Below Grade/Walk Out Wall Designs Paul Ellringer PE, CIH Air Tamarack, Inc. March 2013 Below Grade Wall Designs Outlined in Figure 21 are the preferred ways of insulating below grade walls/ walkout walls. Below grade walls primarily dry to the warm side of the walls. Therefore, the strongest vapor retarder should always be on the cold side (exterior) of the walls. We want to keep moisture out of the wall, but if it gets into the wall, we want it to dry to the interior. Warm side vapor barrier (poly) should never be used. Below grade walls need damp proofing or water proofing on the exterior side (cold side) from the footings to the top of the stem wall. This coating should also be the primary vapor barrier in this wall (no interior vapor barrier). Exterior or integral insulation is preferred over interior insulation. The more mass of the wall we keep warm the better off we are from a moisture and energy standpoint. It is also desirable to install two inches of foam insulation below the concrete slab in the basement or crawl space. This keeps the basement floor dryer and is also more energy efficient. 1
Walk Out Walls We commonly find mold in framed walk out walls in the lower level of houses with large portions of the lower level below grade when a Class 1 vapor barrier like poly (4 or 6 mil) is used on the warm side of the walls. The below grade portions of the walls and the concrete basement slab provide moisture and coldest to the walk out walls. This encourages condensation and mold growth in this wall especially if a Class 1 vapor barrier is used on the warm side. The fix for this problem is to get rid of the Class 1 vapor barrier and use a Class 2 vapor retarder in these walls. Positive pressure negative pressure Most houses in Minnesota tend to be under negative pressure in the lower level or basement this occurs because of leakage in the return air ductwork in the basement 2
RE LL wall has mold all over it Dark spots are mold growth 5 Mold RE LL wall has lots of mold in it 6 3
RE LL above grade wall showing the wall sheathing in good condition mold is primarily in fibrous glass and poly 7 Mold will also grow on the exterior wall sheathing and wall studs discolored areas need to be coated with a mold resistant paint before foaming occurs for interior locations like this Zinsser mold resistant primer (Bullseye) or Perma-White works well. Paints like this are available at Menards or Home Depot or look on web at www.zinsser.com 4
Walk Out Walls The best solution is to insulate these walk out walls with 3 to 4 inches (R21 or 28) of a close cell urethane foam or 2 to 4 inches of rigid foam (pink or blue board). No poly vapor barrier is used on warm side of wall, the foam is both the air barrier and the Class 2 vapor retarder. The foam is covered typically with just gypsum wallboard, no additional vapor retarder needed or wanted. Additional vapor retarders will hurt the performance of the walls. The other options for insulation of the walk out walls (typically fibrous glass) need to use a Class 2 vapor retarder like a vapor retarder paint, Kraft paper, or MemBrain smart vapor retarder on the warm side of wall. Do not use a Class 1 vapor barrier like 4 or 6 mil polyethylene in this wall, the wall will perform better with a Class 2 vapor retarder. Building code allows Class 1 and 2 vapor retarders. Positive pressure negative pressure Mold in the walk out wall typically does not mean the upper level walls are moldy all this comes down to air pressure the negative pressure in the LL pulls humid outdoor air into the walk out wall causing the condensation and mold growth 5
Walkout Wall Mold These problems relate to air pressure, temperature and wall design Change the wall design and fewer mold problems will occur Try to keep the air pressure in the lower level closer to neutral Control indoor relative humidity during the heating season to 30-35 percent and during the cooling season to below 60% Figure 21A Preferred Below Grade Insulation in New Houses Picture Courtesy of Building Science Corporation It is also an excellent idea to install two inches of foam below the slab. 6
