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Renovation 4th Edition

Page 93

by Michael Litchfield


  and forth through the thermal envelope as the

  with mastic.

  seasons change. During summer months, when

  Fiberglass tape and mastic can also be used to the air outside is warmer, it tends to move inward.

  seal gashes in insulated flex ducts, but because

  In colder months, heated indoor air migrates out-

  such ducts “give” somewhat as pressure is

  ward. As water vapor migrates outward along

  applied to them, oriented polypropylene (OPP)

  with the heated air, the moisture condenses as it

  tape and foil-backed butyl tape are often used to

  hits cold exterior surfaces. If that condensation is

  patch flex ducts because both tapes stick aggres-

  sustained and excessive, it can soak and, in time,

  sively. Ironically, you should never use cloth-

  rot the framing. (Warm, moist air escaping

  backed duct tape to seal ducts because heat

  through ceilings causes the picturesque but dam-

  causes its adhesive to dry out and come undone.

  aging ice dams of New England roofs.)

  In response to winter condensation problems,

  VApoR RETARdERS

  builders began installing a polyethylene vapor

  barrier on the living-space side of insulation to

  Any discussion of air-sealing would be incom-

  prevent moist air from migrating into wall cavi-

  plete without mentioning vapor retarders, com-

  ties and condensing there. In winter, vapor barri-

  monly called vapor barriers. As essential as water

  ers work well enough. But in warm months, when

  is to life, excess moisture inside a house can lead

  moist outside air migrates into wall cavities, it is

  to big problems—especially as houses become

  blocked by the vapor barrier and stays in the

  more airtight. Thus, materials that are part of a

  walls. Heating and cooling climates have oppo-

  building’s thermal envelope are chosen, in part,

  site problems when it comes to poly vapor

  on how permeable or impermeable they are to

  barriers. In hot, humid climates, where air

  water vapor.

  conditioning runs constantly in the summer, a

  Permeability is measured in perms: The lower

  polyethylene vapor barrier installed behind

  the number, the more impermeable the material

  the drywall becomes a condensing surface for

  is and the more it will impede moisture’s move-

  moisture.

  ment. The higher the perm number, the more

  These days, there’s an uneasy consensus that

  easily moisture can pass through it. Accordingly,

  vapor barriers should be used sparingly in

  vapor retarders are classified:

  regions of extreme cold, such as Climate Zones 6

   class I (impermeable) vapor retarders

  and higher (see the map on p. 407). That is, vapor

  have permeance levels of 0.1 perm or less and

  barriers should not be used in moderate or hot

  are rightly called vapor barriers. The group

  climates. Take note renovators: If a home has

  includes polyethylene sheeting, rubber

  polyethylene vapor barrier behind the drywall, do

  membranes, sheet metal, aluminum foil, foil-

  not compound the problem by retrofitting foil-

  faced sheathing (insulated and not), and glass.

  faced insulation on the exterior of the building.

   class II (semi-impermeable) vapor

  Moisture in the walls would have nowhere to

  retarders have permeance levels between

  go, and that’s what builders are increasingly

  0.1 perm and 1 perm. Class II materials include

  concerned about. There must be some drying

  oil-based paint, kraft-paper facing on fiber-

  potential for exterior walls, either to the outside

  glass batts, unfaced extruded polystyrene

  or the inside.

  (XPS) panels thicker than 1 in., and traditional

  Better understanding of the movement of air

  stucco applied over #30 building paper.

  and moisture has led to a wider use of class II

   class III (semipermeable) vapor

  and class III vapor retarders. Some builders use

  retarders have permeance levels between

  what’s called the Airtight Drywall Approach to

  1 perm and 10 perms. The group includes

  stop air leaks and the movement of moisture into

  plywood, OSB, #30 building paper, most latex

  walls. And a new generation of exterior latex

  paints, unfaced expanded polystyrene (EPS)

  paints are proving more durable than oil-based

  panels, and unfaced polyiso (polyisocyanurate)

  paints because latex, being semipermeable, is

  foam panels.

  less likely to be lifted off of a substrate by

  trapped moisture.

  Energy Conservation and Air Quality

  397

  Vented and unvented roofs

  Vented attics with insulation on the floor are marvels of nature and

  man. After you air-seal openings in the attic floor and cover it with an

  appropriate amount of insulation, nature does the rest. Rising as it

  warms, air flows up from the eaves and out of ridge or gable vents, car-

  zzzzzz Insulating under the Roof

  Ridge vent

  rying off moisture that may have migrated from living spaces. In cold

  climates, insulation and ventilation combine to keep the roof cold,

  thus minimizing melting snow and ice dams. In hot climes, the same

  combination rids attics of sultry air and moderates temperatures in the

  floors below.

