Holes in the Thermal Barrier
Heat flows through the building envelope in one of two ways: transmission and air leakage. Heat flows through walls, floors, ceilings, windows, doors, and ductwork by conduction, convection, and radiation. When there are air pressure differences, heat is lost or gained through air leaks in these same components.
Heat transmission the building shell depends on two factors: thermal resistance and surface area. Thermal resistance is usually provided by insulation, but it’s assisted by anything that slows air movement through insulation, such as siding and drywall.
Discussions in this article include:
- Current State of Insulation Installation
- Contributing Issues to Thermal Barrier Holes
- Diagnosing the Thermal Barrier
- Thermal Barrier Principles
- Select Insulation Options
- Customer Performance Targets for Insulation
- Improving Thermal Performance
Current State of Insulation Installation
30-40% of heat loss through thermal barrier?
Holes in the thermal barrier are due to missing air barrier or insulation
Contributing Issues to Thermal Barrier Holes
There are several factors contributing to insulation installation problems during construction and remodeling.
Low Priority. For most building professionals, insulation is not high on their list of priorities. It seems so easy to install, not much thought is given to it.
Hard to Add Later. The biggest challenges for retrofitting insulation are gaining access to the building cavities and installing it safely.
Low Motivation. Like many industries where work is hot, dirty, and unpleasant, weatherization contractors often have problems with motivation and employee turnover.
Not Appreciated. In addition, we consumers don’t value their services so we’re unwilling to pay what they’re worth to get it right. This affects installation quality, which is crucial to insulation performance.
Home Energy Audit: Diagnosing the Thermal Barrier
Tip: Diagnosing the thermal barrier is based and an understanding of thermal barrier principles
The process for reducing heat flow follows this path:
- Interview Homeowners
- Conduct Visual Assessment
- Select Insulation Options
- Correct Air Leaks
- Insulate Building Envelope
- Post Insulation Installation Assessment
Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.
As they relate to air sealing, the following questions are asked during the homeowner interview
Are there any hot or cold rooms?
Do you notice any condensation, such as on windows?
Are there any mold or mildew issues?
Do you feel any drafts?
How are your energy bills?: comparatively high, medium, or low?
Is there anything else you feel is relevant?
Answers to these questions help us evaluate the system in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.
Conduct Visual Assessment. To understand where the insulation is and where it should be, all parts of the building need to be looked at: exterior, attic, basement, crawl space, living space, windows, doors, etc. In the process, the presence of insulation, its quality, and type are noted.
Building Envelope. There should be an air tight, insulated barrier at the way around the house, where heated & cooled air is supplied. This barrier is the walls, floors, ceilings, windows, and doors, referred to as the building envelope or building shell. When ductwork is outside this barrier, they too become part of the building envelope because they carry heated & cooled air.
Thermal Barrier. At the building envelope, insulation should be in direct contact with the air barrier, referred to as the thermal barrier. Its purpose is to limit heat transfer through the building envelope by conduction, convection, and radiation.
Ductwork Insulation. An assessment of the insulation around the ductwork is made too, but is discussed as part of the investigation of the heating & cooling system.
Walls. The most difficult area to see is the wall cavities in exterior walls. The concern here is whether there is insulation in the wall or not. The insulation in the wall may not be best, but it is the most expensive area for improving insulation because it’s so disruptive.
Thankfully, there are better place to work on that have a huge impact. Wall insulation seems to be the only area that is close to what it ought to be.
Attic. The greatest area for improvement is in the attic due to temperature extremes, especially when the walls of the living space extend into the attic. We call walls with conditioned air on one side and attic space on the other kneewalls.
Changes in Ceiling Height. Frequently, the wall is wide open where there is a change in ceiling height. Though there might be insulation on the wall in the attic part, attic air circulates in open, uninsulated wall cavities. What is supposed to be there is fire blocking, usually studs cut to cap the hole, to form the air barrier.
Chases. Similar to open wall cavities are chases: box walls fitting around plumbing, ductwork, or a chimney. Often, these cavities are open at the top and there is no insulation on the walls of the chase.
Attic Kneewalls. Very often, insulation batts are friction fit into wall cavities on kneewalls. Even when they are installed correctly, gravity usually takes over and the batts fall out. It’s not uncommon to find crumpled insulation to “fill” the wall cavity.
Missing Insulation. It doesn’t happen often, but it’s too common to find areas of the ceiling where there is no insulation. This usually results from remodeling.
Flat Ceiling Depth. More common is the lack of sufficient insulation ceiling depth to begin with. Current standards are R-30 or more (10+ inches). It’s usually just barely deep enough to cover a 2x4 (3.5-4 inches). When batts are used, it’s almost always inefficient or ineffective because of gaps in the insulation.
Trey Ceilings. A variety of trey ceilings are poplar. Unfortunately, people forget these are made of both walls and ceilings. It’s common to find weak insulation at the corners of the walls and ceilings because ceiling insulation doesn’t overlap wall insulation. In addition, the walls are frequently missing insulation.
