Air Sealing and Insulation

Air Leaks are the easiest, most numerous and cheapest way to decrease waste

Air Leaks are #1 cause of lost energy

"Five Ways your Home Loses Heat"
Heat is lost to infiltration and air loss by over 3 times the amount it is lost due to ceilings. These categories generally stack up this way in terms of % heat loss in a home:
  • Infiltration / Air Leakage: 35%
  • Windows & Doors: 18%-20%
  • Floors & Below Grade Space: 15%-18%
  • Walls: 12%-14%
  • Ceiling/attic energy lost to conduction because of lack of insulation : 10%
(note: this last category of "ceilings" does not include leaks through microscopic cracks in the plaster, lifted drywall tape, moisture damaged disintegration, holes for light fixtures, leaks through penetrations for wires etc.)
The alternate EERE research shows similar air losses categorized differently
That is, put another way, insulation only insulates when air does not move through it.
Consider this example here of the green metal siding that obviously not only did not prevent air flow, it did not stop flying insects such as wasps from building their nests in the fiberglass "insulation" !
This insulation provided almost no benefit. Air filters for engines are sometimes made of fiberglass. Fiberglass provides no air barrier of its own.

Remember to compare these general percentages with the cost for each repair and keep in mind that blown cellulose insulation both insulates and reduces air leakage.
If a wasp can FLY into the insulation, you KNOW the air flows right through it - thus making the "insulation" worthless
Insulation only works if there is an effective air barrier.

An Energy Audit is the key to finding the air leaks

An energy Audit is a detailed analysis of the hidden energy losses that can increase your monthly cost of living by as much or more than a car payment ! So ONE energy audit can save you many times the cost of a new car! Even more importantly, those air leaks sometimes exacerbate allergies to mold, vermin and dust as well as make you unecessarily uncomfortable.

An independent auditor is the best first investment during planning for building upgrades because that auditor works for YOU and not for a commission on a bigger heat pump or more expensive windows etc.

Some tips on getting started with your own lifestyle and building shell "tuneup" (audit).
Energy Star listing of "Common Problems" (most of which will be qualified and quantified by a thorough energy audit.
Energy Star on Audits

Dominion Home Energy Calculator

How much pollution does your energy consumption generate?
(note: to find how much pollution an alternative energy source would save, just plug in the estimated kWhrs of energy replaced by your alternative energy source instead)
Also this includes the power line losses (estimated for your location!)

Home Energy Saver - Do It Yourself audit guide

"Consumers don't want to buy electricity or fuel.
They want warm showers and cold beer !"

Amory and L. Hunter Lovins, Rocky Mountain Institute

A Blower Door measures the air leakage of a house with CALIBRATED meters

A Blower Door

...uses calibrated fans and meters to measure how MUCH your building leaks air. This give you the chance to see exactly how leaky or well sealed a house is. Then after work is done, can see how much the efficiency has been improved.
One caveat to remember is that this simulates a very windy day. There are some instances where a house can be leaky but if the leaks are protected from the wind, it will still hold heat. But a Blower Door is a great place to start with quantifying the effectiveness of your house for holding heat and AC in.

Any audits you get on your building should include the Cubic Feet per Minute and the calculated Air Exchange Rate that comes from using a Blower Door properly.
Did the audit include the CFM and air exchange rate (number of exchanges per hour.) ?

After the blower door has enabled us to both quantify as well as qualify the location, size and nature of the air leaks, bypasses etc., then we can consider controlling the three types of heat loss or gain.

Heat Transfer

Understanding some basics of heat transfer will help you save money and energy consumption.
  • Heat moves from a hot place to a cold place
  • Heat moves in proportion to the DIFFERENCE in temperature. So it costs more to heat a house on a 20 degree day because the heat moves more quickly out of your house.
  • Heat moves in three different ways that must be addressed differently
    • ConVECTion (with moving air) This is the most common way you lose heat (Ex: Hot air coming out of the duct leaks). Also a way to get hot air coming in during the summer.
    • ConDUCTtion (ex: The hot frying pan to your food)
    • Radiant (ex: The Sun shining on you and your house)
Any plan for efficiency, energy conservation and targeted alternative power must take all three forms of heat transfer into account.

Which finally leads us to ... (drum roll) ... the reason you came to this web page ... Insulation

The theory behind insulation is to slow heat transfer. This works regardless of the direction of the heat flow. Heat moves from hot to cold places in proportion to the difference in temperature. So in summer, your air conditioned house loses heat outward to the hot outside. Your house will get hot faster when its hotter outside. In the Winter, reverse the process. Insulation will work the same in either case.

