Building Science Basics - Continued
Moisture
Moisture is not something we think of very often unless we have a roof leak, plumbing problem, or a flood. However, it can lead to bad indoor air quality, mold, or even rot the house around you. In fact, it is the most important factor to consider when we think about house durability. As we built tighter and tighter houses over the last half century, we are still getting just as much water into the houses as in the old days, but it is now harder to dry the houses out. We are now using more insulation in our houses, this slows not only the heat flow, but it also slows the drying out process. Additionally, the engineered materials that are now being used to build our houses are much more sensitive to water damage then the dimensional lumber that was previously used. Therefore, we must prevent the excess water from getting in and allow any that does get in to get back out. There are four ways that moisture moves; bulk, capillary, air transport, and vapor diffusion.
Bulk moisture can come from rain, ground water, or plumbing. Normally, this type of moisture is taken care of by using proper building techniques and normal maintenance. However, occasionally houses do have issues such as flooding. Bulk moisture is the most important because it is a huge amount of water and can do a lot of damage, but it is also the easiest to identify. Fixing the leak (roof, plumbing, or cracks in the foundation) usually takes care of the problem quickly. Then you just need to dry the house out, which can take some time to accomplish.
Capillary action is the water wicking through porous materials or between small cracks. These are often much harder to locate because they may be seasonal or just affect a small area. These issues can be fixed by sealing the cracks, increasing the gap size, or installing a waterproof membrane.
Air Transport is the flow of water, that is held in the air, into enclosed areas through unsealed penetrations and gaps between conditioned and unconditioned areas. A 1 inch square hole can transport 50 pints of water into your walls during the heating season. This water usually comes from occupant behavior, such as cooking, bathing, house plant, and breathing. These problems are solved by ensuring the air barrier is continuous (no holes or gaps) and using spot ventilation (range hoods and bathroom fans) to remove excessive moisture at the source.
Vapor Diffusion is the movement of water vapor in the air through permeable materials. Compared to Air Transport, Vapor Diffusion will on move about 2/3rds of a pint of water during the heating season through unpainted drywall. Using a paint or a primer that is rated as a vapor barrier will help limit the amount of vapor diffusion.
Before we continue our discussion on moisture, we need to define two terms.
Relative Humidity(RH) - is the amount of water that the air currently holds, at the current temperature, compared to the maximum it can hold, at that same temperature. We can change the relative humidity of the air, without changing the actual amount of water in the air, by simple changing the temperature. Holding the actual amount of water in the air constant and increasing the temperature will decrease the relative humidity because the air can now hold more water. Similarly, lowering the temperature will raise the relative humidity because the air cannot hold as much water.
Dew Point Temperature - is the temperature at which, given the current amount of water in the air, the air will reach saturation or 100% relative humidity. Water will condense out of the air below this temperature. When we see steam in the bathroom after a shower, the air is at or close to 100% RH. The mirror is often several degrees colder than the air was in the shower and thus the moisture condenses (fogs up) the mirror.
People like a RH between 30 to 60%. Below about 30% RH, we start to feel too dry. In the winter time we often feel too dry, that is because we have warm moist leaving the house (stack effect) and replaced with drier (actual water content) cold air. As this replacement air warms up, its’ RH drops. We then feel we have to add water back into the air to get it comfortable. By slowing or stopping the uncontrolled exiting air with Airsealing, we will reduce or even eliminate the need to add humidity back into our homes during the winter time.
When the RH goes above 60%, our sense of hot and cold can be accentuated. More importantly though is when the RH goes above 70%, you can expect to see more mold growing. The higher the humidity the more and faster the mold will grow. This can have dramatic negative affect on indoor air quality. Using spot ventilation to eliminate the moisture at the source is the easiest way to solve this issue.
