Insulating Raised Floors in Hot, Humid Climates

Raised floor home in Baton Rouge

Research Findings on Moisture Management

LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates 1

Raised floor homes in New Orleans

Table of Contents

Introduction...... 3 Why does moisture management matter? ...... 3 How much moisture is too much? ...... 3 How does insulation affect moisture levels?...... 3 Three different crawl space types...... 4 Managing rainwater and soil moisture ...... 5 Water vapor movement ...... 5 Experimental study ...... 5 Results ...... 6 Seasonal effect ...... 7 Air conditioning and indoor temperature ...... 7 Interior floor finish...... 8 Types of insulation and resistance to water vapor flow . . . . 8 Summary...... 9 Questions and answers...... 9 Further reading ...... 10

2 LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates Introduction as absorbed moisture. This amount of time the wood spends percentage commonly is called at an elevated moisture level. This document summarizes moisture content. At 80 percent key information based on a relative humidity, wood moisture How does insulation affect cooperative research project content is about 16 percent. moisture levels? conducted by the U.S. Depart- When moisture content increases, The rates of wetting and ment of Agriculture’s Forest wood expands. When wood dries, drying of building assemblies, Products Laboratory and the it shrinks. Expansion/contraction whether they are floors, walls Louisiana State University damage depends on how much or ceilings, can be affected by Agricultural Center. The study the moisture content changes thermal insulation. The job of was supported by the Forest and how sensitive the particular thermal insulation is to slow Products Laboratory, APA–The construction or wood product is down heat flow – to help keep the Engineered Wood Association to such changes. inside of the house warm when it’s and the Southern Pine Council. cold outside and cool when it’s hot This summary is intended for The traditional guideline outside. In addition to its thermal homeowners, builders, architects, for protecting wood and wood resistance, insulation provides insulation contractors, home products from rot or decay is some resistance to moisture inspectors, building officials and to keep the moisture content migration, and this resistance can consultants. The study itself below 20 percent. Studies have vary widely between different (Glass and others, 2010) and shown, however, that mold types of insulation. Insulation’s additional references for further growth can occur on wood at effect on limiting heat flow will reading are given at the end of this moisture content levels above coincidentally make certain parts summary. 15 to 18 percent, and corrosion of the floor assembly warmer of metal fasteners in treated (or cooler) than other parts of Why does moisture wood can occur when moisture the assembly. This is important management matter? content exceeds 18 to 20 percent. because wood tends to dry We generally want our Reaching these moisture content when it is warm relative to its homes to be safe, durable and levels does not mean mold growth surroundings and is prone to comfortable – all while requiring or corrosion will necessarily occur. moisture accumulation when it is reasonable amounts of energy For each of these moisture-related cooler than its surroundings. for heating or cooling. The last problems, a key factor is the thing homeowners want is to find mold or decay in their homes. The key to preventing growth of mold and decay fungi is proper moisture management. It also is essential for preventing corrosion of nails and screws that hold the structure together and avoiding expansion/contraction damage such as cupping or buckling of wood flooring. How much moisture is too much? Wood has a strong affinity for water vapor. At a relative humidity of 50 percent at room temperature, wood holds about 10 percent of its dry weight Figure 1. Example of an open crawl space.

LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates 3 of construction is not typical and is risky in flood hazard areas. Building codes require that raised floor foundations in flood hazard areas permit floodwaters to move through the space under- neath the building. That can be achieved in closed crawl spaces with breakaway panels or vents that normally stay closed but open when floodwaters exert pressure. The long-term ability of these devices to remain sufficiently airtight to provide an essentially closed crawl space has not been demonstrated. Furthermore, in the event of a flood, the crawl Figure 2. Example of a wall-vented crawl space. space will flood, and the perimeter walls will inhibit drainage and drying after the flood. In addition, Three different and perimeter walls typically are potential floodwater contaminants crawl space types covered with a vapor barrier, and and mold growth, which may occur subsequent to flooding in a For the purpose of discussing the crawl space may be provided closed crawl space, will be coupled moisture management, crawl with conditioned air. A number with indoor air. Because of these spaces can be classified into three of studies in various climates have hazards, the closed crawl space is different types. shown this type of crawl space can remain safely dry, but this method not advisable in flood-prone areas. 1. We refer to open pier-and- beam foundations as open crawl spaces. (See Figure 1, page 3.) Open crawl spaces may have a continuous wall on just the front side and be open on the other sides. 2. We refer to crawl spaces with continuous perimeter walls that include vents to the outside as wall-vented crawl spaces. (See Figure 2.) 3. Finally, we refer to crawl spaces with continuous perimeter walls with no vents as closed crawl spaces. (See Figure 3.) A closed crawl space, with regard to air and water vapor movement, is effectively part of the interior space and is intended to be isolated from the ground Figure 3. Sketch of a closed crawl space with conditioned air supply. and the exterior. The ground [Illustration: Dennis Livingston, Community Resources. Reprinted with permission of the U.S. Environmental Protection Agency.]