Insulation slows heat transfer by impeding the three different way that heat moves (see heat transfer)
The most effective insulation is a vacuum. That is why we used to have "vaccuum thermos" containers before we came up with advanced closed cell foam that could be blown into shapes.
In a vaccuum, the heat cannot move by either convection or conduction.

The next most effective insulation is air. STILL air. If the air moves either through an area, then it is transferring the heat by convection, like in your oven. Air gaps that are too large can have "convection currents" that go in a loop to carry the heat from a cooler window pane to a hotter one. Or inside a wall gap. If there is a leak, then the air just carries the heat with it through the air gap. That is why leaks are so important to heat loss control.

So the goal is if you can't have a vacuum, to keep the air STILL. That is why double paned windows and foam and air barriers with fiberglass work better than single pane windows or fiberglass with no air barrier.
Fiberglass is used on engine air filters. Fiberglass does not stop air flow. So without an air barrier, fiberglass is nearly worthless, only slowing the heat loss slightly.

Closed cell foam works by trapping cells of air. Open cell foam also traps some air, but not as much. Styrofoam encapsulates tiny tiny air bubbles in plastic.
Storm windows work by slowing or stopping air flow.
Cellular shades work by trapping a cell of air.
Exterior insulation works by trapping air outside stone and mortar so the air inside can use the stone as a "thermal battery" and store heat.

Eventually entropy wins and everything ends up the same temperature. Insulation only slows that process down.

So insulation must control
  • Radiant Heat gain and loss
  • Conduction of heat from one object to another
  • Convection of that heat through the air currents

  • Cell Shades insulate your windows

    Cellular Shades

    Cell shades are like adding a styrofoam panel to your window ... in-effect superinsulating your windows. They can also come with a track to block air from flowing around the shades.

    [Click here for a demonstration]

    Cellular shades are especially helpful on the north side of a house where you do not get any solar gain in the winter and thus want to keep your heat in, but cellular shades can also help control excess heating from the Summer Sun and keep your AC inside and the Summer Heat out!

    Commonwealth Solar can install cellular shades for maximum cost-effective window insulation.



    Heating and Cooling Ductwork is a MAJOR potential loss point for heating and air conditioning!
    • Air leakage through gaps in ducts. The heat is lost with the air moving (convection).
    • Air leakage through (convection) interface of duct to wall/floor/ceiling (registers)
    • Conduction of Heat through walls of ducts
    Air leaks are the leading loss point for heating and air conditioning.
    The first goal of any efficiency and conservation effort must include checking the ductwork for leaks.

    Burying the ductwork where possible under cellulose insulation will help tremendously with both causes (conduction and convection) of heat loss from ductwork.

    We have even seen very "affordable" installed ductwork that was LEFT WIDE OPEN, typically at the end of a house. We usually find those when the occupants complain that an end room seems to get little or no air flow from the registers.
    But where the ducts are in the attic or even (horrors!) out in the outside air ... then proper testing, sealing and then insulating those ducts is vital.

    If the house seems to be cheaper to heat with electric space heaters, this is a very important piece of evidence that your ductwork is hugely wasteful.

    Two studies showed that ducted homes had 70 percent more air leakage and used 17 to 22 percent more energy for space heating.

    • Consider that the square footage of exposed air is often just as large in the ductwork as the entire ceiling of the building! The duct has four sides and travels the length and width of most buildings. In spite of this huge square footage of your conditioned air to the attic and/or outside air, most ducts have no insulation or only R3 for half an inch!

      So putting ductwork in the attic is like adding another whole attic of heat loss in the winter or heat load in the summer.  So a duct exposed to the heat and cold of an attic with no insulation or only R3 of an inch or fraction of an inch makes heating or cooling a  house with ducts in the attic like heating and cooling an entire additional building. Is your wallet up to that?
    • The solution is to either place the ductwork in the conditioned space (not in the attic) if you are installing a new system from scratch for an upgrade of a historic house or building a new building.
    • If the ductwork already exists in the attic, then seal and insulate as much as absolutely possible. Check that ductwork periodically because often some of the sealing will fail over time.
    • Also pay attention to how well sealed the registers are to the wallboards, ceilings and floors especially because a leaky return register can draw in air from the inside of the wall or even outdoors which might include mold, illness causing irritants like fiberglass fibers, and vacuum air from  the basement or crawlspace through  the inside of the wall where holes in the base allow air to come up from below.
    Oikos noted that one study showed that "by repairing 43 percent of the duct leaks, they cut infiltration by 63 percent." and that "heat loss from the average ducted air distribution system reduces the overall system efficiency by 30 percent."

    An independent auditor is the best investment because that auditor works for YOU and not for a commission on a bigger heat pump or more expensive windows etc.


