Insulation Installation Derby KS
In the previous installment we reviewed how to determine the optimum amount of insulation, considering the nature of the construction, the fuel type and the climate. In this installment we will consider a buildings thermal envelope, the type of insulation appropriate to each surface, and how to install it properly.
The Thermal Envelope
Building scientists define a buildings thermal envelope as the collection of building surfaces (walls, floors, ceilings, etc.) that separate the conditioned (heated and/or cooled) spaces from the outdoors. In Illustration 1, the surfaces best insulated with fibrous insulation (generally either fiberglass blanket or batt, or short-fiber blown or sprayed fiberglass) are shown in pink. Surfaces better insulated with rigid foam (generally closed-cell extruded polystyrene) are shown in blue. As in the case of a hot-air balloon, which a heated building in winter resembles, there must be no holes -- either actual or as gaps in the insulation through which heat or hot air can escape.
Conspicuous in their absence are the exterior doors and windows. They, too, are part of the thermal envelope. Of course R-value is one of the prime considerations in purchasing windows and, less often, doors. I omitted them only because we generally dont add to their existing R-values.
Note also that none of the floors in contact with the earth is insulated except at the perimeter. This is not because the earth is a good insulator, but because it is such a large thermal mass. It takes so much heat to change the temperature of the ground that its temperature remains essentially constant year-round at a depth of 10 feet. Heat loss and gain by conduction are proportional to temperature difference, and the temperature difference between a building and the earth beneath it is small compared to that between the building and the outdoor air. The exception is at the building perimeter, where the heat path to outdoor air is short. Note areas F (crawlspace wall) and H (sunspace slab perimeter).
Before we consider insulation further, we must confront the issue of vapor barriers. No subject causes more confusion and controversy between builders and building scientists than the proper role of vapor barriers. These are materials or films designed to prevent the passage of water vapor (the gaseous state of water). Here are the issues.
First, as with perfume, the smell of cooked cabbage or any other airborne gas, water-vapor molecules always move (diffuse) from areas of higher concentration toward areas of lower concentration. Because airs ability to contain water vapor decreases as air temperature drops, air contacting a surface colder than itself will generally deposit some of its water vapor as liquid water on the cold surface (think of your car windows fogging in winter). Although we complain about the dry air in our homes in winter, in absolute terms the air inside a heated home contains more moisture than the colder air outside. The concern in heating climates is that indoor air may find its way into an insulated cavity and condense moisture on the cold sheathing, providing ideal conditions later in the year for dry rot. The solution is obvious -- make sure the interior surfaces contain a continuous vapor barrier.
In cooling climates the situation is reversed. There the problem is warm humid outside air sneaking into building cavities and striking the backsides of air-conditioned interior surfaces. The moisture deposited inside cooling-climate walls often leads to mold and rot. Thus, in cooling climates the vapor barrier goes on the exterior of the thermal envelope. Just remember, vapor barriers always go on the warm side of building surfaces regardless of climate.
What constitutes a vapor barrier? The rate at which water vapor passes through a material is the materials water-vapor permeance, measured in perms. One perm is defined as 1 grain of water vapor transmitted per hour per square foot per 1 inch of mercury-vapor pressure difference. All you need to know is that materials with a rating of 1 perm or less are considered to be vapor barriers.
Polyethylene sheeting (poly) and the paper and foil facings of blanket and batt insulations are all vapor barriers. Closed-cell foams and plywoods with exterior glue are also vapor barriers. The one I like best, however, is specially formulated latex primer paint. You may have to look around for it, but it is well worth the effort. Glidden is one manufacturer that offers such a product.
Illustration 2 shows typical details for insulating a wall in a heating climate. Note that two different vapor-barrier solutions are shown.
The conventional approach is to staple a continuous sheet of 4- or 6-mil polyethylene over the inside faces of the framing members before installation of the drywall or other surface finish (any joints are taped or formed over studs). Problems I have with this approach are:
. What to do about the gaping holes due to electrical boxes,
The question of longevity of the poly material, and
. What to do if the poly is ripped during construction.