Roof-Wall Joints. Another vulnerable point is where the roof goes over an exterior wall. Many times, wind has blown insulation out of the way. Normally, the height of the roof deck is too short here, so the corner of the wall and ceiling ends up with little or no insulation.
Sloped & Vaulted Ceilings. Sometimes there are insulation problems over sloped or vaulted ceilings. They include missing, crushed, or crumpled insulation.
Light & Cabinet Soffits. Soffits over cabinets and light cavities often have issues because the framing is attached to the ceiling. The problem is that most times a cap over the soffit is missing, leave sections of the wall uninsulated.
Garages. The space between the garage ceiling and the room above often has an number of issues. First, the floor is not blocked just below the walls, allowing air to pass through. The floor depth here is usually deep: deeper than insulation is usually made and definitely deeper than the required insulation depth.
Insulation Support. When the floor cavity isn’t completely filled or the insulation isn’t otherwise supported, it falls down and away from the subfloor, effectively making the area uninsulated.
Wire Support Failure. Typically, this area is filled with batts, using wire insulation supports to hold it place. To make the insulation stay, the batts are crushed and the supports often fall out anyway due to structural drying, seasonal changes, and vibration.
Effective R Rating. The current rated insulation requirement for insulating this floor over the garage in some parts of the country is R-19.
Based on engineering analysis, Home InSight has determined R-19 batts usually results in effective cavity insulation value of R-5 on the day it’s installed when put up with wire supports. It’s expected to go to R-0 sooner rather than later, and is not durable enough to last the expected life time of the building.
Unfinished Basements & Crawl Spaces. The subfloor over a basement or crawl space is usually even across the whole floor assembly. The real question is whether the foundation walls or the subfloor should be insulated. The answer depends on practicality, future plans, and budgets.
Subfloors. Typically, the subfloor is insulated with batts. The issues with insulating a subfloor this way are identical to the description for subfloors over garages. The difference is the batts are accessible and deterioration is visible.
Walls. When the subfloor isn’t insulated, foundation walls should be. Cripple walls (framed walls extending from the foundation to floor above) usually are. However, it’s usually friction fit batts that don’t fit or stay in the wall cavities.
Concrete walls are usually exposed, allowing the wall temperature to be the same as the soil temperature outside.
Separation of Finished and Unfinished Spaces. It’s common to find unfinished areas of a basement, usually for storage, water heaters, and heating & cooling equipment. The unfinished areas are usually not intentionally heated or cooled. Frequently, the walls separating the two areas are not treated like exterior walls. Often the wall is uncovered on the unfinished side.
Cantilevered Floors. An often overlooked area is extensions of floors and bay windows beyond the floor below. Often, the area above the wall is open. If there is insulation, is often not enough or is installed incorrectly.
Windows. Windows are unique and a special case of the thermal barrier. The assessment of the windows is discussed in the article on Window Gains.
Doors. Compared to other features of the house, the surface area of exterior doors is relatively small. The main entry door tends to be decorative. Mostly, presence of the door is noted along with whether there is a storm door installed. The door should be properly installed and fit in its frame.
Select Insulation Options
After the visual assessment, the location of the thermal barrier has to be decided. Then there are a variety of ways to insulate each section of it.
Air sealing and building insulation decisions are made together, because the insulation needs to be against the air barrier.
Locate the Desired Thermal Barrier. The first thing that has to be determined after the assessment is where the thermal barrier is and where it should be. Depending on the style of house, the location may be obvious or result from a series of other decisions that need to be made. When it’s not obvious, several cycles of decisions maybe needed to figure it out.
When the house is box with a flat ceiling over a slab floor with the ductwork inside the building, the location of the insulation is obvious. Leave it where it is.
Attics. When ductwork runs through the attic, choices become more complicated depending on how to the ductwork is insulated. It may make more sense to insulate the roofline, depending on health considerations, and how air leaky the ceiling an ducts are. Some people are sensitive to fiberglass and other types of ceiling insulation.
When there are a few attic kneewalls, a decision about whether it cheaper and easier to insulate the roofline is a consideration. It may make more sense to enclose the attic, if certain types of furnaces or water heaters are not present.
Unfinished Basements. Unfinished basements could go either way. When the surface area of walls is less than the surface area of the floor, the better solution is almost always better to insulate the walls. If there are combustion appliances in the area, a special type of mechanical closet needs to be created: a combustion closet.
Crawl Spaces. Crawl spaces are interesting. The preferred solution is to close the vents, insulate the walls, and spread a vapor barrier over the walls and ground. This is not always possible, especially when moisture issues cannot be reasonably resolved. The presence of combustion water heaters and furnaces are a concern too. Ductwork in the area is a major consideration too.
When the subfloor is lumber rather than sheets, heavy air leakage through the floor is almost guaranteed. Here, it may make a lot of sense to spray the subfloor with at least a light layer of foam to air seal and insulate the floor at the same time.
Walls. Depending on the age building and air leakage through walls, wall insulation may be considered. If the wall cavities are empty, the can be filled with blown insulation, especially is the siding is easy to remove or repair afterward. Some types of insulation, such a dense-pack cellulose, provides a good air barrier too.