Insulating a House |
| Tips on Insulating an Existing House As a "first cut" evaluation in deciding what items are most worthy for energy efficient retrofits, start with the "Low-Energy Retrofit-Priority Checklist" which is adapted from the Environmental Building News (July 2007.) This checklist assumes that you are starting with a fairly standard existing house relative to energy features: for example, an uninsulated basement: 2x4 walls insulated with R-11 fiberglass; a flat ceiling insulated withR-19 fiberglass; insulated-glass windows or single-pane windows with stormwindows; a relatively leaky 5-7 air changes per hour at 50 pascals (ACH50), and an atmospherically vented furnace or boiler. Within each category, the checklist starts with easy, low-cost measures, and includes progressively morecostly or difficult measures. Note that some later measures obviate the need for earlier measures, so this should not be considered a step-by-step action list. When the starting conditions are different—a better-insulated house or ahouse with no insulation at all—the cost-effectiveness of different measures will differ, but many of the strategies will still be applicable. Once you have selected those items from the priority list, you may want to seek a professional energy audit to get a better idea of the energy use at present and the value of the energy savings. Alist of energy raters is available at the Alaska Housing Finance Corporation Web site: www.ahfc.state.ak Before proceeding to do energy efficient retrofits, carefully review the following sections by building element to be sure you understand the necessary detailsrequiredtodoa"bestpractices"job.Sequencing of vapor barriers, placement, careful installation of insulation to eliminate voids, use of appropriate caulks and sealants, selection of materials, and material compatibility are all crucial details which when done right ensure a very satisfactory energy efficient result. Understanding of the house as a system is very important to taking the correct and effective steps in a retrofit. A good review of building science concepts describing how a house works can be found in Chapter 2 "Building Science" of the Alaska Residential Housing Manual (© 2007 AHFC), which is available in either hard copy or CD from the Cooperative Extension Service 474-5211. Important cautions are knowing where to place vapor barriers to prevent moisture problems, understanding heat flow, andproperties ofvariousinsulations.All these concepts and properties are covered well in the Alaska Residential Housing Manual. Ceiling Lack of ventilation and vapor barrier in the ceiling is often evident by moisture stains and water leaking out of electrical outlets during the spring thaw. Glaciering at the eaves is another sign that warm air is leaking into the roof cavity or that it is inadequately insulated.The following steps should be taken if additional insulation is installed in the ceiling. Lift the existing insulation and check for the existence of a vapor barrier. The vapor barrier usually consists of a clear sheet of polyethylene (Visqueen). Older houses may have a doublesheet of rosin paper cemented to an asphalt coating. Sometimes the rosin paper may be coated on one side with a thin sheet of aluminum foil. The vapor barrier must be securely fast enedunderthe ceiling joists to be effective. The aluminum foil cementedontoarock lathisnot adequatebecause the joints are not sealed. All air sealing should be completed BEFORE any new insulation is added to the attic. If there is no vapor barrier evident, then a vapor barrier must be installed before placing any type of insulation. Vapor barriers may be installed using the following guidelines. There is no satisfactory technique for installing and sealing a polyethylene sheet between the joists. If urethane is used without an additional vapor barrier, specification of water vapor permeability of the urethane should be provided by the applicator in writing. It is best touseapolyethylenevaporbarrierforurethane foam applications. If any loose fill (cellulose, mineral wool, fiberglass) insulation is used, a 6-mil polyethylene sheet must be installed on the underside of the ceiling and sealed along the seams and edges withnon-hardeningcaulkingcompound.Then, 1 inch x 2 inch nailers should be installed under the vapor barrier. These may be covered with acoustical tile or sheet rock. Wherepossible,allopeningsaroundplumbing vent stacks, plumbing walls, electrical wiring, lightingfixturesandchimneysshouldbetightly sealed against water vapor and air migration into the roof cavity. Recessed lighting fixtures should be removed and the opening tightly sealed against warm air and water vapor leakage. Inspect exhaust fans located in the attic before installing insulation. Replace if needed (very likely). All access openings and stair wells from the interior of the house into a cold roof cavity or attic should be tightly sealed against migration of warm