This article was originally published in the March/April 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1995
Kneewalls, dormers, side attics, top attics, and sloped ceilings in the top story of a building can lead to unexpected air leakage and insulation performance problems in any house, old or new. Cape Cod-, Mansard- and Gambrel-style houses typically have half-stories with these troublesome features, and the same types of thermal performance problems are often created in other homes when an attic is finished and converted to a living space. Fortunately, given the proper approach, these problems can become energy savings opportunities during a remodeling or energy retrofit project.
Key Junctures Are Key to Leakage
Dormers and kneewalls, which are among the most common features found in 11/2-story homes, require numerous transitions in the framing called key junctures (see Figure 1). Key junctures are openings in the frame of the building often hidden from view at intersections between walls, floors, porches, and ceilings. When viewed with an infrared scanner, there is no question that they are energy losers. But builders, remodelers, and contractors are generally unaware that key junctures cause significant thermal problems. Even heavily insulated houses, newly insulated houses, and houses with energy-efficient windows and plastic vapor barriers in the walls and ceilings can perform poorly when key junctures are left untreated.
The Beast's Lair--the Floor/Kneewall Transition
Key junctures can be found in any home; however, 11/2-story houses and houses with converted (finished) attics usually have more of them, and generally have one of the leakiest types--the floor/kneewall transition. The floor/ kneewall transition is the joist cavity between the ceiling of the full floor and the kneewall of the half story above it. Rarely does anyone install blocking to seal the joist cavity beneath the kneewall. As a result, heated air flows through the joist cavity out past the insulation into the side attic. It is important to note that this is not only conditioned air from interior rooms; it also may be air flow due to convective looping between the joist cavity and the side attic. Convective looping results when the warm uninsulated surfaces heat cold attic air in the joist cavity causing it to circulate and flow out into the side attic.
A lot of conditioned air can escape through the floor/kneewall transition because the joist cavity is usually interconnected with service penetrations and other leakage paths in all the interior walls. Also, the floor/kneewall transition opening is large--typically 8-10 in. high (at least the height of the joist)--and runs the length of the house.
Because of the stack effect, pressure differences are usually higher across attics than other building components. This pressure is a strong driving force for leakage, underscoring the importance of successfully sealing attic air leaks. Even if the floor of the side attic is insulated, often existing insulation merely disguises heat loss problems. Batt insulation by itself slows, but does not block, air flow, and we typically find that any batt insulation in or near the floor/kneewall transition becomes very dirty from dust as air streams through it (see Figure 2).
Fixing these side attic gaps can be tricky because they are often hard to get to, but an untreated floor/kneewall transition can have a beastly impact on the energy bill. It's like leaving a hidden window wide open year-round.
Remodel with a Plan
Remodeling projects make household living space more beautiful and functional. Converting the attic or raising the roof are economical ways of gaining more living space. But the remodeler must carefully plan insulation and air sealing treatments to avoid and correct serious thermal performance problems, control building moisture, and improve indoor air quality.
Fortunately, it is usually easier to access key junctures during a remodel, as kneewalls go up or the attic is finished off. This is the best time to insulate and fix air leakage problems and bypasses that cause convective looping, because it is relatively easy either to seal joist cavities before erecting the kneewall, or to cap the wall cavities with a top plate before installing attic floor boards. An understanding of the problems that arise with dormers and side attics in 11/2-story houses will help the top-of-the-house remodeler to plan an approach that keeps the beast at bay.
Air Sealing and Moisture
When retrofitters tighten abuilding, maintaining adequate ventilation is always a concern. Rather than just stopping air sealing at a preset air tightness level, controlling moisture involves source control, mechanical ventilation with fans, and proper use of air/vapor barriers. Installing a quiet, powerful bathroom fan controlled with a humidistat or timer will provide more reliable fresh air flow, and an effective air barrier helps keep wet air from traveling into attics and other building cavities where it may cause moisture damage. This can include ice dams and other problems (see Ventilating Attics to Minimize Icing at Eaves, p. 35). Plastic vapor barriers are often used when insulating open walls and ceilings during remodeling projects. Unfortunately, the barrier will do little good if one fails to seal the key junctures.
Taming the Beast
The key to taming the beast is lining up the house's pressure boundary with its thermal boundary. The pressure boundary is the barrier preventing infiltration of outside air and exfiltration of inside air. The pressure boundary should be on the warm side of the insulation, which should be positioned in line with the thermal boundary, that is, the boundary between the conditioned area and the unconditioned area of the house. Air sealing at the thermal boundary can reduce convective looping, thereby enhancing insulation performance and improving energy savings significantly over conventional methods.
