Thermal Control in Buildings

This article gets a little technical but the overall basic concepts are important to understand. Traditionally built homes leak warm/cold air at large rates, causing increased energy cost for your home. Simple solutions like the type of insulation or the method of insulation help to mitigate those leaks and therefore saving you money. The following article from buildingscience.ccm will help to understand the basics. We at Lythgoe Design Group specialize in green and energy efficient design, therefore can help you with any new project, retrofit, or addition to increase energy efficiency and save you money. -LDG

by John Straube

Background

Heat flow can be a transient or a steady process.  In the transient state, temperature and/or heat flow vary with time.  Steady-state heat flow occurs when the temperature and heat flow reach a stable equilibrium condition that does not vary with time. Depending on the particular problem, the assumption of steady-state conditions may provide sufficiently accurate predictions of actual heat flow and temperature conditions. However, for some problems the assumption of steady flow can result in significant errors.
Heat flow can occur in one, two, or three dimensions.  In almost all real situations, heat flow occurs in three dimensions but, from a practical point of view, it is often acceptable to simplify considerations to only one-dimensional, or series, heat flow.
Heat transfer occurs by three primary mechanisms, acting alone or in some combination:

• conduction,
• convection, and
• radiation.

Changes in moisture state, although not strictly an energy transfer mechanism, must also be considered since these state changes absorb and release heat energy, i.e., latent heat.

 

Figure 1: Conduction of heat through a solid.

Conduction is the flow of heat through a material by direct molecular contact.  This contact occurs within a material or through two materials in contact.  It is the most important heat transport mode for solids; it is sometimes important for liquids, and it is occasionally important for gases.
Convection is the transfer of heat by the movement or flow of molecules (liquid or gas) with a change in their heat content.  This is an important heat transfer mode between fluids and solids, or within fluids.
Radiation is the transfer of heat by electromagnetic waves through a gas or vacuum.  Heat transfer by this mode therefore requires a line of sight connection between the surfaces involved.  All objects above absolute zero radiate heat energy; it is the net radiative heat transfer that is the heat transfer of interest.  Radiation is mostly of importance for heat transfer between solids and within highly porous solids, but radiation between high-temperature gases is occasionally of practical importance.
State change, sometimes called phase change, occurs at a constant temperature but still entails the movement of energy.  For example, evaporation absorbs energy and condensation releases energy.  This  energy is sometimes called latent heat.
The mode of heat transfer often changes during the process of heat flow through and within building systems.  For example, the sun transmits heat by radiation to the earth, where it can be absorbed, for example, by a brick wall.  The heat is then transferred by conduction through the brick and transferred to the indoor air by convection and to the indoor surfaces by radiation.

 