Air leakage, into a cold attic in the winter time, can lead to a problem that looks like a roof leak but is not. What can happen is the warm moist air enters the attic through leaks in the ceiling, the air cools until it goes below its’ dew point temperature, then water will condense out of the air on whatever surface is nearby. This condensed water will then drip back onto the ceiling drywall and appear to be a roof leak, when in fact it is actually an air leak in your ceiling. This will most often happen in areas with a lot of water available to go into the air, such as a bathroom or even a kitchen (fixed with Airsealing).
Indoor Air Quality
Indoor Air Quality (IAQ) issues in a house arrive from a large variety of sources: Smoking is by far the worst cause of bad indoor air, which is why it is banned in most public buildings. Cooking is often one of the next things on the list of causes for bad indoor air. Did you remember to turn on the range hood last time you cooked, or was it simple too noisy or worse yet, a recirculator type. All the cleaners, soaps, shampoos, and other household chemicals definitely have an effect on indoor air quality. Soil gases like Radon. Pets of all variety may shed or have dander (although cat dander is one of the worst). Even the materials the house and furniture are made of (particle board, OSB, carpeting, paints & stains, etc.) can off gas (often formaldehyde). That new car smell that everyone knows, it is the off gassing (bad stuff) of various material (plastics mostly) that the car is made of, would you want to live in that environment very long, certainly not. Those people with allergies, asthma, or otherwise have a compromised lung function suffer the most from these problems. The longer people are exposed to bad air, the worse their problems can become.
Fresh air (mechanical ventilation - intake and exhaust) is the primary solution to iaq issues, but don’t forget, if you don’t bring it in - you won’t need to get it out. Therefore, choose carefully when it comes to furniture, carpeting, and cleaners. A simple rule to follow is - if it stinks it is not good.
For more info: Indoor Air Quality issues
The Three Fixes
Airsealing - is the service you probably have never heard of, but can't live without. The best and simplest explanation of what airsealing is is to think about winter time and wearing a sweater outdoors on a cold and windy day. How much good is that sweater doing to keep you warm, probably not much, because the wind goes right through it. Now put a windbreaker on over the sweater and you quickly warm up. Why? Because you stopped the wind (airflow) through your insulating layer (the sweater). Similarly, by airsealing the house, you stop the airflow through your insulation (allowing it to do its' job of resisting conductive heat loss), plus you are stopping the convective loss of the warm air leaving the house and cold air entering the house (assuming it is winter time). This is the two for one benefit of airsealing.
For more info: Airsealing Section
Insulation - ALWAYS follows airsealing. Why? Because the insulation can't do its' job if air can flow right through it. A nice thick blanket of insulation in the attic, with proper airsealing, will even out the a lot of the temperature differences through out the house. The walls also need proper insulation, with knee wall (those wall section that open to the attic) needing special care to ensure the insulation is doing its' job.
For more info: Insulation Section
HVAC - heating, venting and air conditioning, plus things that create heat (eg. water heater) or move air (eg. bathroom fan). Once the first two fixes are complete, the HVAC system can be properly sized for the house. In America, we think bigger is better, but for HVAC, bigger is worse. Why? Because the unit will short cycle, meaning it turns on for a short period of time and turns off, and repeat. Unfortunately, this leads to very uneven heating or cooling throughout the house. The other problem is that the entire system (equipment and duct work) must be properly designed to work together. Most often it is not and that results in a typical house only getting about 57% of the rated output of the equipment. Thus a 93% efficient furnace usually only delivers about 53%. Put another way, for every dollar you spend on heating, you only 53 cents worth of actual heating in return. The best systems run at about 95%, with a 96% efficient furnace, would give around 91 cents to the dollar (there are always some losses). HVAC Airflow & Efficiency Testing can find these installation errors that cause poor performance. The other area of concern for HVAC is ventilation. How much and what is the air source? Mechanical ventilation is the solution (there are a variety of options).