Diagram courtesy of Building Science Corporation Figure 21B Weep Screed used with Stucco New Anodized or galvanized 24 gauge metal flashing extends up 4 inches behind weep screed and 6 inches below grade. Grade Treated wood Area behind metal is covered with asphalt water proofing a minimum of two inches on each side of the crack Foam or Fibrous Glass Insulation Concrete Foundation Figure 21C Proposed Addition of Metal Flashing when below grade wall insulation is outside plane of wall 7
Anodized or galvanized 24 gauge metal flashing with hem at bottom extends up 4 inches behind WRB and also 6 inches below grade. Rigid vinyl could also be used. Foam or Fibrous Glass insulation Grade Weather Resistive Barrier building paper or drainage housewrap 4 inch overlap horizontally and 6 inch vertically Area behind metal is covered with asphalt two inches on each side of the crack Damp-proofing is on masonry wall and extends to top of foundation wall and to bottom of footing Concrete Foundation Figure 21D Proposed Addition of Metal Flashing Siding or cladding Ground slope 5-10% - ground 5 feet from house is 6 inches lower Impermeable Backfill (Clay Soil) 8-12 inches Granular backfill Filter fabric Weather Resistive Barrier (WRB) - building paper or drainage housewrap overlapped 4 inches horizontally and 6 inches vertically. Anodized or galvanized 24 gauge metal flashing with hem at bottom extends up 4 inches behind WRB and 6 inches below grade. Rigid vinyl sheeting can be used in place of metal. Foam insulation installed outside of damp- or waterproofing. Damp-proofing is on masonry wall and extends to top of foundation wall and to bottom of footing. Clean coarse gravel Perforated drain pipe Extruded polystyrene Granular capillary break (no fines) Figure 21E - Recommended Below Grade Wall Design 8
Cap Flashing Building paper Foam insulation with protective coating extending a minimum of 6 inches below grade Back leg of cap flashing is 4-6 inches high Horizontal leg needs to cover foam insulation Drip edge needs stick out ½ inch do not caulk here sheathing stud wall sealant/caulk between flashing and sheathing/ top of foam Masonry foundation Figure 21F Cap Flashing Figure 21G Cap Flashing That Extends Below Grade 9
Figure 21H Cap Flashing That Extends Below Grade to protect foam Below Grade Wall Designs Figures 22-23 are preferred ways of insulating below grade walls on the warm side of the masonry. Figure 24 is a wall design that is not recommended. Figures 22-23 are more durable than Figure 24. Figure 24 is more likely to trap moisture inside the wall cavity. Figures 22-23 have 20 times as much drying ability (can handle more water either bulk or water vapor). Bulk water (liquid) is also more likely to penetrate the wall cavity in Figure 24 than in Figures 22-23 (bulk water more likely to stay in the masonry wall). With all other conditions being the same, Figures 22-23 are about 100 times more likely to be successful, dry, and mold free basements than Figure 24. 10
A 10% slope is a drop in elevation of 6 inches at a distance of 5 feet from the foundation Ground slopes Away at 5-10% Clean coarse gravel Perforated drain pipe Granular backfill Filter fabric Urethane foam (Closed cell open cell is not recommended) insulation to a thickness of 1 1/2 inches (R-10) to 2 inches (R14) to create an air barrier/vapor retarder. Studs are at least ½ inch from the masonry wall foam is applied around and behind the studs to a final thickness of 1 ½ inches from the wall, foam goes under bottom stud ½ inch. Interior gypsum wallboard installed on 2X4 studs ½ inch off floor two inches of XPS foam insulation below slab Figure 22 Recommended Below Grade Wall Design With Internal Spray Foam Insulation Spray in foam goes behind studs for ½ to 1 inch 11