air and water vapor. Access openings into a cold attic should be placed in the gable ends of the roof rather than in the ceiling. When placing additional insulation in the roof cavity, special precautions must be taken not to restrict air movement over the insulation at the eaves, particularly with trusses constructed of 2 inch x 4 inch top and bottom chords. It may be desirable to place a 2 inch x 24 inch strip of rigid urethane under the eaves instead of blanket, batt or fill insulation. New products are available for this purpose, also. The plate (top) of interior partitions should be vapor proofed with vapor resistant paint and the edges sealed with caulking, when no other vapor protection has been provided. Stud Frame Wall Lack of insulation in a wall may be evident by blistering of paint on exterior siding, frost or condensation behind furniture and drapes, or staining of sheet rock nailheads.Follow the steps listed below to insulate an existing wall. Remove a section of exterior siding and sheathing in several locations and determine if the wall is insulated and/or vapor proofed. No insulation should be blown into the wall until it is properly vapor proofed. The wall may be vapor proofed by installing a 6-milpolyethylene sheet over the existing interior wall covering. The vapor barrier should be sealed at all edges and seams. The polyethylene may be covered with sheetrock or paneling. Cont'd on page 5. This is an extreme example of a phenomenon called "ghosting of the studs" where the surface of the wall on the inside of the stud is cooler and therefore condenses out moisture and other vapors on the cool surface first. In the case shown, the problem is fairly horrible with the resulting mold on every cool surface of the inside of the building. Retrofit would solve this by insulating on the exterior or the interior and using good vapor barrier sealing to keep moisture from getting to cold surfaces. The existing condition may also be a result of a fairly tight air seal causing exceedingly high relative humidity The wall is being covered on the exterior with Tyvek© homewrap and the siding then installed such that cellulose insulation can be sprayed inthe wall cavities and below the windows as the siding is attached. The finished wall is then sealed and the insulation is contained by the Tyvek© on the exterior. This system worked extremely well but it only worked well becausethe new window openings were sealed with foam sealantand the vapor barrier caulked to the inside of the window frame. The vapor barrier itself was of good quality, and had the confidence of the retrofit team that it did indeed stop most of the moisture at the inside wall. These two photographs (Figures 2 and 3) are indications of one way to amply retrofit a wall of a house to achieve a good result. The wall shown in Figure 3 resulted in approximately an R-27 wall when it was originally about R-11. This nearly tripled the insulating value of the wall. The interior of the wall may also be vapor proofed by a vapor resistant wallpaper. Regular vinyl wallpaper may be waterproof and washable, but not necessarily vapor proof. The permeability should be specified by the manufacturer and not be greater than 0.750 perms. After the wall has been properly vapor proofed, mineral wool or cellulose may be blown into the wall through 1 1?2-inchto2-inchplugscutthrough the exterior siding and sheathing in every cavitybetweenthestuds.Cavitiesunderwindowsshould also be insulated. Amoreexpensiveanddrasticmethodistoremove all interior wall covering, place 31?2 inches of insulation between the studs, cover with 6 mil polyethylene and then install new gypsum board or paneling. Basement An uninsulated basement can cause a large portion of the heat loss in a house. Heat loss may be evident by melting of snow along the foundation wall.The following steps may be taken to insulate a basement of an existing house.It is always more effective and less likely to cause basement moisture problems if the basement is, and can be insulated on the outside of the wall, and below grade (below the soil surface). There are excellent extruded polystyrene rigid board insulations which are ideal for this application. Insulating basements and heated crawlspaces from the outside prevents moisture problems, and keeps inside wall surfaces warmer and dry. It is highly preferable to any interior insulation on basement walls, and should be the preferred option whenever possible. Always protect the outsidesurfacefromphysicaldamageandinsects with a permanent protective sheathing. The basement of an existing house may be insulated from within by installing 2 inch x 4 inch nailers at 16-inch centers on the wall. Place 2 or 31?2 inches insulation between the nailers. Over this place a 6 mil polyethylene vapor barrier. The vaporbarriermaybecoveredwithgypsumboard and suitable paneling. Foamed plastic may be sprayed between the nailers. However, a polyethylene vapor barrier should be placed over the insulation and nailers. Also, rigid foam plastic insulation board may be used. Closed Crawl Spaces A masonry crawl space of a home can account for 45 per cent of the heat loss depending on the temperature required to maintain a warm floor surface temperature. Heat loss is often evident by melting of snow along the foundation wall. The guidelines below may be used to insulate a closed crawl space. 