of air per minute.

Other factors that effect the rate of cooling are the orientation of the room, the area of the glass, the type of glass, the number of hours of sunlight, and the "thermal mass" of the space.  In most U.S. locations, a westerly orientation will be the hardest to control because the room is warmest when the sun directly impacts the glass.  When the sun is at a steep angle to the glass, much of the light is reflected off.

The "thermal mass" of the space refers to the materials in the space which absorb the heat from the air and then return it to the air as the space cools.  Slab floors and the moist earth in planters are particularly good at absorbing heat.  In a south facing room, these "mass" components will absorb the heat during the morning, keeping the room relatively cool. And they will give off that heat, "pour it back into the space" as the room cools in the evening.  It is primarily because this flywheel effect that even big fans can’t quickly cool down the space.

A good rule of thumb for sizing the ventilation system is to calculate the area of the longest wall or roof glass and multiply it by 8. Divide that by 800 to determine how many TC1000’s that you need.

Growing Things

May 2000

Sunrooms have changed greatly in the past twenty years. Their identity has been somewhat elusive. Even the name "sunroom" isn’t a sure thing.  Sometimes they are called solarium, atriums, Florida Rooms, sun porches, and even hot houses and greenhouses.  Before the Oil Embargo in the late 1970’s, most hobby greenhouses or sunrooms had a single layer of lapped glass. This kept the structural load on the roof low and allowed breezes to blow through the room to keep the plants happy. In many of the rooms, large sections of the glass roof opened up to cool the space in summer, and the roof was coated with a white wash. In winter, a furnace kept the space warm and produced CO₂ which also kept the plants happy.  Most of these rooms were owned by wealthy folks where the gardener tended the plants and made sure the house had fresh flowers.

The new, tight energy efficient rooms may be great for heat and light, but they are not so good for growing things. Many of them become plant morgues where pots of dried and shriveled poinsettias and spider plants get stored.  Plants need more than just light. They need air movement and nourishment.

"The greenhouse soil must be light, rich and aerated. Minerals, air and organic matter are the three necessary components for good soil. The most important minerals are: 1. Nitrogen which comes from decaying organic matter, cottonseed meal, fishmeal, bonemeal, and manure; 2. Phosphorous which comes from ground phosphate rock and steamed bonemeal; 3. Potassium which comes from wood ashes and granite dust. Air for the soil is found in the pockets contained in peat moss, vermiculite and sand. When soil becomes dense and compacted, aeration is lost and the plants drown.

Plants are also effected by the temperature in the space. In order for tomatoes, cucumbers and eggplant to fruit, they ideally need to grow in a temperature range between 55° and 85° F. Leafy greens grow best in a 45° to 75° F range. There are different temperatures at different heights in the greenhouse. During the winter start seedlings high up near the peak. The coolest spot is on the floor near the glass. During the summer, the room should be shaded to keep out the harshest of the sun. Take advantage of internal shading for growing plants like berries.

Plants need fresh air to replenish oxygen needed for respiration and carbon dioxide needed for photosynthesis. If there is not sufficient air movement in the greenhouse, a layer of stale air surrounds each leaf and slows growth. Air movement through ventilation is the most effective way to control humidity and temperature, and thus to promote the propagation of vigorous, disease-resistant plants."  (Susan Yanda, The Solar Greenhouse Garden, Second National Conference on  Energy Conserving Solar Heated Greenhouses)

Thermal Mass

October 2000

Thermal mass is a critical factor to consider when predicting the comfort performance of a sunroom or solarium or greenhouse. Thermal mass works like a temperature flywheel. All of the components in the room absorb heat (or coolth) and then return that heat (or coolth) back to the space when the temperature swings the other way. For example, consider a brick that starts the day at 50ºF. The air temperature surrounding the brick is also 50ºF. If the surrounding temperature rises to 55ºF, the temperature of the brick will absorb some of that heat and rise to 55ºF. How quickly it rises to 55ºF, depends on the "heat capacity" of the brick. Every material has a heat capacity number that indicates how much heat the material can absorb depending on the rise in temperature. Air, for instance, can’t hold much heat, only .018 BTU per cubic foot for each degree the temperature rises. A brick can hold 24.6 BTU’s per cubic foot per degree F rise. Water can hold 62.4! So if you have a barrel of cool water, it will take a long time to warm up and a long time to cool down. The brick will respond more quickly, but the air will have the fastest response of all.

If you added up all the cubic feet of brick, dirt, water, tiles, wood, cotton (furniture), etc. in your sunroom, and multiplied each of those components by their heat capacity number, you could calculate the total thermal mass in the space. Why would you want to do such a thing?  It depends on how you want to use the room. If the space will be used to grow things, for example, a fairly large amount of thermal mass is needed to limit the temperature swings and keep the space warm into the night. If the space is needed to generate heat for the house, using it as a solar collector, a "light" mass is best. Don’t leave the heat in the sunroom. Draw it quickly into the house.

The same thing is true when using a cooling fan. A room with a "heavy" mass will take a long time to cool once it gets hot even after the sun goes down. You should also note that the slow temperature change also applies to warming the space once the mass gets cold. If you’ve ever been on one of those old stone castles or cathedrals, you have experienced "heavy" thermal mass first hand!

For most sunroom installations, the thermal mass of the room is much less of a design factor than the type of glass or how it is furnished. Nevertheless, for a complete, system design approach, the mass is an extremely important element for the final product to work the way the owner expects it to.