Impermeable Backfill (Clay Soil) 8-12 inches Ground slopes Away at 5-10% 2 inch thick extruded polystyrene foam insulation all seams and penetrations taped or caulked to create an air barrier/vapor retarder. Foam insulation is not installed between the studs. Install ½ inch minimum of foam below the bottom stud Clean coarse gravel Granular backfill Filter fabric Interior wall gypsum wallboard Installed on 2X2 or 2X4 studs ½ inch off floor Damp-proofing Capillary break Perforated drain pipe Granular capillary Break (no fines) XPS insulation two inches thick Figure 23 - Recommended Below Grade Wall Design with internal rigid foam insulation XPS insulation is continuous XPS is not installed between the studs All seams in the XPS are taped or caulked Seam between floor and XPS is Caulked Studs installed in front of XPS Can be used to hold it in place XPS thickness should be 1 to 2 inches thick Photo courtesy of Building Science Corporation 12
This wall design has limited drying to the interior and the wall is likely to stay wet most of the year below grade walls need to dry to the interior to stay dry it is bad to trap moisture in the wall 4 or 6 mil poly on both sides of stud wall with fibrous glass insulation in the wall cavity Gypsum Wallboard attached with nails or screws which penetrate the poly Figure 24A Below Grade Wall Design with one or Two Layers of Poly Which is not Recommended see next slides Figure 24B - Mold infested fibrous glass insulation in below grade wall diaper needs changing Standard Problems with These Designs 13
Figure 24C - Mold growth on treated studs and back of poly these designs do not work Figure 24D -Basement walls are saturated with moisture 14
Black spots Are mold Growing In the fibrous glass Figure 24E These designs limit the drying ability of the wall to the inside to function properly below grade walls need to have drying ability to the inside of the building Venting the soil gasses from below the slab is an excellent way to lower the moisture content below the slab. This will also help control radon levels in the basement. one inch foam insulation Figure 25 15
1X2 or 2X2 treated wooden spacers, 6 inches long are fastened flat on the concrete floor a minimum of ½ inch from the masonry wall (can be up to 1 ½ inches away if needed to straighten out the wall). Attach spacers to concrete floor with two masonry screws or glue conc. slab 48 inches on center Figure 26 Rebuild of Basement Walls This wall can be built using 2X2 or 2X4 studs. When using 2X2 spacers, a 2X4 stud is installed on edge across the top of the spacers, mounted vertically 16 inches on center. The new stud wall is installed ½ to 1½ inches from the concrete block wall. The top of this wall is fastened to the 1 st floor joists above. 2X2 or 2X4 studs 2X4 studs installed flat if 2X2 studs are used Figure 27 - Recommended Rebuild of Basement Walls with Spray Foam Insulation 16
Foam insulation is applied under the bottom wall stud. Mold sensitive materials (wall studs) are enclosed in foam insulation and protected from water penetration from the outside and from the inside. Interior water must be over 1 ½ inches deep before the studs will get wet and start to rot. Three inches of foam is applied to above grade and rim joist areas of the house. Below grade walls need 2 inches of foam 17
Figure 28B Never stuff fibrous glass insulation in a rim joist area Mold Mold Figure 28A - Fibrous glass in the rim joist is always problematic and encourages mold growth 18
Figure 28C Closed cell urethane foam to a thickness of 2 ½ inches (R-19) in the rim joist, can be left exposed without a thermal barrier 4 inches of pink or blue foam could also be installed here Figure 28D Closed cell urethane foam to a thickness of 2 ½ inches (R- 19) in the rim joist, can be left exposed without a thermal barrier 4 inches of pink or blue foam could also be installed here must be sealed with caulk or foam to make an air barrier discolored wood needs to be coated with Zinsser Perma White or other mold resistant paint 19
Install gypsum wallboard (paper-less preferred) ½ inch off the concrete floor. Figure 28 - Recommended Rebuild of Basement Walls with Spray Foam Insulation References Building Science Corporation: www.buildingscience.com or 70 Main Street, Westford, MA 01886, Phone 978-589-5100 Minnesota Department of Commerce Energy Information Center: http://www.commerce.state.mn.us/pages/energy/ InfoCenter/EnergyGuides.htm Univ. of Minn: www.buildingfoundation.umn.edu/ FoundRuleWWW/default.htm Air Tamarack Corporation: or 20
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