Determining the Thermal Boundary Location
The first step in planning an insulation and air sealing approach for a 11/2-story home is to decide how best to define the thermal boundary for the half-story. For side attic(s), this involves deciding to define the thermal boundary at the kneewall and side attic floor, or defining it at the slope and gable end walls (see Figure 3). Insulating the kneewall and side attic floor and sealing the floor/kneewall transition is often recommended as the best option. However, it is not always the best approach for energy efficiency.
Often the kneewall area contains leaky access doors, ductwork or built-in closets in addition to the floor/kneewall transition. When that is the case, it may be more practical and economical to define the thermal boundary at the slope and install insulation and an air barrier on the slope. Keep in mind that the air barrier on the slope must be durable and cover a very large area, which makes it labor intensive and more expensive to install. In many cases it will be easier to insulate the kneewall and side attic floor and seal the floor/kneewall transition and other kneewall leaks rather than install an air barrier on the slope.
The decision to locate the thermal and pressure boundaries at the slope or at the kneewall and attic floor will be influenced by initial cost, energy cost, and moisture control needs. It should be made on a case-by-case basis, after examining the house. Using blower doors to test existing air leakage into an attic can also help with this decision. (See Pressure Diagnostics for Side Attics, p.31).
Treating the Beast
Methods and Materials
When air sealing, one should pay careful attention to details like materials selection, surface preparation for sealant, fastening, durability, and interaction with insulation. Some of the quality materials to use include:
Sealing the Floor/Kneewall Transition
If it is accessible, the floor/kneewall transition may be sealed with dense-blow cellulose or rigid material, including polyisocyanurate insulation, sheet rock, plywood, or waxed cardboard. Rigid material can be cut and fit between joists and mechanically fastened to the sole plate of the kneewall. Carefully seal every joint between the material and the joists, kneewall, and ceiling below with gun-applied sealant or urethane foam. Fasten rigid insulation with large headed fasteners, such as roofing nails.
If the side attic is floored, dense-blow cellulose can be used or the flooring may be cut away temporarily to gain enough access to use waxed cardboard. Cut and fold the cardboard, fit it between attic joists at the floor/kneewall transition and fasten it to the joists with a hammer stapler.
Plywood can be used for air sealing the floor/kneewall transition and other big openings when remodeling and finishing off the attic. At that time there usually is more room to cut, fit, and seal the plywood because kneewalls, flooring, wall board, and other finish materials have yet to be installed. Another technique that works during renovations is the use of wood blocking. Cut the blocks from the same stock (typically 2 x 8) as the joists, fasten by nailing through the joists into the ends of each block. Then seal all the joints with foam or caulking.
The floor/kneewall transition may be sealed with blown cellulose installed at a density of 3.5-4 lb per cubic foot. For floored side attics drill through the floor boards or plywood next to the kneewall and blow the cellulose straight down. Build cellulose up to the proper density (see Sidewall Insulation and Air Leakage Control, HE Jan/Feb '90, p.13) to create an effective plug of cellulose at the floor/kneewall transition.
Joist Cavities at Gable Ends
The joist cavities between floors at each gable end are also significant thermal bypasses, particularly if the first-floor ceiling was constructed with furring strips between the joists and drywall (or plaster). These locations, which are often uninsulated, should be dense-packed carefully from the outside or insulated with fiberglass and sealed with rigid materials and foam during remodeling. For those 11/2-story houses where the walls are already insulated, a more practical approach involves tubing the insulation into the gable-end joist bays from inside a side attic.
Sealing these cavities improves wall insulation performance and helps to ensure that conditioned air in the joist cavity won't simply escape out the gable end when the floor/kneewall transition is sealed.
On Cape Cod in Massachusetts most of the 11/2-story houses have mini-kneewalls on the back wall of the house between the first and second floors. Sometimes called eyebrow roofs, they break up the wall line and add interest and beauty. However, without the proper equipment they are a beast to fix. These mini-kneewalls are not even big enough to fit a tank of foam inside. There is no space for a person to effectively use standard air sealing treatments.
High density cellulose can be blown and packed into the entire mini-kneewall cavity, including the floor/ kneewall transition, using a plastic tube at the end of an insulation blowing hose. Provided it is properly installed it will resist air flow and retain its rated R-value. This eliminates any air infiltration through the juncture and insulates at the same time. Access for blowing insulation into the mini-kneewalls may be gained by drilling through the roof or via the soffit. If the remodeling project involves raising the roof to create a shed dormer, the mini kneewall area can be blocked off and sealed with plywood during rough framing as described above. Another means of access that works well when remodeling is to drill holes in the sub-floor adjacent to the mini-kneewall and blow the cellulose in prior to installing flooring or underlayment.