  Codes require a net-free ventilation area (nFVA) of 1 sq. ft. of vents

  for each 300 sq. ft. of attic space, equally divided between eave and

  ridge vents. But building scientist Joe lstiburek takes issue with that,

  opting instead for more ventilation at the eaves—say, a 60/40 split,

  with the eaves getting the greater proportion. lstiburek reasons that

  giving eaves more ventilation “will slightly pressurize the attic. A

  Closed-cell foam

  depressurized attic can suck conditioned air out of the living space,

  or batts

  and losing that conditioned air wastes money.”

  As near perfect as vented attics are, though, they have weak points

  Prefab vent

  energy-wise. you can pile insulation as high as you like in the middle

  baffle

  of the floor to attain required R-values, but the tight spaces where roof

  Soffit vent

  slopes meet sidewalls are a problem. do your best to air-seal and pile

  insulation over top plates—without blocking soffit vents and the like.

  Installing rigid insulation over wall sheathing can reduce energy loss

  Venting the roof deck. If you want to turn the attic into conditioned

  in this vulnerable juncture, but it’s a prohibitively expensive fix unless

  space or you’ve got a room with a cathedral (vaulted) ceiling, insulating and

  you already need to strip siding for some other reason.

  ventilating become more complex. The venting path is essential y the same—

  air flows up from the eaves and out at the ridge—but creating a vent channel

  under the roof deck can be chal enging. (The IRC requires 1 in. o
f airspace

  under roof sheathing; Joe lstiburek cal s for 2 in. of airspace, minimum.)

  Instal ing a prefabricated vent baffle (above) is certainly the fastest and

  zzzzzz A Vented Soffit

  perhaps the most cost-effective way to create a vent channel: Staple prefab

  Roof deck

  Rafter

  baffles between rafters and use canned spray foam to air-seal baffle edges.

  Vent-holes

  drilled into blocking

  Insulating under the roof is the conventional way to go and probably the

  between rafters

  most affordable if the roof is in good shape. your heating zone, your budget

  and the depth of your rafters will decide what type of insulation to use.

  Filling joist bays with closed-cell spray polyurethane foam will provide the

  greatest R-value per inch, but it’s the most expensive option. If you have

  2x10 or 2x12 rafters, you may be able to reach requisite R-values, with

  Insulation

  some combination of batts between the rafters and XpS rigid foam under

  Ceiling joist

  them. Furring out rafters to gain additional depth is another option, albeit

  Fascia

  Drywall baffle

  a labor-intensive one.

  Insulating over the roof may be a more attractive option if (a) your raf-

  Blocking

  ters aren’t deep enough, (b) you can’t afford to spray-foam insulation, or

  Top plate

  (c) your roof is worn out and you need to strip it anyway before reroofing.

  The built-up roof shown on the facing page relies on an array of 2x4 purlins

  Soffit vent

  (horizontal pieces) that capture the foam panels installed over the existing

  roof deck and provide nailing surfaces for materials installed over the foam,

  as well as and an array of vertical 2x4 spacers that create vent channels

  To keep air flowing from the soffit to the ridge, it’s

  important to keep attic insulation from covering the

  over the foam and under the new plywood sheathing (a second roof deck)

  vent to the exterior. Typically, builders use rigid foam to

  to which the new roofing will be nailed. A very complicated assembly,

  weigh down loose insulation along the sidewalls or

  which should be attempted only by seasoned builders.

  install combination baffles/air chutes to ensure airflow.

  398 Chapter 14

  An unvented roof is often the only viable option when roof framing is

  complicated, such as when there are hips, valleys, dormers, or skylights

  that would prevent eave to ridge ventilation; when the house has no soffits

  and hence no soffit vents; or when adding eave vents would clash with the

  zzzzzz unvented Roof

  house’s architectural style. unvented roofs also make a lot of sense in high-

  wind or high-fire areas, where vents might admit drenching rains or embers.

  Here again, the choice of what insulation to use and whether to install

  it over or under the roof deck depends on the condition of the present roof,

  the depth of rafters, the R-value you must attain, and cost. Cost aside, the

  most straightforward route is spraying closed-cell polyurethane foam on to

  Drywall

  the underside of the roof deck. If you need additional R-value, install XpS

  foam panels to the underside of rafters, and then cover the foam with

  drywall. Correctly done, this assembly creates a premium air-and-thermal

  barrier. Again, it’s a complex roof that needs to be impeccably detailed

  by a pro.

  Closed-cell foam

  of course, there are caveats. (There are always caveats.) unvented roofs

  in the snow belt may still need some type of venting to keep the roof

  Foam air-seals

  and insulates

  cold and prevent ice dams. And some roofing manufacturers won’t honor

  top plate

  warranties if their shingles are installed over unvented roofs. For more on

  this complex and constantly evolving topic, visit Joe lstiburek’s website:

  www.buildingscience.com.

  zzzzzz Insulating over the Roof

  2x4 spacers at 2 ft. o.c.