Figure 2: Convection and radiation

Insulation

All materials and layers in a building assembly have some resistance to heat flow.  However, some materials with a k-value lower than about 0.05 to 0.07 W / m ∙ K are deliberately used in building assemblies for their ability to retard the flow of heat.  These building products are called thermal insulations.  Insulations are usually solid materials (so-called body insulation), but radiant barriers that control only radiation heat transfer across air spaces are also available.
Since conduction is a major mode of heat transfer and still air is a low-cost insulator, insulation products tend to be low-density materials (i.e., porous materials with a large proportion of voids filled with air) and / or made of low-conductivity elements.  For example, glass of 2500 kg/m³ density and 1.0 W/ m ∙ K thermal conductivity is spun into fibers and formed into a batt of about 16 kg/m³ density and 0.043 W/ m ∙ K thermal conductivity. Fiberglass batt insulation is widely used as insulation despite the high conductivity of glass because of the very high percentage of pores filled with air.  This type of thermal insulation is approximately 99.4% air.
Foam plastic insulations have a lower percentage of air voids than glass fiber batts, but are made of lower conductivity plastic material. Soft wood of 500 kg / m³ density and 0.11 W/ m ∙ K thermal conductivity is produced to produce cellulose insulation of 60 kg / m³ density and 0.042 W/ m ∙ K thermal conductivity.
Most materials with high strength have relatively high density, and the strength of most building materials (e.g., concrete, wood, plastic) drops along with drops in their density.  Hence, the need for low density (or more accurately, high porosity) reduces the structural capacity of most insulation.  Accordingly, low-density insulation layers—such as glass fiber batt, and foamed plastics—are used to control heat flow in most modern building enclosures, while high-density, high-strength, high-conductivity materials such as steel studs, and concrete are used to support structural loads.  In the past, building materials such as adobe, log, and low-density brick were used in a manner that combined both moderate insulating and acceptable load-bearing functions. Buildings constructed of these materials had thick walls, both to provide a reasonable level of resistance to heat flow and to provide sufficient strength.
Within porous insulations like fibers and foams, all three modes of heat transfer actually occur simultaneously. At low densities, the effective conductivity is high since convection and radiation can move heat through the relatively open space. At high densities, convection and radiation are suppressed, but conduction through the increasing proportion of solid material becomes important.  Therefore, an optimum density can be chosen: one that varies with the type of material. For glass fiber A (a very fine glass fiber), the optimum density is about 2 pounds per cubic foot (30 kg/m³).  However, since the cost increases as more material is used, the density of glass fiber batt is more commonly less than 1 pcf (15 kg/m³).  If higher strength is required (as it is for low slope roof, curtainwall, and exterior basement applications), higher density fibrous products of 3 to 8 pcf (50 to 125 kg/m³ density) are available. Foam plastic insulations provide better R-value if higher strengths are needed. For example, extruded polystyrene with a density of only 2 pounds per cubic foot can easily resist pressures of 10 psi (or 1440 psf).
The wide range of values for thermal conductivity, and the inverse relationship of strength to thermal resistance, may be appreciated from the thickness required to achieve a certain level of thermal resistance.  Figure 3 is a plot of the thickness of various building materials required to achieve a thermal resistance of RSI3.5 (R20).
Rockwool fiber is thicker than glass (since the spun rock contains more impurities than glass), and hence conduction plays a larger role at lower densities.  Rockwool products therefore tend to use higher densities to achieve the same thermal performance as spun glass.  This extra density provides these products with greater strength and more resistance to convection and radiation effects.  Even though more material is used for the same thermal resistance, rockwool products compete in applications that require these properties.

 

Figure 3: Comparison of the thickness of various materials required to achieve R20 (RSI3.5).

Thermal Bridging

Heat flow deviates from one-dimensional at corners, parapets, intersections between different assemblies, etc.  When heat flows at a much higher rate through one part of an assembly than another, the term thermal bridge is used to reflect the fact that the heat has bridged over / around the thermal insulation.  Thermal bridges become important when:

• they cause cold spots within an assembly that might cause performance (e.g., surface condensation), durability or comfort problems
• they are either large enough or intense enough (highly conductive) that they affect the total heat loss through the enclosure

Thermal bridging can severely compromise thermal control and comfort in some building types. Heat flow through steel stud walls and metal curtainwalls is dominated by heat flow through the metal components.  Failure to break these thermal bridges can reduce the R-value of the insulating components (the insulated glazing unit or batt insulation respectively) by 50 to 80%.  Filling the voids in concrete block masonry with insulation is not very effective: adding R15 insulation to a 12” block with increase the R-value of the wall by about R2. Wood framed walls are not as badly affected, but reductions of 10 to 20% are common. A separate BSD discusses thermal bridging in more detail.

Figure 4: Best Case R-values for Walls with no extra framing for windows, floors or partitions.

 

Figure 5: Solving thermal bridging through studs by using insulating sheathing.

Heat Loss to the Ground

The ground temperature under and next to a building is generally very close to the annual average temperature. This means that the temperature difference between the inside of most buildings and the ground is not that large, although it is much steadier.  Hence, less insulation is needed to control heat flow to or from the ground. Nevertheless, some insulation is still required in many climates zones (DOE Zones 3 and higher). In many cases, heat flow control for slabs, crawlspaces, and basements is limited by that needed for control of moisture and comfort problems, not energy. 

 

 

Figure 6: Temperature profile through the ground over the year in a cold climate location.

Air Leakage

If cold air leaks out of the building during cold weather, it is of course replaced with cold air. This cold air must be heated up to make it comfortable.  In warm weather leaking air is replaced with hot air that needs to be cooled and dehumidified. The energy impact of air leakage is significant and must be considered since it is often an important heat loss/gain component of modern buildings. For example, air leakage can account for 30% of the thermal flow across the enclosure in a well-insulated modern home.
The use of a complete air barrier system is required to prevent unintentional air leakage. Several BSD’s and Guides address this critical issue.
Airflow can reduce or bypass thermal insulation in other ways than by just flowing across the enclosure.
Convective loops can form within highly air permeable insulation (low-density fibrous insulations) or small gaps around insulation (possible with rigid board insulation or improperly installed batts. 