It is important to install a minimum insulation of R-19 and to provide fastening other than just stapling when adding fiberglass batt insulation to a kneewall. The problem is that the kneewall is open on the cold (out) side because the kraft paper backing that comes with insulation must be installed on the warm (in) side of the insulation. After a few years, the glue holding the paper on loses its adhesiveness and as a result the insulation can, and often does, fall down. The insulation will stay in place much longer if you use plastic mesh, wires or twine stapled in place. Pennsylvania energy consultant Linda Wigington recommends a method that is better still. Her approach is to cover the entire cold (out) side of the insulation with house wrap, another layer of batt insulation, or rigid insulation. This holds kneewall insulation in place and protects it from wind intrusion, which can seriously degrade the R-value. House wrap is particularly important for kneewalls because these vertical walls are open to the attic on the outside and exposed to wind blowing in through soffit vents.
Sealing Wall Cavities at the
Surprisingly enough, it is very common to find both interior and exterior wall cavities open to the attic, even in new houses. Frequently gable end walls and interior partition walls that reach up to sloped roofs are constructed with no wood, plaster, or sheetrock to block air from flowing out the top of the wall into the attic. As a result, the thermal performance of the wall is seriously degraded by convective looping, even if it is insulated. In effect, unconditioned-air flow through wall cavities can make an interior wall more like an exterior space between rooms. Wall bays with no top plates may be sealed at the juncture of the ceiling using dense blow cellulose or rigid materials and foam sealant.
Another common problem occurs in walls beside dropped ceilings, where we often find top plates that are out of line with the pressure boundary (ceiling)--the wall may have a top plate above or below the plane of the dropped ceiling. In this case, rigid materials should be used to create an effective seal above the dropped ceilings in the same plane as the rest of the ceiling.
Installing a Rigid Air Barrier
This treatment is recommended for those cases where you choose to define the thermal boundary at the slope of the side attic rather than at the kneewall and side attic floor. Remember that the gable end walls must also be sealed and insulated.
You can apply foil-faced rigid insulation such as polyisocyanurate, minimum 3/4-in. thick, which extends along the rafter in the slope from the top plate of the kneewall down to the top plate of the first floor wall. It should be fastened with roofing nails and sealed at all seams and edges with caulk or foam to establish a complete, continuous air barrier. Foil tape, contractors tape (3M-brand #8086 or equivalent), or duct mastic may be used for sealing the seams.
Install rigid air barriers on the warm side of existing insulation only. Tyvek or an equivalent type of house wrap should be used for applications where the air barrier has to be installed on the cold side of insulation. This is acceptable because house wrap will block air flow but won't trap moisture vapor in the insulation. When needed, install 1/2-in. gypsum board or drywall to protect the rigid air barrier from physical damage or if required by local fire and building codes.
When using a rigid air barrier on a slope that joins with attic flooring, it is very important to seal the floor joist cavity at the eaves so that a continuous air barrier is connected with the top plate of the first floor wall. Dense blow cellulose is, once again, often the best material for this application.
Ventilation for Slopes
There is a fair amount of controversy about when to provide ventilation air space between insulation and the roof deck. That topic deserves more attention than we can give it here. A rigid air barrier should reduce ventilation needs because it helps to ensure that no moisture gets past the insulation. However, for cases where ventilating insulated slopes is deemed necessary, David Keefe (of Building Tune-Ups in Vermont) recommends the following method. Install 1-in. by 1-in. lumber running all along each rafter where it meets the roof sheathing. Then fasten sheets of plywood over these strips of wood to create a sealed cavity between that plywood and the sheathing. The plywood is caulked and sealed so that the cavity is only open to a soffit vent at the eave and to the top attic or to a ridge vent. The slope then may be insulated with batts, wet spray, or blown insulation. Material costs for this method are comparable to using Styrofoam baffles. It also improves insulation performance compared with typical practice because it keeps the cold, windy ventilation air completely isolated from both the insulation and the pressure boundary.
No Tame No Gain
Energy retrofitters and remodelers who think it's sufficient to simply insulate their side attic kneewalls and floors may be in for a long, baffling battle against high energy bills caused by hidden air leaks and bypasses. Understanding key junctures and these techniques to fix them can help you tame the beast upstairs by providing a more systematic approach to remodeling, insulating, and air sealing.
David Connelly Legg, from Auburn, Massachusetts, is an independent consultant and trainer serving utilities and weatherization agencies in the Northeast.
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