Building Science Basics

The following information will give you an overview of the various aspects affecting your house that we look at as Home Performance Inspectors. This list and the information presented is by no means all inclusive, but it should give you a basic understanding of how your house works. The suggestions that we come up with from our inspection of your house are designed to help control these aspects within your house. Once implemented, you will have a more comfortable home with better indoor air quality that will probably save you energy and money too.

The House as a System

Most people normally think of their house in terms of the various separate components; the furnace, the dryer, the water heater, the stove, the windows, the lights, etc. As Home Performance Inspectors, we think of your house as a system of interconnected components. Therefore, when we look at your house, we think in terms of how one or more component might affect another component. For example, we know that the dryer, range hood, and bath fans can affect how your water heater vents its’ exhaust gases, or the reason your teenager room maybe cold is because they keep the door closed all the time and heated air cannot get in if not enough air can get out (the closed door inhibits the flow out of the room). Furthermore, we understand that other things, like location and orientation, can also affect how your house performs. Although we cannot change your house’s location or orientation, we can suggest ways to increase your comfort.

This video discusses 'What IS Home Performance?'.

The following sections take a little deeper look into each of the 4-3-2-1 system of Home Performance.

Heat Transfer

You have probably heard that heat rises. Unfortunately, that is not quite right. Hot air rises, that is because it is less dense than the surrounding air. As for Heat, it is a form of energy and it flows from areas that are warmer to areas that are cooler. The bigger the temperature difference, the faster the heat flows.

Heat moves mostly in one of three ways: conduction, convection, and radiation. Actually, it moves in some combination of all three methods, but that is beyond the level of this explanation.

Conduction

Heat moves from molecule to molecule. It is the only type of heat flow within a solid. Whenever you touch an object and it feels warm, heat is flowing from the object into you. Similarly, if the object feels cool, heat is flowing from you into the object. Insulation is used to stop/slow Heat flow by Conduction.

Convection

This is heat flow within a fluid (either liquid or gaseous). The fluid will develop currents (either natural or forced) that move the heat around. Thus a forced air furnace moves warm air around by the force of the blower motor. Convection is also the one responsible the "drafts" in the house. These "drafts" are actually either from the hot air rising and finding exit in you ceiling (fixed with airsealing), thus cold air enters lower in the house, or they can come from natural convective currents near cold windows (the air near the window loses heat becomes denser and descends while warmer air rises elsewhere in the room, this cycle continues as long as there is a temperature difference.).

Radiation

Heat moves from one object to another object, not in direct contact, without heating the intervening medium. The Sun heating the Earth without heating the vacuum of space is the best example. Another is if you have been in a supermarket walking down the freezer aisle and felt cold, heat flow by radiation is to blame. You are losing heat to the freezer, but the air temp around you has not changed. Low-E glass is one form of a radiate barrier that is strongly encouraged. There are other types, but the long term effectiveness of these is a big question mark. They start out working well, but as they get dirty, their effectiveness decreases rapidly. Cleaning them is not a real possibility, so they are not currently recommended.

Air Flow/Leakage

Airflow is created by air moving from areas of higher pressure to areas of lower pressure. That sounds simple enough, but there are some complicating issues. First, air goes where it wants, it can be hard to keep conditioned air where you want it to be and unconditioned air out of where you do not want it. Secondly, air is lazy and takes the path of least resistance, this means it often does not go where you want it to end up. Next, 1 CFM IN = 1 CFM OUT (a cfm means cubic feet per minute and is referred to as a flow rate). What this equation really means is that for every chunk of air that leaves a house an equal chunk of air must somehow enter the house at about the same time. There may be warm buoyant air exiting through holes in your ceiling and incoming air may come from your crawlspace or garage (breathing air from either of these sources is not what you want, particularly for prolonged periods). It could also mean that when you turn on your commercial grade range hood that is supposed to take out every hint of smoke, it does not do so because not enough air can get into the kitchen fast enough. In other words, just because the fan is making a lot of noise does not guarantee that it is actually moving air.

So where does the high and low pressures in your house come from? There are a variety of sources; stack effect, reverse stack effect, wind, air handler (furnace), and venting equipment (bath fans, range hood, and the dryer). We will explain each of these next.

Stack Effect

For the house in general, we think of this as predominately a cold weather event. This is where the hot air rises to your ceilings creating an area of high pressure. An area of low pressure is also created low in your house that sucks in air to keep the balance. If the high pressure air finds an exit, say through your ceiling light fixture, the area of low pressure, low in your house, sucks in air to balance the amount of air. Often times the incoming air enters at the rim joist or from the garage. The taller the buildings the stronger the Stack Effect that will be produced. Airsealing will not eliminate Stack Effect, but it will reduce or eliminate the air leakage that it causes.

Reverse Stack Effect

This is predominately a warm weather event. The cooled air from your air conditioning sinks in your house creating a high pressure area low in the house and thus a low pressure area is created high in the house. Given some leakage out down low, hot air gets sucked into the house from the attic through leaks in your ceiling. Airsealing will not eliminate the Reverse Stack Effect, but it can reduce or eliminate the air leakage that it causes.

Wind

There will be positive pressure on the windward side of the house and negative pressure on the downwind side of the house. This is where leaks in doors and windows really play a bigger role, but by ensuring that the weather stripping is in good shape will take care of this problem. A bigger concern with wind is wind washing of your attic insulation. Wind washing is where the wind moves the loose blown attic insulation away from where it is supposed to be and leaves patches of your attic near the eves with little or no insulation. This problem can be resolved by using insulation baffles.

Air Handler

Most houses have supply registers in virtually every room, but only have 1 or 2 centralized returns. The system works okay as long as all the interior doors are left open. Once the interior doors start being left closed, pressure imbalances start to develop between the rooms with only supply air (they get pressurized) and those areas where the returns are located (they get depressurized). The doors are undercut to help relieve these pressure imbalances, but are insufficient to do the job fully. The result is the pressurized rooms leak even more air to the outside and the depressurized areas need even more air to make up for the air lost due to leakage. Balancing the pressure in the rooms by adding dedicated returns, jumper duct, or transfer grilles, along with doing airsealing will reduce or eliminate the leakage.

Venting Equipment

Most venting equipment (dryers, range hoods and bathroom fans) suck air out of the room and send it outside. This depressurizes the house slightly and air must somehow make its’ way back into the house. If the incoming sources are limited, then the outgoing air will also be limited, thus you will not be getting ventilation that you need to remove the moisture or kitchen odors. Even more important though is, where is the incoming air coming from (your attached garage? Your crawlspace?)? These unknown sources of can be a significant source of poor indoor air quality. You want to add a known source of air that can be filter to help eliminate the depressurization and improve indoor air quality. This known source of supply air is normally ducted into the return air side of your air handler (furnace) to solve the problem.

Remember: Energy Efficiency Stops Here!

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