 

Figure 7:  Convective air loops that reduce thermal control of insulation.

 

Figure 8: Wind-washing, the flow of wind through air permeable insulations,
can reduce the thermal performance of insulations.

Intentional ventilation has the same energy penalty as the same quantity of unintentional air leakage. Hence, the amount of ventilation should be no more than needed (see ASHRAE standards for guidance).  In extreme climates, much of the energy required to heat/cool ventilation air can be recovered in heat recovery ventilator. Energy recovery ventilators also reduce the impact of humidity.

Solar Radiation Through Windows

Solar gain through windows exposed to either the direct sun, or reflected sun (reflected off the particles in the sky, creating diffuse radiation, or reflected off a surface) can dramatically affect the heat flow in a building.  Hence, the building energy flows must account for the solar gain through windows.  This amount of heat can dominate the performance of a modern building with relatively high window coverage (i.e., above 20 to 30% window to wall ratio).
The Solar Heat Gain Coefficient (SHGC) is the window property used to rate the amount of energy allowed through windows. The SHGC is the fraction of incident solar radiation that passes through a window and becomes heat inside the building. For example, if the SHGC of a glazing unit is 0.50, and the sun is shining on the window with an intensity of 500 W/m², 250 W/m² will enter the building.
The lower the SHGC, the less solar heat that the window transmits through and the greater its shading ability. In general, south-facing windows in houses designed for passive solar heating (with a roof overhang to shade them in the summer) should have windows with a high SHGC to allow in beneficial solar heat gain in the winter. East or West facing windows that receive large amounts of undesirable sun in mornings and afternoons, and windows in houses in hot climates, should have a low SHGC.
Solutions to control this form of thermal control include reduced window area, projecting horizontal shading (most effective on the south), exterior operable vertical shade, and solar control coatings on windows. Interior shades have a relatively small impact, but have the important role of controlling glare and providing privacy.
Passive solar heating design is used to capture the heat of the sun in a beneficial manner—this requires that most of the windows face south, and that window area be limited to collect only as much energy as needed for heating and to warm storage. Modern passive buildings have better control of the thermal losses in cold weather and hence have almost normal ratios of window to wall area.

Interior Heat Gains

In a well-insulated building, the interior heat generated by occupants and activities can be quite important. In cold weather, this interior heat offsets the heat required to warm the space. In warm climates this heat adds to the cooling load.  In smaller buildings (or buildings with a large enclosure surface area to interior floor area ratio) such as housing, interior heat gains do not play a large role in most cases.  Only in very well insulated homes or mild heating weather (i.e., around 10 ºC or 50 ºF) do interior heat gains form a significant proportion of heat flows in a small building.
Large boxy buildings (that is, those with a small ratio of enclosure surface area to floor area) are often dominated by internal heat gain.  Thermal flow in properly insulated commercial office buildings generally is dominated by heat gain and loss through windows at the perimeter (that is, within about 30 feet of the perimeter) and by interior heat gains in the core.  By employing moderate areas of high performance (U<0.3 or 1.25 W/m²ºC) windows in a well insulated opaque enclosure, many commercial buildings will require little or no heating in below freezing weather when occupied.

Conclusions

This is just a summary introduction to the large topic of thermal control. Various Building Science Digests discuss specific topics of insulation, air barriers, ventilation, passive solar design, glazing, and thermal bridging. Despite the many specific situations that will arise in the building industry, it can be generally concluded that the control of heat flow in buildings requires insulation layers penetrated with few thermal bridges, an effective air barrier system, good control of solar radiation, and management of interior heat generation.

Ice Dams

The Problem

The more snowfall, the more likely the problem. If the climate is very cold, there are less likely to be problems.  Hence, ski resorts tend to be perfect areas for ice dams.
Simply put, ice damns are ridges of ice and icicles caused by meltwater from further up the roof re-freezing lower on the roof. The “dam” created by the ridge of ice along the eaves can trap further meltwater and result in significant leakage under and through the roofing, especially shingles and metal roofing. This leakage can cause damage to the sheathing, the roof structure, or the ceiling and walls below. Large icicles along the eaves can become a danger to people below if they fall.

 

Figure 1: Ice dam at a typical roof.