1. Excavate a trench along the wall to the depth of the footer for placement of insulation. Attach 2 or perhaps 3 inches of rigid polystyrene or urethane board on the interior of the masonry or concrete foundation wall depending on the severity of the climate. Sprayed-on urethane may also be used on the exterior of the foundation, as can extruded polystyrene. Spray on foams need durable protective sheathing to keep from damaging the foam. The foundation vents should be replaced with permanent closures. For additional comfort and fuel savings, the floor of the crawl space may be covered with a 2 inch layer of rigid polystyrene. A 6 mil polyethylene vapor barrier should be placed under the insulation. The insulation should be covered with 4-to 6-inches of sand and gravel. This system also can aid in reducing radon induction into a crawlspace or basement. Floors Over Unheated Crawl Spaces A floor over an unheated crawl space should be provided with as much or greater insulation than the ceiling, since it is the closest surface upon which wework, playand relaxduringwakinghours.However, it is still not possible to attain an ideal floor temperature without installing insulated skirting around the foundation, which must be done with extreme caution in permafrost zones. Slab-On-Grade Slab-on-grade is not recommended for Alaska except in a basement. Insulating the floors and foundations of a house with a concrete floor is difficultatbest.Ideally,inanewhome,theperimeter of the floor area should be insulated with at least 2 inches of polystyrene or urethane, with a 48 inch strip laid around the perimeter. It is very costly to provide sufficient heat to raise the floor surface temperature comparable to room air temperature. A cold floor results in stratification of air such that the thermostat must be set at 75° F to maintain a comfortable temperature at the floor. This can result in temperatures near the ceiling as high as 85° F. (See also the Manual on Frost Protected Shallow Foundation Systems noted on page 4.) The following steps may be taken to insulate the foundation wall of a slab-on-grade house. 1. Excavate a trench around the perimeter of the house and install 2 inches vapor resistant extruded polystyrene high density board. The insulation should extend at least 32 inches below the surface. Foamed-in-place urethane is excellent, but it necessitates excavating a 4 foot wide trench to assure uniform foaming of insulation by the applicator. A rigid urethane or polystyrene board only requires a 1 foot wide trench to place the insulation. The foam plastic insulation above grade should be plastered or in some way protected against deterioration by ultravioletlightofthesunandmechanicaldamage by rodents, dogs and other pests. 2. A floating slab should be insulated by placing rigid insulation along the edges. To insulate an existing concrete slab-on-grade, the following steps may be taken. Install 2 inch x 3 inch or 2 inch x 4 inch treated (all weather wood) sleepers over the existing slab. A3?4 inch space should be left at the ends of sleepers to allow for possible expansion due to moisture adsorption. The space between the sleepers may be insulated with 2 inches of polystyrene or urethane rigid board or foamed-in-place urethane. A wood subfloor and/or finish flooring should be placed over the nailers. A 3?4 inch clear space should be left around the perimeter of the subfloor and finish flooring to allow for possible expansion. A 1?4 inch opening should be left behind or under the base molding to facilitate natural removal of water vapor that may condense out under the floor, particularly during summer when the heat may be turned off. Exterior Retrofit of Vapor-Sealed InsulationInsulation products with low water-vapor permeability are used for exterior retrofit of insulation. These insulations include closed-cell polystyrenefoams and foil-faced plastic foams which are usuallyavailable in 2 foot x 8 foot or 4 foot x 8 foot sheets. Both types of insulation are excellent vapor barriers and, therefore, must be applied with special precautionstotheexteriorofstructures.Otherwise,moisture could accumulate in the wall and be trapped by this new exterior vapor barrier. Refer to manufacturer’s recommended installation procedures whenever you use these types of insulation, and contact the Cooperative Extension Service at 1-800 478-8324 for further information. Visit the Cooperative Extension Service Web site at www.uaf.edu/cesand Rich Seifert’s homepage at www.uaf.edu/ces/faculty/seifert |
Author AHFC Consruction Guidelines