  Two layers of

  rigid foam insulation

  5/8 -in. plywood

  (new roof deck)

  Drywall

  Vent strip

  Rafter

  Spray foam air-

  seals and insulates

  Doubled 2x4

  top plate.

  purlins at perimeter

  Energy Conservation and Air Quality

  399

  soffit-to-ridge ventilation can prolong shingle life

  Increasing Control ed

  and make upper-floor rooms appreciably cooler.

  Ventilation

  Keeping vent channels open from soffit to

  ridge is essential to keeping air flowing.

  After air-sealing the house, you may need to add

  Continuous soffit vents are typically screened to

  controlled ventilation—some mixture of mechan-

  keep animals and insects out of the house. Inside

  ical ventilation (fans) and natural convection—to the attic, it’s equally important that as you air-

  exhaust stale air, remove excess moisture, and

  seal the perimeter and insulate the attic floor, you

  avoid furnace backdrafting. To moderate temper-

  prevent insulation from clogging vent holes

  atures under the roof and get rid of moisture that (intakes) drilled in the blocking (see “A Vented

  has escaped from living spaces, attic and roof

  Soffit” p. 398) or drifting down into soffit vents.

  vents work like a charm as long as uprising air is

  The simplest way to keep insulation where it

  allowed to flow freely. Better yet, this venting is

  belongs is to install baffles where attic floor joists

  free. Looking at mechanical solutions, whole-

  meet the rafters.

  house exhaust fans, and a few passive intake

  Standard how-to advice is to construct baffles

  vents don’t cost much.

  on site by cutting up rigid-foam panels—without

  SoFFIT-To-RIdGE VEnTIlATIon

  saying exactly where or how you attach the

  baffles in the tiny triangular spaces under the

  If air-sealing and insulating attic floors are the

  roof. Moreover, the spaces between rafters almost

  first steps to reducing excessive moisture and

  always vary, so precutting baffles outside the attic

  heat in an attic, increasing ventilation is the

  won’t work. In a 30-ft. by 40-ft. attic, you’ll need

  second. Nothing exhausts moisture or cools the

  roughly 80 lin. ft. of baffles, which could take

  area under a roof as effectively as soffit-to-ridge

  days to fabricate, fit, and seal.

  ventilation, as discussed here and in chapters

  AccuVent, a baffle fabricated out of recycled

  5 and 7. (Gable-end vents help but are usually 1 ft. plastic, seems a good solution to some of the

  to 2 ft. below the highest and hottest air.) As a

  problems just mentioned. Unlike rigid-foam baf-

  bonus, in winter, cool incoming air can prevent

  fles available at many home centers, AccuVents

  snowmelt and ice dams along eaves. In summer,

  are flexible and less likely to fracture when fit

  when unvented roofs can reach 150ºF to 160ºF,


  into the cramped triangular spaces where sloping

  roofs meet walls. Staple the flexible vent’s lower

  end to the inside of a top plate, roll its upper end

  Both an air chute and a baffle that

  snug to the roof deck and staple it, and then seal

  zzzzzz A prefab Baffle/Air Chute

  restrains attic insulation, the

  its edges with spray foam. Ridges in the back side

  AccuVent seems a good solution if

  of the vent keep it spaced about 1 in. from the

  roofing nails sticking down

  sheathing, so air can flow over it. It’s both a baffle

  through the sheathing don’t

  and an air chute, in other words.

  impede its installation.

  InSTAllInG An EXHAuST FAn

  Baffle

  The least expensive mechanical ventilation is a

  whole-house exhaust fan—similar to a bath fan

  Seal baffle edges with

  but rated to run continuously. (This fan is in

  expanding foam.

  addition to a bath fan and a kitchen range hood.)

  It’s hard to imagine an exhaust fan as small as

  Batt or loose-fill

  insulation

  50 cfm ventilating a whole house, but that size

  can vent up to 1,500 sq. ft. Moreover, Energy

  Star–certified fans such as Panasonic’s FV series

  do so cost-effectively, with a 15-watt fan running

  8 hours a day costing less than $10 annually to

  operate. To avoid depressurizing the house and

  causing backdrafting, it’s wise to install two or

  three passive intake vents as well. The cost of

  fan and intakes is modest—only a few hundred

  dollars—so spend a little more and have an

  HVAC specialist size and locate components for

  you and help you choose a 24-hour automatic

  timer to control the fan.

  400 Chapter 14

  P R O T I P

  Bath-Fan BEllS And WhIStlES

  There are complex formulas

  Removing moisture is the primary function of a bathroom fan—good to remember

  for sizing bath fans, but a good

  when considering all the extra features you could buy. First, get a quiet fan: 2 sones to

  rule of thumb is 1 cfm (cubic

  3 sones is tolerable, 1 sone is quiet and there are even 0.3-sone fans. Next, consider

  foot per minute) per square foot

  switches. Because fans usually need to continue venting after you leave the room, get

  for bathrooms 100 sq. ft. and

  smaller. For bathrooms larger

  an electronic switch with an integral timer so the fan can keep running after the light

 

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