4 LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates Managing rainwater higher temperature to lower Experimental study temperature. For example, when and soil moisture Conditions were monitored in a building is air-conditioned and Site grading and management a dozen Louisiana homes – eight the outdoor climate is hot and of roof runoff can largely in New Orleans and four in Baton humid, water vapor migrates determine how wet the soil Rouge. Eleven of the 12 homes through the building shell from becomes under a house. In were located in flood hazard outdoors to indoors. This is general, the soil around the areas and were constructed with referred to as inward vapor drive. foundation should be graded open pier foundations. The sole so water drains away from the Inward vapor drive means home in the sample with a wall- building. Wet soil under a raised water vapor will be absorbed by vented crawl space incorporated house can supply a large amount the subfloor from the outside. a vapor-retarding soil cover, in of humidity to crawl spaces, This absorbed moisture will accordance with conventional especially in wall-vented crawl migrate through the subfloor recommendations. The 11 other spaces. and will dry to the inside. When homes (with open crawl spaces) An established method of the rate of wetting is higher than did not incorporate soil covers. the rate of drying, moisture will limiting evaporation of moisture Air temperature and humidity accumulate in the subfloor. If the from wet soil into wall-vented were measured with data loggers moisture content gets too high for crawl spaces is to cover the soil placed indoors, outdoors and in too long, problems like mold and with a vapor retarder such as crawl spaces. Moisture content rot can occur. polyethylene, typically 0.15 and temperature of the wood or millimeters (6 mil) or thicker. The To protect the subfloor from plywood subfloor was measured, use of soil covers in wall-vented moisture accumulation, the typically twice each month. crawl spaces is based on a large insulation under the subfloor Monitoring started in October body of research. If site conditions should be selected to provide 2008 and concluded in October allow rainwater to wind up on top enough resistance to the inward 2009. of the soil cover, however, the soil vapor drive. cover may be counterproductive. Results from this study showed humidity levels (on an absolute scale) in open crawl spaces were essentially the same as outdoors. This means evaporation from the soil under an open crawl space is overpowered by a high rate of air exchange between the crawl space and the outdoors. This finding suggests if a house is to be built on a site with poor grading and drainage, an open crawl space would be preferable to a wall-vented crawl space. Water vapor movement Water vapor generally diffuses from an area of higher concentration to one with lower concentration. This often corresponds with migration from Figure 4a. Example of rigid, foil-faced polyisocyanurate foam.

LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates 5 All insulation systems were nominally R-13, except the batt insulation, which was nominally R-19. Houses in New Orleans originally were insulated with fiberglass batt insulation. Contractors removed batt insulation from half of the floor and replaced it with rigid foam or spray foam insulation. Floors in the Baton Rouge houses were insulated entirely with rigid foam and/or spray foam. Results The main results are summarized here, followed by a discussion of the main factors Figure 4b. Example of closed cell spray foam. affecting subfloor moisture levels and the implications. The sample of 12 houses E. Same as D, except with the For all 12 houses the pre- included six different insulation addition of one coat of a spray- dominant vapor drive systems: applied vapor retardant paint was inward from May A. 2-inch-thick, rigid, foil- coating (nominal perm rating through October (when air faced polyisocyanurate foam less than 0.5). conditioning was running). insulation installed below F. 6.25-inch, kraft-faced During the other months, the the floor joists. All seams fiberglass batts installed difference between indoor and were sealed with foil tape, between floor joists with the outdoor water vapor pressure penetrations were sealed with kraft facing up against the (a way of expressing humidity spray foam and rim joist areas subfloor, supported by metal on an absolute scale) was were insulated with spray foam rods (Figure 4d, page 7). small. type D below (Figure 4a, page 5). B. 2 inch average thickness of approximately 2 pounds per cubic foot closed cell sprayed polyurethane foam below the subfloor (Figure 4b). C. 2.6 inch average thickness of medium-density (1 pound per cubic foot) open cell sprayed polyurethane foam below the subfloor. D. 3.4 inch average thickness of low-density (0.5 pounds per cubic foot) open cell sprayed polyurethane foam below the subfloor (Figure 4c). Figure 4c. Example of open cell spray foam.

