The Home Owner's Guide to HVAC

The Envelope and Green Technologies

Rob Turner P.E.

• Publisher: Page Publishing, Inc.
• Year: 2021
• Print ISBN: 9781662440106
• eBook ISBN: 9781662440113
• Topic: Architecture
• Subtopic: Architecture General

How green is green? The average homeowner is usually well-informed about which products and concepts are considered "green, " but they seem to have less of an understanding of how they compare, and which things work the best for the lowest investment. Did you know that 40 percent of the average home power bill is dedicated to air conditioning in warm climates? Further, energy is spent on cooking, hot water, clothes dryers, and such. Did you know that solar hydronic panels are way more efficient than solar voltaic panels? How about instantaneous water heaters? Are they good? Homeowners need to make dozens of decisions about which products or services they need in order to make their home the most efficient it can be for the given budget. Do you have mold or humidity problems? If you want to learn about things like the home envelope, HVAC, hot water, and electrical systems, this book is for you. It is written in plain English, and covers all the bases. Also, given are tips on saving the environment from an engineer's perspective. The author is a big fan of the environment, but does not like "greenwashing." The generic definition of greenwashing is the practice of making an unsubstantiated or misleading claim about the environmental benefits of a product, service, or technology. Pros and cons of various "green" products are discussed so that the homeowner can understand the smartest purchases and practices. One of the things you will learn is that the envelope of the home is just as important as the HVAC. You have to understand both to be successful. The envelope is thoroughly discussed as well as HVAC, plumbing, electrical, and the various green technologies.

Chapter 2

The Envelope and Humidity

When considering humidity, the envelope of the home is probably more important than the air conditioner itself. Humidity can cause real damage to the home. Mold is a dirty word. It is the subject of lawsuits, and it terrifies contractors and homeowners alike.

New houses are built very “tight.” This means that they probably have spray foam insulation, the best windows and doors, and a vast array of weather stripping and seals. Some people say that homes can be too tight. This is a very rare occurrence in my opinion because contractors are humans, and humans make mistakes and don’t always do every facet of a job perfectly. Even in new homes, there will always be cracks, crevices, and imperfections that will allow humid air to flow into the home. Basically, if a home was “too tight,” then the exhaust fans would not run with the doors closed. Think of trying to draw air out of an empty plastic milk jug. When you run an exhaust fan, it discharges a certain quantity of air—let’s call it 100 CFM (cubic feet per minute). If you exhaust 100 CFM of air out of the building, and the fan is running properly, there will be 100 CFM of hot, humid air that will enter back into the home to replace that air. The objective is to get the new home as tight as possible so that there are minimal cracks, voids, and crevices for air to flow in and out of the home.

Older homes usually have more cracks and crevices, and the air flow is higher in and out of the home. When the wind blows on a house, it creates a positive side and a negative side on the home. This can cause a lot of air movement through the home. This air movement brings in a lot of heat and moisture from the outdoors that the air conditioner has to overcome. Leaving a door open on a hot and humid August day can bring in a lot more moisture than you think. Leaving the door open on a hot day for ten to thirty minutes when there is a gentle breeze can bring in water vapor into the house in quantities better described with gallons than cups. The air conditioner will eventually drain this water back out of the house, but it can get into a situation where the air conditioner is fighting a losing battle.

The problem is that a dry home is like magnet for moisture. Humidity has a natural tendency to even itself out over spaces. So if it is extremely humid outside, and you open the door, there is a powerful force that drives moisture into your home over a short period of time. This is one reason large buildings frequently have a vestibule, or two sets of doors, at the entrance. The intent is to enter the vestibule, and close the outside doors before entering the second set of doors. This prevents direct communication of humid outside air with dry indoor air. But aside from doors, this force (called vapor pressure) will constantly drive humidity through all the cracks and crevices of your home in the summer. In most cases, the home is tight enough to only leak enough humidity such that the air conditioner can keep up with the battle and stay ahead of the game. Homes that lose the battle usually end up having humidity problems.

Have you ever heard of “sick building syndrome”? Sick building syndrome is most commonly when you have humidity problems in the home that lead to mold issues and other air quality issues. Humidity is everything when it comes to air quality problems in the home. Mold spores develop when the humidity is sustained at 60 percent or higher RH (relative humidity). So the goal is to get your home’s relative humidity to below 60 percent, where 50 percent RH is ideal. The best solution to humidity problems is a good envelope. A good envelope will minimize the amount of hot humid air that will enter the house. The second best solution is a good A/C system. The reason I say the A/C system is second best is that improving the envelope usually reduces the struggle of the A/C system. So with less humidity in the home, the A/C system can be more effective.

Figure 7: Mold damage as a result of high humidity.

There are many types of envelope systems that can be implemented in the walls, floors, and roofs of a home. For example, a wall may be made up of vinyl siding on the outside, then Tyvek wind barrier, then OSB sheathing, then studs with batt insulation, and then sheetrock on the very inside. There are dozens of these systems which use different products for walls, and they can be modified for brick, stucco, and all the other wall types. I won’t cover all these wall systems and window systems but will instead cover the general building insulation, the attic, and the crawlspace. These areas are the most common areas for problems.