For more info: HVAC Section
The Two Main systems of the house
Envelope (or Enclosure) - There are two important barriers within your house. These are the thermal barrier and the pressure barrier. The thermal barrier is made up of the insulation and is intended to restrict the heat flow. The pressure barrier is made up of the drywall, doors, windows, and various other construction materials and is intended to restrict airflow. These two barriers should be continuous, properly aligned, and fully touching. These two barriers together make up the (living) envelope of the house. Air inside the envelope is what we condition and air outside the envelope should be unconditioned. Unfortunately, in many house the barriers have gaps, are misaligned, or are not in full contact with each other, resulting in uncontrolled heat flow and/or airflow. This uncontrolled heat flow or airflow results in the house being less comfortable, as well as having possible air quality issues.
HVAC - This was discussed above.
Control=Comfort
By controlling heat flow, airflow, moisture, and the indoor air quality, we can gain a comfortable, healthier house. Here is the general order to be done, assuming no overriding priorities, major problems exist, or budget concerns. Airsealing the ceiling on the highest level is always first. This stops the heat loss by convection to the attic and allows the insulation in the attic to do its’ job of reducing heat loss by conduction. Furthermore, it stops moisture from getting up into the attic and causing problems, as well as the need to add humidity to the house in winter time. Next up is airsealing the bottom of the house, this closes off where the air usually gets into the house (and out of the house in the summer time). Then we spend time airsealing the area in-between with particular attention to any connections to the garage (IAQ issues). Once airsealing is done, we start adding insulation at the top and move downward. The airsealing and insulation together give us the biggest bang for our buck since they are relatively inexpensive to accomplish.
Next the HVAC systems are worked on. By tightening up and insulating the house, you will not need as big of a furnace or air conditioner. The only way to know what you need is to run a computerized calculation of Manual J (heating load - joules), Manual S (equipment selection), Manual D (duct design), and Manual T (termination - registers). These calculations will tell you exactly how big the system needs to be along with duct sizes for each room. Anyone telling you that he knows what your house needs without running these calculations is talking out of his backside! Once you have this information, it can be decided how much of your existing duct work can be used and what needs to be improved. By buying a smaller system, you may save enough money to help justify the improvements to the duct at very little extra cost. It is also important to make sure the duct get sealed using mastic (not tape), this is a permanent flexible sealing that helps ensure you are getting the air where you want it to go. Most importantly, you will be more comfortable with a system that is designed properly, something not many people have in their house. By the way, if you have swapped to a high efficiency furnace, you should also install a direct vented water heater at the same time. The old atmospherically vented water heater is more likely to back-draft if you do not have make-up air ducts installed.
The V in HVAC stands for ventilation. Now that the house is tighter, you need to have enough fresh air getting into the house to keep the good IAQ that you have working to get. This ventilation can be a HRV (heat recovery ventilator) or a combination of spot ventilation, continuous house ventilation, and supply ventilation, or the best, a combination of the HRV and spot ventilation (range hood), plus make-up air.
Windows and doors are last on the list because of the high price tag and relative low pay back (from an energy perspective). This is not to say you should not replace them, there are certainly times that it makes a lot of sense. For example, if you are doing a major remodel of a room, then replacing a couple windows in that room in conjunction with everything else being done makes sense. Another case where replacing windows makes sense is if the windows are in such bad shape that they cannot be repaired. If your windows have condensation between the panes, the seal has been lost. In this case, it makes more sense to just replace the glass (Sealed Glass Unit - SGU) rather than the whole window. Even if you have single pane windows, it makes more sense to just add high quality storm windows that will cost you a third of what a full window replacement would cost and perform nearly as well as a new dual pane window. Regardless of which option you chose, make sure the new glass has a low-e coating. The low-e coatings helps to keep the heat energy inside the house in the winter and outside in the summer.
For more info: Windows and Doors Section.
Final thoughts
Remember, changing stuff in the house will change how the house operates and the interaction of the various components. Therefore, the house needs to be retested after changes have been made, particularly airsealing. Some of the changes can actually result in the house being less safe, thus requiring the need for mechanical ventilation to ensure there is enough fresh air. Retesting should be done to catch these unexpected and unsafe interactions so they can be fixed.
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