6 LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates Air conditioning and indoor temperature For a given type of insulation and interior floor finish, subfloor moisture content generally increased with decreasing indoor temperature during summer. That is, the cooler the air conditioning was keeping the temperature indoors, the wetter the subfloor. The potential for low air conditioning set-point temperatures to cause problematic moisture accumulation in floors over crawl spaces in the southeastern United States has been recognized for decades (Verrall 1962). A cautious designer should Figure 4d. Example of typical fiberglass batt insulation. select floor insulation that can accommodate lower-than- average temperatures during Air temperature in open Indoor temperature during the air conditioning season crawl spaces was very close summer. without resulting in moisture to outdoor air temperature. Type of interior floor finish. accumulation in the subfloor. These crawl spaces were On the other hand, homeowners Type of under-floor slightly warmer than outdoors in hot, humid climates can insulation. in cold weather and slightly reduce the risk of seasonal cooler than outdoors in warm Seasonal effect moisture accumulation if they weather. In most cases, a seasonal trend set the thermostats controlling In contrast, the wall-vented was observed of higher subfloor their air conditioners as high crawl space was considerably moisture content during summer as they feel is practical. Houses and lower subfloor moisture in the study with summertime warmer than outdoors in cold weather and considerably content during winter. This is a indoor temperatures of 78 cooler than outdoors in warm result of the subfloor being cooler degrees F or higher did not weather. than the crawl space during the show elevated subfloor moisture months when air conditioning levels, regardless of the type In all crawl spaces, water of floor insulation. Higher air vapor pressure essentially was is running and warmer than the crawl space during the winter. The conditioner thermostat settings the same as outdoor vapor will, along with reducing the pressure. seasonally varying temperature differences between the subfloor risk of moisture problems in Moisture conditions within and the crawl space are amplified subfloors, result in less energy plywood or solid wood by the thermal insulation, which consumption. Use of ceiling fans subfloors were found to is located between the subfloor and stand-alone dehumidifiers depend on several variables: and the crawl space. can improve summertime Season of the year. comfort levels in homes with higher air conditioning set-point temperatures.

LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates 7 Interior floor finish insulation was not reliable for foam had no discernable effect For a given indoor temperature preventing summertime moisture on subfloor moisture content. It and type of insulation, accumulation in subfloors. should be noted, however, that summertime subfloor moisture Batt insulation, although subfloor moisture contents under content generally was higher giving lower subfloor moisture carpeted floors never became under an impermeable floor finish contents on average than open elevated, due to the moderately such as vinyl than under carpet. cell foam, also gave some elevated high vapor permeability of the Vinyl is very impermeable and moisture levels (above 20 percent carpet. prevents inward drying of the moisture content). The glass fibers In contrast, if the floor finish subfloor. Carpet, on the other do not provide much resistance was vinyl, the vapor retardant hand, is much more permeable to to water vapor diffusion, but the paint applied over open cell water vapor. Hardwood flooring kraft paper facing, right below foam appeared to result in lower with polyurethane finish and the subfloor, does provide some subfloor moisture contents on ceramic tile are less permeable resistance. The kraft facing average (relative to an otherwise than carpet but considerably becomes more permeable as identical floor system with the more permeable than vinyl. relative humidity increases, vapor retardant paint omitted), Impermeable floor finishes, however, and in the southeastern but some individual moisture by inhibiting inward drying of United States, the outdoor readings still exceeded 16 the subfloor, raise summertime relative humidity commonly is percent moisture content. The subfloor moisture content. above 80 percent. data regarding the effect of Types of insulation and In a few instances, open cell vapor retardant paint were not resistance to water vapor flow foam was finished with a coat of conclusive, and further research is needed to determine whether For a given indoor temperature vapor retardant paint. One reason the combination of open cell and type of interior floor finish, for choosing this combination foam and vapor retardant paint higher subfloor moisture content is that open cell foam plus paint can be a reliable strategy for during summer was found with is less expensive than closed preventing summertime moisture more permeable insulation. cell foam. If the floor finish was accumulation in subfloors in this Greater permeability allows for carpet, the application of vapor climate. water vapor to migrate through retardant paint over open cell insulation and into subfloor materials. Foam board faced with aluminum foil is essentially impermeable to water vapor. Closed cell spray foam insulation is somewhat impermeable. These types of insulation showed good performance, preventing summertime moisture accumulation in subfloors. In contrast, open cell spray foam and batt insulation are much more permeable. Open cell foam gave subfloor moisture contents above 20 percent in some cases when vinyl flooring was present and the air-conditioned indoor temperature was relatively low during the summer. This type of Raised floor home in Baton Rouge 8 LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates Summary the wall-vented crawl space decreasing permeability of the Twelve houses in New Orleans was considerably warmer than interior floor finish (wetter and Baton Rouge, La., were outdoors in cold weather and subfloor under vinyl than under monitored over a one-year period. considerably cooler than outdoors carpet) and increased with In all 12 houses the predominant in warm weather. In all crawl increasing permeability of the vapor drive was inward from spaces, water vapor pressure was under-floor insulation (wetter May through October (when essentially the same as outdoor subfloor with open cell sprayed air conditioning was running). vapor pressure. polyurethane foam than with During the other months, the Moisture conditions within closed cell sprayed polyurethane difference between indoor and plywood or solid wood subfloors foam). Foil-faced rigid foam and outdoor water vapor pressure was generally showed a seasonal trend closed cell sprayed polyurethane small. of higher moisture content during foam exhibited good performance, keeping subfloor The air temperature in open the summer and lower moisture content during the winter. moisture content within crawl spaces was very close to acceptable levels. In contrast, outdoor air temperature. These Subfloor moisture content during summer generally increased with open cell sprayed polyurethane crawl spaces were slightly warmer foam and fiberglass batt than outdoors in cold weather decreasing indoor temperature (the lower the air conditioning insulation were not reliable for and slightly cooler than outdoors preventing summertime moisture in warm weather. In contrast, kept the temperature, the wetter the subfloor), increased with accumulation in subfloors.