The Causes

The fundamental causes of ice dams are well understood. If part of a roof becomes warm enough to melt snow that is lying on the roof, the snow will melt, and water will run down a sloped roof. If the water subsequently encounters a cold surface, the water will revert to ice. Therefore it can be said that the cause of ice dams is a difference in temperature on the roof surface where the upper part of the roof is warmer than the lower.

Photograph 1: An ice dam; note the snow melting on the slope above the ice dam.

Photograph 2: Not just a problem for sloped roofs.

Most explanations of why ice dams form are actually explanations of why the roof becomes warmer in one spot than another spot lower down. There are several reasons, and most can be a particular cause in a particular case. Often a combination of mechanisms causes the differential temperature.
The most common causes, in no particular order, are:

• insufficient insulation or thermal bridging;
• air leaking into the space below the roof membrane;
• a source of heat in roof such as a poorly insulated duct, hot-water piping, etc.;
• a difference in snow thickness, especially when combined with solar radiation.

The cause and some solutions to each will be examined below.

Cause: Insufficient Insulation

If there is insufficient insulation between heated space and the roof sheathing, significant amounts of heat can flow to the underside of the sheathing.  Snow also insulates the roof membrane, increasing its temperature. This scenario will raise the temperature of the roof-snow interface above the melting point even at cold outdoor temperatures. The result is many hours of snow melting with sufficiently low temperatures to ensure rapid freezing (Figure 2).

  Figure 2: The process of ice dam formation caused by poor insulation.

The greater the snow depth on a roof (which is a function of the snowfall, the slope and friction of the roof, and the weather elements that encourage evaporation), the more its insulating effect on the roof membrane. For the same amount of insulation on the inside, an increase in snow depth will increase the temperature at the interface between the snow and the roof. Once enough snow has been accumulated, melting will begin. Once the meltwater reaches the eaves, the heat loss from the underside of the eaves and the fascia ensures that the water refreezes. Even when enough insulation is specified, problems arise where heat flow through large framing members and compressed insulation at truss heels results in lower installed R-values.

Solution: Better Insulation / Ventilation

The more insulation provided on the interior the more unlikely it is that enough snow will collect to cause melting. How much insulation is enough? The amount depends on the snowfall and the climate of course, but it is recommended that a minimum of R-30 should be provided below ventilated attic roof membranes, R-35 below ventilated cathedral ceilings, and R-40 below unventilated cathedral ceilings. These insulation levels should be increased for very cold climates (DOE Zone 6 or higher).
Another way to keep the sheathing cold is to ventilate with outdoor air (Figure 3). Although ventilation will remove some of the heat that passes through the ceiling, very low levels of insulation and thermal bridging cannot practically be offset by ventilation. Ventilation is best used as a supplement to good insulation as ventilation can be used to remove the little heat that flows through a well insulated roof.

Figure 3: Solutions to poor insulation.

Better framing practises and high-heel trusses can be used to reduce the likelihood of local areas of poor thermal performance. Alternatively, in cathedral ceilings for example, adding exterior rigid insulation to the framing members (but below any ventilation) can be used to blunt thermal bridges and increase the total thermal insulation level. For example, 2x8 rafters filled with cellulose insulation (>R-30) plus 50 mm (2”) of extruded polystyrene insulation in an unvented cathedral ceiling will meet the R-40 minimum recommendation and avoid thermal bridges at the rafters.

Cause: Air Leakage

Warm air leaking through the ceiling plane or air from pressurised space-conditioning ducts bypasses the thermal insulation installed and will directly increase the temperature of an attic space or the underside of the membrane.
Not only does air leakage into the roof cause warming and melting of snow, in colder weather it often causes condensation on the underside of the roof sheathing. The two processes combined can cause serious wetting and decay of the sheathing and roof structure as well as leakage into the space below (Figure 4).
Plumbing stacks, recessed lights, chimneys, electrical service penetrations in partition walls, and attic hatches all can on their own, or in combination, allow sufficient air leakage into the ceiling space (cathedral or attic) to cause serious condensation and ice damming problems (Photograph 3).

 

Photograph 3: Ceiling penetrations in a cathedral ceiling.

Air leakage from ducts placed in the attic (a bad idea for many reasons) can also acts as a significant heat and moisture source and cause both condensation and snow melting.

Figure 4: Ice dam formation process caused by air leakage.