Home Insulation. There are many different types of insulation for homes. Insulation is a one-time cost that affects the power bill for the life of the home. People today have actually used straw as a “green” insulation. This might work well in Arizona, but it is a dangerous insulation type to use in the humid South. If any water vapor gets into the straw, it can become a mold factory. So this is not recommended in the hot humid South. Other contractors are doing some interesting things with concrete or metal walls with Styrofoam insulation, but this is not yet at the tried-and-true level of being a gold standard in home construction. There are a few other choices like rock wool and rigid insulation, but they are not as common. Let’s focus on the more common types of insulation. I would say the four main types of tested insulations in use today are batt insulation (the pink stuff you roll out in the attic), blown-in/loose-fill insulation, closed cell spray foam, and open-cell spray foam.

Batt insulation is the oldest technology, but if installed properly, it is a very good insulation. One common product is the pink stuff from Owens-Corning, which is fiberglass insulation that can be rolled out. This insulation can be used in walls, attics, and the crawlspace (in the floor system). Batt insulation can come non-faced (just the pink stuff) or faced with a vapor barrier. This facing is usually a brown paper material on one side. I just used the words vapor barrier. These words are used a lot in home construction. The intent of the words is that it is a barrier that slows down moisture transmission. It is actually incorrect to use the word barrier because a barrier implies than no moisture can get through. A piece of metal is an example of a true vapor barrier. Water cannot penetrate solid metal. The more appropriate words to use are vapor retarder. Having said that, the industry almost always uses the words vapor barrier.

If using batts, the vapor barrier is recommended. There is a lot of debate on which way to face the vapor barrier in a wall, whether it be towards the inside or towards the outside. I have heard that locating the vapor barrier towards the colder environment is best. I am not going to go any further into this debate, but will focus on the choice of major types of insulation. So, as for batt insulation, installation is the most important aspect. The installer must be very diligent to cover 100 percent of the wall cavities, the attic space, and the crawl space. If a one-square-foot patch of batt insulation is left out of the attic, it is not just a small hole where a small amount of heat and humidity can come through, but rather it should be thought of like it is a hole in a bucket of water.

Batt insulation is the least-expensive choice, but it has to be installed perfectly with 100 percent coverage in order to work at an optimal level and prevent humidity problems. Current code (zone 3) requires that twelve inches of batt insulation be installed in the attic. This is an R-value of 38. The best way to accomplish this is to roll out a six-inch layer of R-19 in one direction and another layer of R-19 in the other direction. This cross grid pattern helps seal up various holes more completely at the floor of the attic. Care must be taken to fully fill the points where the roof meets the floor of the attic. This example is for a ventilated attic. This concept as well as other attic concepts will be discussed below along with the insulation strategies.

Batt insulation, in a thinner version, is usually what is installed around the ductwork. If there is dripping under the ducts, whether they be in the crawlspace or the attic, you should check the duct insulation for tears, missing patches, or air leaks. These will cause dripping as the cool air can leak out and condense. Ducts in the attic or crawl space should have three inches of this duct insulation.

Figure 8: Batt insulation with paper facing.

Spray foam insulation is a superior product for humidity control in my opinion. This insulation can be sprayed into the walls, under the floor, and in the attic. It expands into all the nooks and crannies for superior coverage. It has an inherent vapor retarder built in that is superior to most other products. There are two types: open cell and closed cell. There is huge debate within the industry as to their differences, pros and cons. I won’t get too deep into the debate, but I will share some general information.

First, closed cell is more efficient per inch (twice the R-value per inch as compared to open cell). It dries hard to the touch, and won’t let water drip through. It has a superior vapor retarder. It gives additional structural strength to the wall or roof, and has good sound dampening characteristics. Closed cell is very difficult to remove and is not recommended for historic buildings. Closed cell is the most expensive.

Open cell insulation can achieve the required R-value with thicker applications. It is superior for sound dampening. It has a very good vapor retarder but not as good as closed cell. If put in a roof, it has a very good vapor retarder built in, but will let water drip though over time so that roof leaks can be identified. Open cell does not greatly increase the structural strength of the structure. It expands more than closed cell, and it dries with a “squishy” surface, i.e., it can be compressed and it will expand back to its original shape. It is more easily removed, but still must be carefully considered for historic buildings with historic roofing timbers. Open cell is less expensive as compared to closed cell. Open cell is my personal recommendation.

Spray foam of both varieties have some drawbacks. There is a very particular chemical mixture and application process. If put on to thickly or too quickly, the curing process will develop too much heat and could cause cracking in the insulation. This is a very expensive problem to fix. If the chemical mixture is incorrect, the product will not cure properly. Whe...

Table of contents

• Introduction
• The Envelope and Humidity
• HVAC
• Plumbing
• Electrical
• Bogus Green Concepts and Products
• How to Save Money in Your Home
• Engineer’s List of Ten Things You Can Do to Save the Environment