What is Thermal Bridging?
To make your home green, you should understand a little building science to make wiser choices.
I have touched upon building science and thermal bridging in other posts; however, I feel that one post on the subject can make a homeowner a bit wiser when making green choices for their home. Using science or scientific methods and concepts has always gone into home building. These principles shine through in Roman engineering. In more recent years, there has been a renewed vigor in analyzing buildings by scientific means to improve their functionality, hence you will see more use of the term “building science”. One aspect of this science is studying how heat moves through a structure.
By studying how heat flows through your home, we can stop it, in order to keep the homes warm or cool depending upon the season. We all are aware that insulation helps; however, we may not realize that heat can travel through various objects, eventually defeating our insulating efforts. The term we use to describe how heat travels is thermal bridging, because the heat can travel via a bridge from one area to another past your insulation. Wood and other materials out of which we make our houses happen to be conductors of heat. By creating more massive walls, like a really thick adobe, we slow down the progress of heat, so we say these walls are a thermal mass. Different materials transfer heat at different rates, so we can plan out our structures to limit thermal bridging.
Here is what happens. Imagine a hot summer day with all of that sunlight hitting your exterior wall. The wall heats up, so the journey of the heat begins through that wall. Let us say that you wall has a typical amount of insulation in the stud (framing) cavities, an R19. This will slow down the heat, but now the framing is heating up. The wood transfers this exterior heat past your insulation to your interior wall. Now the heat has free reign into your cooled home. This is not a quick process, but it does reduce your energy efficiency. The reverse happens during winter, which is why your interior walls of exterior areas will be cold to the touch.
In new construction, there are methods of preventing the heat from traveling along the framing. A home inspector can only tell that this has been done during the farming inspection. Once the wall covering goes on, there is no way to see if thermal bridging has been deterred. Most new homes address the thermal bridging issue to one degree or another.
What about an existing older home?
My home, built in the sixties, has no insulation in the walls, and it had very little in the attic. Adding insulation to the walls can be hard and costly, so I focused on the attic. One way to reduce thermal bridging is to cover the ceiling joists with insulation and make sure that you have insulation in every cavity between framing members. A popular means of insulation for do it yourselfers is the rolled batts. If your batt is 18″ in width, but you joist spacing is 24″, you have a six inch gap. That gap can greatly reduce the effectiveness of your insulation. Another problem with rolled insulation is that you might have so much, and the last framing space that you come to has a few inches before you get to the next piece. That tiny gap also reduces the efficiency of the insulation. These are two fairly common problems that I see in my home inspections. Another is only insulating up to the framing for the ceilings. This leaves the wood framing exposed, allowing heat to transfer. You cannot cover up the framing holding your roof on, but you can cover the joists. This helps reduce thermal bridging, which will be our main goal in an older home.
As I said, my home has no insulation in the walls. The easiest and most effective method would be for me to take down the sheetrock to install insulation. This would be good to do when remodeling, but otherwise, few homeowners will undertake this task. What can we do? As building science is progressing, manufacturers are studying ways to improve products to help older homes. There are paints which can help slow down heat, so that may be your first step. On the interior, you can remove electrical outlet covers, and place one of these specially designed insulation pads behind them. My joke is next hang a tapestry on the wall. This was actually the function of tapestries on castle walls.
On an old home, your most inefficient locations will be the wall openings. You will have heard of adding weather stripping to doors and windows, and that is a good first step, but let us consider how heat can travel to the other side of the wall. Most older homes have solid wood doors, so there is not much that we can do, except paint the surface. The real problem will lie with the windows. If the house has an aluminum frame, single paned window, you have a problem. Aluminum is like a wick, pulling heat from the unwanted surface to the area where we do not want it. A single pane (sheet) of glass also is not the best at stopping heat. Replacing windows is the best option, but be sure to recycle the old windows to be green. Most new aluminum windows will have a vinyl cladding to help reduce thermal bridging. If buying new windows is not an option, we could consider a few steps. Painting is the simplest choice. Adding strips of wood over vinyl over the aluminum can help. Neither of these options are perfect, but the begin the path of reducing heat transfer. The next big concern would be the glass panes. Modern windows are engineered to reduce heat transfer, but adding a second pane of glass can reduce heat transfer. There are companies which manufacturer products that can be installed by the homeowner. These are custom products in most cases, so it may be better to buy the new window. Look at the costs. What I do like about this product is you are using the existing window. I did rig up my own plexiglass second pane, and this helped.
The next choice to reduce thermal bridging may be a decorating one. Wood and vinyl shutter help reduce thermal bridging; however, there are insulated shades which can do a great job. My wife and I have been going from room to room to add either shutters or new shades. These can be an expense, but you could pick the most important rooms to do, and keep working at adding them over time. I prefer wood shutters over other materials for shutters, but these can be expensive. When examining curtains and shades, ask for insulating shades and thicker materials. They may cost a bit more, but again, plan out decorating one area at a time.
Now that you are thinking about reducing thermal bridging, look at your own home to consider how heat will travel through the materials, and how you can stop it. New green / eco products for the home seem to be offered all of the time, so look around you favorite stores to see what is on hand. There may be somebody right now thinking how to manufacture some retrofit product to help you.
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