Questions and answers impermeable floorings with more permeable floorings. 1. The study results indicate that open cell Although the study did not investigate the effect of sprayed polyurethane foam and fiberglass batt drooping batt insulation, we would expect that drooping insulation are not always reliable for raised floor batts pose an additional risk due to humid air bypassing systems in this climate. Is there a suitable retrofit the kraft vapor retarder, leading to increased moisture for a raised floor system in which either open cell accumulation in the cool subfloor of an air-conditioned foam or fiberglass insulation is already installed? home. We advise making sure all batts are held in full contact with the subflooring. Answer: The study did not address this issue directly. The study did find, however, that properly Likewise, risk of moisture problems in homes with sealed foil-faced rigid foam insulation installed below open cell foam subfloor insulation may be reduced by the floor joists (without any insulation in the joist the same strategies (higher thermostat settings and spaces) prevented summertime subfloor moisture more permeable flooring). Although the study did not accumulation. This performance is attributed to the investigate the effect of multiple coats of vapor retardant vapor-impermeable aluminum foil facing and the paint over open cell foam, it is possible this strategy air-sealing details at all edges and penetrations. We would result in lower summertime subfloor moisture therefore expect this type of insulation to be a suitable levels. retrofit for a raised floor system already equipped with 2. The study results indicate closed cell spray fiberglass insulation. As long as the existing insulation foam is a suitable insulation for raised floor and subfloor have not been exposed to elevated systems in southern Louisiana. Should the floor moisture levels, it would not be necessary to remove the joists, as well as the subflooring, be covered with insulation. closed cell spray foam? If it is not feasible to add foil-faced rigid foam (due Answer: The study did not address this issue, and to obstructions, affordability, etc.), the risk of subfloor we therefore cannot make explicit recommendations. wetting may be reduced in batt insulated floors by It could be argued from building science principles, keeping the air conditioning thermostat setting at however, that covering the joists in wall-vented crawl 78 degrees F or higher and replacing vinyl and other spaces with closed cell foam is likely to keep them drier

LSU AgCenter Pub. 3187 Insulating Raised Floors in Hot, Humid Climates 9 during summer months. As the study indicated, in a infestation depends on location of the joists and whether wall-vented crawl space, crawl space temperature can they are preservative treated. Joists that are in contact be noticeably cooler than outside temperature during with piers – or near perimeter walls in the case of summer months while water vapor pressure in the crawl wall-vented crawl spaces – have the potential to serve space is very close to that of the outdoor environment. as infestation routes. If joists are not pressure treated, This results in high relative humidity levels in the crawl spraying them with borate preservative coating will space. Under these conditions, the joists are likely to substantially lower infestation risk. Homeowners who reach higher than desirable moisture levels. Therefore, have contracts with pest control operators for termite isolating the joists from crawl space conditions by inspection should follow the contract terms. covering them with closed cell foam could reasonably be 4. What is a suitable time of year to install closed expected to limit the peak moisture content they reach cell spray foam insulation? during summer months. In contrast, in open crawl spaces, both temperature and vapor pressure are very Answer: In a new home that is not yet occupied, similar to outdoor conditions, and thus seasonal peak the season for installation would not appear to matter, moisture content of the floor joists is expected to remain although it is important to ensure that the floor system in a safe range. For this reason, covering the joists in is adequately dry before installing the insulation. A floor open crawl spaces with closed cell foam is not expected deck that was constructed with wet lumber or that was to provide substantial benefits. exposed to rain before the building was enclosed should be allowed to dry. In an existing occupied home that is 3. Will covering floor joists with spray foam air-conditioned during the summer, installation would increase the risk of termite infestation? be best done during late fall, winter or early spring. The Answer: Covering the joists with spray foam floor system moisture content at time of installation will can interfere with performing periodic inspections be less important if the interior floor covering is vapor- for termites. The degree of risk concerning termite permeable.