Vapor-barrier paint is my favorite approach. The vapor barrier here is two coats (safety in numbers) of latex vapor-barrier paint, applied over the drywall and under the finish treatment of paint or wallpaper. Foam gaskets seal the electrical cover plates to the painted wall, forming a continuous vapor barrier. If anything ever happens to this vapor barrier, you will be able to see it and rectify the problem.
Effective vs. Nominal R-values
Most people know that a wall framed of 2x6 studs, 24 inches on-center, and filled with 6-inch fiberglass batts is an R-19 wall. But is it? Not always.
There are two reasons why not. First, the insulated cavity comprises but 90.8 percent of the total wall area, less around areas with doors and windows. At least 9.2 percent of the wall area is occupied by solid wood framing, the R-value of which is 6.9 (1.25 per inch for spruce), not the fiberglass 17.6 (3.2 per inch).
Even greater are the effects of air gaps, whether caused by settling (an issue with blown fibers without binders), interference from wiring or poorly fitted batts. In the case of a batt that is 1 inch smaller than the framing cavity in both height and width, the gaps represent only 5 percent of the wall area. However, they are even more damaging than the framing because the R-value of a 4-inch air space is only 1.0, versus 17.6 for the batt.
The net effects of the framing and air gap reduce the walls R-value from the nominal 17.6 to an astounding 8.9. The lesson here is that the care with which fiberglass blanket and batt is fitted is more important than the insulations R-value per inch.
Now that we have identified the issues, it is time to look at how one might best insulate each of the surfaces comprising the thermal envelope of Illustration 1.
Attic Floor Open attic floors present the ideal opportunity for optimum levels of insulation. Because incremental additions of insulation require no additional framing and occupy no valuable living space, optimum levels are very high. Unless the space is to be used for storage, an attic floor presents no problem -- simply remove a few boards and blow loose-fill insulation under the floor. Additional insulation can be installed over the floor in the form of loose fill or batts.
Providing a conventional poly vapor barrier in existing houses is problematic. The issue of a vapor barrier has two parts:
1. Sealing the ceiling surface is easily accomplished with vapor-barrier paint; however,
2. Sealing openings -- such as electrical boxes for ceiling fixtures, spaces around chimneys, and openings to interior walls below -- must be addressed on an individual basis. As Illustration 1 shows, the spaces between roof and attic floors and walls must be ventilated to the outside, the most effective ventilation being provided by a combination of continuous ridge and soffit vents.
Sloping Ceilings At first glance, a sloped ceiling appears to be the same as a sloped exterior wall. So why do we need to treat them differently? Two reasons:
1. Being designed to shed water, the roofing material is most often a nearly perfect vapor barrier on the wrong side of the cavity, and
2. In order to prevent snowmelt and ice dams, the underside of the roof sheathing needs to be kept at outside air temperature. The cold roof provides an alternative solution.
We cant simply blow loose-fill insulation into the ceiling cavity, nor should we simply fill the rafter spaces with blanket or batt. The solution is polystyrene attic rafter vents that provide continuous ventilation channels above the insulation.
Cold Roofs The cold roof offers a completely different approach to insulating roofs. A thick (usually 3- to 4-inch) layer of rigid, closed-cell foam is sandwiched between the ceiling and the roof sheathing. The foam is inherently a vapor barrier, and its high R-value keeps the roof sheathing at a reasonably low temperature. Under a deep layer of dry snow, which is a pretty good insulator, some melting will occur, but refreezing does not occur until after the water has drained from the roof. Thus, the cold roof prevents both condensation and ice dams.
Exterior Walls Illustration 2 shows two separate options for the vapor barrier:
1. Poly installed just behind the drywall
2. A combination of vapor-barrier primer (two coats) and gasketed switch and receptacle boxes. A third alternative is the use of faced insulation products. The foil or impregnated kraft-paper facings are true vapor barriers, but the facings also prevent you from seeing how well the insulation is fitted to the cavity and around wiring and plumbing.