    Foam outside wall that will keep out freezing air temperatures in the Winter as well as a air and vapor barrier for leak reductions

    Exterior Insulation

    Usually a brick or concrete foundation and/or walls are insulated inside. However when possible, its actually better to insulate on the OUTSIDE.

    Exterior Insulation usually with foam panels provides a wonderful opportunity to use the "thermal mass" of your building to your advantage. Thermal mass is any heavy mass (like the thick walls of older municipal buildings, or a cave, for example) that change in temperature very slowly. Thus as you go into Summer, the cooler condition of the thick walls decreases your need for Air Conditioning. And then as you enter Winter, the left over heat from the Summer decreases the rate at which your house cools off.

    Insulating the outside of a thermal mass, that is, a thick brick or concrete wall and/or foundation now means that effect is amplified because you have insulated your walls from the extreme heat of early Summer and the extreme cold of early Winter. As you run your Air Conditioning or Heating, it is now heating a wall that averages the outside temperature as well as absorbing as a kind of "thermal battery" your efforts to cool or heat your house to the temperature you want.

    An added benefit is that in some cases there are many microscopic holes in the mortar that allow diffused air to be drawn through the brickwork!
    So instead of "pointing" the joints in brick, another option that gives you energy efficiency benefit is to put on exterior insulation.
    (For example, in one blower door test, we found a steady blast of air coming down a chimney. This was in spite of the fact that the top of the chimney was capped solidly. It turns out the mortar had failed to some degree, it needed pointing, and so the air was coming into the house through the mortar!)

    Interior Storm Windows

    Interior storm windows are very easily made of "clear" pine or other more attractive wood and slips into the inside of the window to create an air barrier for far less than a new replacement window. It also allows you to keep the historic windows but stop the air loss.
    Also interior storms can be constructed for rental historic office or residences cheaply enough to save more than what was spent on the storm windows. There are also more permanently attached magnetic interior storm windows.

    Interior storm windows stop the air leakage that accounts for MOST of the lost heating or air conditioning. Then shade cloth in the summer prevents too much radiant heat from the sun coming in. Cell SHades are like instant retractible insulation for your windows
    Cellular shades can insulate the windows from conducted heat lost in both winter (nighttime) and the summer (daytime).

    Generally interior storm windows are made on a wooden frame sized to the window minus about 1/8th of an inch. Then weatherstripping is applied to the outside after PolyOlefin ("heat shrink") film is wrapped around that frame. That weatherstripping provides both the air seal and the friction to keep the interior storm window in place.

    Commonwealth Solar can manufacture interior storm windows.

    Interior Storm Window is a cherry frame with Poly Olefin (heat shrink) film wrapped around it

    Incandescent Lights are mostly resistant strip heaters


    Illumination makes up a huge part of any typical energy bill for a building.
    Lighting is included in the insulation section because so often the heat of that light bulb PUMPS YOUR HOT OUT OUT THROUGH A BYPASS IN THE CIELING. As you can see in the graphic ...
    • Incandescent lights are essentially heaters that also make light since 90% of the energy goes into making heat. There are other ways to illuminate your work and play space.
      • The worst problem case are the recessed incandescent lights or halogens that are each a potential fire hazard and "smoke stack" that will pull heated or cooled air right out of your living space! Recessed Lights are MAJOR leak point!

        A professional energy auditor will pay a lot of attention to recessed lighting.

        Many times there is no insulation or sealing on the recessed lighting containers and this becomes the equivalent of several wide-open windows!!!

      Misconceptions that Misappropriate your Money

      There are many common misconceptions and occasionally misinformation  that will waste your money and give you very little to no comfort increase for the dollar.
      Once you understand some basics, you can see where a trained energy detective auditor can make a huge difference for much less investment.

      Here's just a selection of four examples:

      Example 1: Fiberglass insulation. Air filters are often made of fiberglass. You can often SEE where a leak of air has passed THROUGH the fiberglass insulation by the dirt that it has filtered out of the passing air.

      Fiberglass insulation only works when there is no air flow or air pressure. Typically an air barrier of some kind ensures no airflow so that the insulation can work.

      Dusty dirty fiberglass tells you that either
          no air barrier was installed,
               or was badly installed,
                      or has failed.

      Using blown celluloses insulation instead of rolls of ineffective fiberglass does a much better job of filling in gaps and stopping air flow.
      Commonwealth Solar provides the service of blowing cellulose insulation  and simultaneously sealing your attic and walls after a proper audit

      For more information on controlling air flow with insulation and vapor barriers see Building

      foil bubble insulation can reflect heat back toward YOU
      Example 2: Misconception: Radiators are inefficient. A radiator will lose the most heat to whatever is closest and coldest. Heat goes from a hot place to a cold place.
      So what is the coldest place that is closest to your radiators?
      The wall of course!