Solution: Air Sealing

Although the solution is rather simple to state, it is quite difficult to achieve in practice. Great care must be taken to seal the ceiling air barrier (often the polyethylene vapor barrier is used as an air barrier or the drywall; see Photograph 4). Table 1 provides a list of air leakage locations and means to ensure they are sealed.
Caulking can be used around small openings and gaps, but expanding polyurethane or acrylic foam should be used around openings more than about 1” (see Photograph 5). For large openings, drywall can be used as the air barrier with taped or caulked joints used to complete the seal. Great care and attention to detail is essentially the only solution to air leakage, although expensive answers such as spraying the entire ceiling with expanding foam (NOT dense-pack cellulose, or blown in batts) can provide the required airtightness quite easily.

 

Photograph 4: Polyethylene sheet sealed as air barrier around bathroom fan housing.

Photograph 5: Air-sealing with expanding polyurethane foam – much easier to do before the interior finishes are installed.

Adding ventilation openings is likely to increase the problem by allowing the leaking air more easy access to the outdoors. In some cases, adding a very large amount of ventilation can help by diluting the moisture content of the leaking air and reducing its temperature. However, it is very difficult and bad practise to ventilate away gross air leakage. Once the majority of the air leaks have been sealed, ventilation may be beneficial.

Table 1. Locations of Attic Air Leaks and Sealing Methods 

Problem: Heat Sources in Roof Space

If ductwork, hot-water piping, or any other source of heat is located above the majority of the ceiling insulation, it can act as a heat source.

Solution: Insulate and Remove

Locating heat sources above the insulation is bad practise, since it causes increased heat loss and energy consumption. If the sources are already there, adding a significant amount of insulation and an airtight blanket around them will reduce the source of heat dramatically. (Note: the addition of foil-faced R-6 insulation is NOT sufficient ductwork insulation in cold climates). As always, ventilation can be used to remove any small amount of residual heat that is not blocked by the insulation.

Cause: Snow Thickness Variation

Wind speeds are often higher near ridge lines and scour snow from the roof. Snow catchers at the base of metal roofs hold back sliding snow near the eaves. Exposed shingles or metal can easily be heated to 70 F (40 C) above ambient air temperature and cause melting of the snow immediately next to the exposed roofing.  Water will run down the roof and can freeze at the eave, which is cooled from the underside (Figure 5). If the temperature is cold enough and the snowcover thin enough the meltwater may form a relatively harmless thin sheet of ice below the snow. In other situations, large ice dams and icicles can form.

 

Figure 5: Ice dam formation process due to uneven snow thickness.

Solution: Ventilation, Insulation, and Waterproofing

Keeping the underside of the roof sheathing close to the exterior temperature is the best solution. This can be achieved with good ventilation and good insulation acting in concert. Even if all good design practises are followed this process can still occur. For this reason it is recommended that a waterproof membrane (preferably self-sealing to accommodate roofing nails) be provided. This membrane should come up the roof high enough to resist a 6 to 8” height of water above the edge of the wall insulation below (Figure 6). Hence, steep pitched roofs will require narrower strips of this membrane than shallow roofs.

Conclusions

Differential temperatures below the roofing membrane cause ice dams. There are several causes of this temperature difference – poor insulation levels, air leakage into the roof, heat sources in the attic, and solar heating of bare roofing. There are primarily two problems caused by ice dams – water leakage and dangerous ice conditions. Some solutions solve one of these problems and some solve both.
There are three broad strategies that can be used:

1. Stop excessive heat from flowing to the underside of roof.
2. Remove the excessive heat by ventilation.
3. Provide a waterproof roof membrane that will not leak under standing water and protect pedestrians from falling ice.

In many cases a combination of strategies is used. 

 Figure 6: Waterproof “eaves-starter” or “peel and stick.”

 

Photograph 6: Self-adhering membrane over the entire roof surface – an appropriate solution in some climates.

Blunting thermal bridges by providing full depth insulation and exterior insulation over the framing is the solution to poor insulation levels. The most common, most damaging, and often the most obvious problem to solve is excessive air leakage. Air sealing requires care and attention to detail during design and construction. Removing ducts and pipes from the attic or at least insulating them well can control this source of accidental heat.
Ventilation removes heat from under the roof membrane. Since ventilation cannot remove a lot of heat, it can usually only be used once the sources of heat have been removed or reduced drastically. If one can be sure of removing air leaks and thermal bridges, ventilation may be unnecessary.
Finally, since the sun can cause some melting on exposed upper portions of a roof, one typically needs to provide a layer of self-sealing waterproofing at the eaves of all sloped roofs.  This provides some back-up protection in the event that other strategies are not perfectly implemented.