My favorite wall uses sprayed-in short-fiber fiberglass with the paint/gasket vapor barrier. The insulation contains a latex binder that adheres to the sheathing and framing, and prevents the fibers from settling over time. Spraying the insulation fills the cavity completely, eliminating gaps around and behind wiring and plumbing, as well as reducing air movement within the cavity.
Kneewalls The attic kneewall is similar to the sloping ceiling, except there is no need for the ventilation channels. With unlimited space, R-19 fiberglass batts can be installed between 2x4 studs.
Crawlspace Walls If part of your conditioned space is over a crawlspace, you have a choice of where to stop heat flow:
1. Inside the crawlspace wall (as shown),
2. Outside the crawlspace wall, or
3. Between the joists of the floor above the crawlspace.
I have shown option 1 in Illustration F (left) because it is the simplest to implement in an existing building.
First, in order to reduce the flow of water vapor from the earth, a continuous 6-mil polyethylene vapor barrier is laid over the ground with edges lapped up the wall 6 inches or so. The box-sill area is filled with either fiberglass batt or tightly fitted blocks of closed-cell foam. Cleats are screwed up to the floor sheathing between the joists for the purpose of stapling up faced insulation batts. R-19 batts are hung from the cleats, and their side flanges stapled together to form a continuous vapor barrier. The bottoms of the batts run out onto the poly for several feet, with the batts held against the wall by lengths of 2x4.
Option 2 consists of fastening sheets of 1- to 2-inch-thick extruded polystyrene to the outside of the concrete wall, protected with a thick layer of special cement/latex coating formulated for the purpose (available at any concrete supply outlet). This option is highly effective but is recommended only for northern termite-free areas, because the space between the foam and concrete provides an ideal pathway for termites.
Option 3 is simple before the floor sheathing is installed, but difficult after. Since a plywood subfloor qualifies as a vapor barrier, all that is required to insulate the floor is to fill the joist spaces with faced R-30 (9-inch) batts, with their flanges stapled to the joist bottoms. To prevent the facing from acting as a wrong-side vapor barrier, slash it every few feet at right angles to the joists. If animals are a problem, protect the insulation with 1-inch-mesh chicken wire (fencing) stapled to the joist bottoms.
Basement Walls Before insulating a full basement wall, we must consider the use of the basement. If it will never be used as finished space, it can be insulated in the same ways as crawlspace walls. If we plan to turn it into living space, however, we must finish the walls in addition to insulating. Illustration G on page 62 shows the most obvious solution, that of framing a wall inside the basement wall. This wall is treated exactly the same as the kneewall. Careful attention must be given the box sill and the space above the wall between the floor joists. The illustration shows cleats against the subfloor. Either the insulation facing or a separate poly vapor barrier can be stapled to the cleats and the framing to create a complete vapor barrier.
An alternative is covering the concrete wall with extruded polystyrene panels, held in place with 24-inch on-center strapping fastened to the concrete through the foam with hardened concrete screws. The fire code requires covering the foam with a 15-minute rated material, such as half-inch drywall. In this case it is simplest to fill the box sill with tightly fitted scraps of the foam. These pieces need not be covered with drywall if the ceiling is also finished in drywall.
Slab Perimeter The exposed perimeters of slabs are most commonly and easily insulated with extruded polystyrene. In new construction, the slab is undersized by the thickness of the foam so that the foam is trapped under the sill. In a retrofit situation, aluminum flashing is placed under the siding and over the top of the foam. Exposed areas of foam must be protected from destructive UV rays, so they are covered either by painted sheets of pressure-treated plywood or the cement/latex coating mentioned in crawlspace walls, option 2.
In Illustration H, note that the foam extends to the bottom of the slab and then outward. The rule of thumb is that the foam should extend down plus out a total distance equal to the depth of frost. Following this rule assures that the frost will never reach under the slab to heave it. This is an excellent and proven way of protecting a shallow slab in northern areas.
Again, in termite country, we must be concerned about the foam providing a hidden pathway for the little buggers. A solution is to lay an aluminum or galvanized-steel termite shield over the top edge of the foam before the sill is fastened down.
In our next installment, we will review multiple types and functions of windows and doors, continuing our discussion of the thermal envelope.