      So by placing pennies worth of reflective foam insulation behind your radiator you dramatically lessen the lost heat to that cold exterior wall!

      Example 3: Solar Attic Fans. An attic stays hot long after the sun goes down. Also the purpose of ventilation is to remove moisture (if there is any) as well. A solar activated fan typically does NOT have any moisture sensor.

      Far better to
      • install a ridge vent and/or
      • bury your ductwork in blown cellulose insulation and/or either
      • get a reflective roof such as TPO with insulation under the membrane or
        • seal the roof in something like the silver or white coating that also has the benefit of protecting your roof tiles, metal or membrane from the sun's UV and heat.
        • This will also reduce the heat load on whatever is inside your attic, such as your heat pump air-handler. The less your air handler has to fight, the less you pay and the longer it lasts.
      Example 4: "Houses must breathe". A house must stay dry. You do the breathing. Properly handling the moisture is the key. A musty smell and dripping windows are your hints that there is a problem. Building Performance Institute literally wrote the DOE book on moisture management health issues from mold.

      [More on energy wasted by ductwork
      in the attic and how you can save that]

      More On why a Professional Energy Audit
      should always be a part of
      purchasing or upgrading a house.

      There are many other misconceptions for which there are tricks of the trade.
      Call Commonwealth Solar to have an energy detective find your energy thieves and help you spend the money you already use for energy much more effectively!

      Stack effect is just like a smokestack, the hot air rises in the house, drawing in cold air at the bottom and blowing it out the leaks in the attic

      Stack Effect

      "Stack Effect" is all about convection ... the air carrying your heat or air conditioning away through leaks and the air flowing up through the inside of your house from the bottom and then out the top ... like a smoke stack!

      How does the smoke get out of a chimney instead of coming into the house? Because hot air rises and creates suction at the bottom pulling the smoke up and out the top of the stack.

      The same thing happens in any box ... including your house!
      That is why you feel a draft at the bottom of a door more than the top. And why the basement leaks are SO obvious when you find them. But you can't seem to feel any leaks on the top floor ceilings.

      That is also why the windows in the middle make less difference (in still air) than air-sealing the basement, crawl space and attic.

      That is also why air leaks and infiltration is the #1 cause of lost cooling and heating.
      And also why open fireplaces are a major source of lost heat ... and why they often have glass doors in front of them.

      One tip that this is rampant in your building is extra-dry air leading to chalking skin and nosebleeds.
      Combustion of air in your living space is then exhausted along with the moisture and the cold air coming in is relatively dry thus dramatically drying out your living space air and causing dry skin and  your nose.

      TIP: CLOSE THE CHIMNEY DAMPER When its not in use!
      "According to the Department of Energy, a lit fireplace sucks about 24,000 cubic feet of furnace-heated air up your chimney each hour.
      Bonus: It's replaced by cold air that comes in the opposite direction through the same opening, causing your furnace to work extra hard to keep your house toasty.
      Crack a window in the room where the fireplace is located and then close the door, so it doesn't suck too much warm air from the rest of the house.

      TIP: Provide a "Fresh Air Vent" for your combustion devices (gas fired hot water heater, furnace, fireplace, etc.)

      The orange is the Thermal Envelope" and also where insulation should usually be placed

      Thermal Envelope (Shell)

      The thermal envelope is that border between your inside temperature and the outside temperature.

      Put another way, the thermal shell describes the part of the building that separates your occupied conditioned (heated or cooled) space from the outside. For example in the attic, your "roof" comes in TWO parts. The outside part that keeps out the water ... and the floor of the attic that separates the thermal areas. The attic is vented to the outside and the insulation airseals and insulates that from your top floor rooms.

      Note that the thermal envelope is not always the same exact place as the weather (liquid) envelope.

      In this picture, the orange parts are generally where you would place insulation.

      An exception is that, if you can, by placing the insulation on the outside of the thermal mass (bricks) you can help make the house more efficient by using that thermal mass to even out the temperature swings. Thus your heater and Air Conditioner now works against your cliamates's AVERAGE temperature instead of that day's temperature.

      Here's a link to a  great example house ...

      Wind Pressure

      Most of the time you are better off focusing more money and time on the basement and attic. However if your building is exposed to a lot of high wind conditions, it may make sense to spend extra attention on exterior storm windows, interior storm windows, possible window replacement (but check their air leakage rates !!!!) and of course air sealing on the upwind and downwind sides of the building.

      If you have any further questions, please email and ask so that we know what we need to add to this appendix !


      Note: Last Update 06/02/2021

      CommonWealth Solar, LLC

      12433 Autumn Sun Lane • Ashland VA, 23005 • (804) 216 5371