Air Sealing in New ConstructionKL rev1.ppt · • box sills, • rim and band joists • After plumbing and electrical rough-in • cantilevered floors, overhangs, • cape details,

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AIR SEALING IN NEW CONSTRUCTION

SEALED & DELIVERED

Logan Brown

Keith Levenson

Better Buildings by Design Conference

Burlington, VT

February 13, 2008

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Why air seal at all?

• Prevent ice dams and moisture damage

• Reduce heat loss

• Control indoor air quality (IAQ)

Why don’t we “just build them like we used to?”

• Old buildings were Energy Hogs

• Hard to maintain comfortable temps and humidity levels

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Air Sealing Gives You IAQ Control

• GOAL: Best IAQ with least amount of energy loss

• STRATEGY: Tight shell with exhaust ventilation at the sources of

pollutants and moisture:

• Stove top

• Bathroom

• SIDE BENEFIT: Longer lasting house

• keeps moisture out of walls and attics

• reduces ice dams

• prevents frozen pipes

• avoids paint failure

Air Sealing also helps the building meet RBES, VT ENERGY STAR

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Can buildings be too tight?

• Short Answer: NO

• But, they can be under-ventilated

• A tight building is easier and cheaper to heat, cool, and

ventilate

• A tight shell gives you better control over indoor air quality

• A tight shell will eliminate the need for “creative” HVAC

distribution

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The Air Sealing Goal

A continuous & effective air barrier

*This is the basic intent of the Thermal

Bypass Inspection Checklist

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Some terminology

• Air barrier (www.airbarrier.org)

• Stops air flow through a building envelope under normal

pressure conditions.

• Air will always flow from high pressure toward low pressure.

• On most buildings, sheetrock is the effective air barrier.

• Vapor barrier (or retarder)

• Stops or slows water vapor diffusion; can be flexible (poly), or

rigid (foam or painted sheetrock).

• Water vapor diffuses:

• from high concentration to low concentration,

• from warm to cold

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More terminology

• Thermal Boundary

•Separates conditioned and unconditioned space. It is

made up of an air barrier, optional vapor barrier, and

insulation.

• To be an effective Thermal Boundary, the air barrier

must be in contact with the insulation*

*This is #1 on the Thermal Bypass Inspection Checklist

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Insulation

• Fiberglass does not stop air

flow

• Loose blown cellulose does

not stop air flow

• If air moves through

insulation, it is not insulation

-- it is filter medium

• Some types of insulation are

also good air barriers

• Rigid foam

• Spray foam

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Moisture Migration:

Diffusion vs. Air Leakage

- Diagram from Building Science Corp. Builder’s Guide for Cold Climates, 2001

Water vapor diffusion through 4’ x 8’

sheetrock wall (no vapor barrier)

Air Leakage through sheetrock wall

with a 1 square-inch hole

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Stack Effect

When it’s cold outside, a

building behaves like a

chimney:

Warm, moisture-laden air

leaks out the top of the

building…

… and cold air infiltrates at

lower levels.

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Which leaks do I seal?

• All leaks play a role, so seal everything

• Because of “stack effect,” the top floor leaks are usually

outward. Sealing top floor leaks stops warm, moist air from

leaking into walls and attics, reducing heat loss AND moisture

damage.

• On lower levels, cold, dry air tends to leak inward. Sealing these

leaks reduces drafts, and reduces heat loss by slowing the

overall air change rate.

• With a tight shell, an exhaust fan can depressurize the building

enough so that almost all air leakage is inward. Moisture

damage is minimized.

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Different houses require different levels of effort

There are two general categories of houses:

• Simple

• Complex

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House with simple shape

• Footprint of the foundation, 1st floor and 2nd floor are the same

and aligned with each other

• Air sealing is generally straightforward

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House with complex shape

• Footprint of the foundation, 1st floor, and 2nd floor are different

and/or not aligned with each other

• Requires more creativity, thought and planning to air seal

effectively

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Another type of house

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Sealing leaks at the top

• Chases

• Attic hatch

• Plumbing and wiring penetrations

• Interior partition wall top plates

• Electric boxes and bath fan housings

• Balloon framing

• Recessed lights

• Tongue and Groove Ceilings

• Ducts in Attic

• Flues and Chimneys

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Using plans to guide air sealing

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Chases

• Seal with or rigid foam or sheetrock and minimally expanding foam or

two-part expanding foam with fiberglass backing

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Attic hatch

• Weatherstrip the trim lip

that the cover sits on

• Also caulk the outside

edge of the trim lip at the

sheetrock:

• Insulation must be in

permanent contact with

hatch

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Plumbing and wiring penetrations

• Minimally expanding foam works well

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Interior

partition

walls

Diagram from Field Guide Residential

New Construction, CSG, 2000

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Interior partition wall top plates

•Easy way: bead of caulk on top

plate before hanging sheetrock

•Or seal gap from the attic

with foam or caulk

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About minimally expanding foam

• Use high-quality gun with adjustable bead size

control - Pageris, Purfil, Hilti, Handi-Foam

• Avoid can with a straw - too hard to control, more

expensive in long run

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Electric junction boxes

Can be sealed from the attic:

Or from the inside, with caulk or mastic

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Bath fans

•Seal metal housing to sheetrock

with caulk or minimally expanding

foam

•Fan will perform better by drawing

only from room instead of attic:

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Balloon framing

• Stud cavities open to attic - no top plate

• Blocking recommended at ceiling level

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Recessed lights

• Use IC-rated, and “Washington State” approved (ASTM E-

283 tested) box

• Seal box to sheetrock with caulk, or use gasketed trim kit -

no foam

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Tongue and Groove Ceilings

• Impossible to air-seal all the T & G joints

• Install T&G over painted sheetrock or other rigid air barrier

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Polyethylene does NOT make a good Air Barrier

The most durable, effective air barriers are RIGID

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Ducts • Keep ducts out of attic or exterior walls

• Seal duct at sheetrock with foam or caulk

• Seal all duct joints with mastic, or PVC glue

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Flues and Chimneys

• Avoid installing chimneys and flues through attic by

using direct-vent heating systems and gas fireplaces

• Use non-combustible material to air seal: metal or

sheetrock and fire-rated caulk

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Flues and Chimneys

Seal masonry chimney chases with metal and high

temperature caulk

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Air sealing and Fire Code

• Work together - Code officials often want fire caulking

in the same place we want air sealing

• Code officials will often accept rockwool stuffed “tight”

as draft stop … This WON’T stop air

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Basement and Foundation Air Sealing

•Slab Floors

•Below-grade walls

•Walkout Basements

•Mud Sills and Band Joists

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Slab Floors and Below-Grade Walls

• Below grade: no pressure difference, so no air flow

• Stop diffusion of moisture, soil gas, and radon with vapor barrier

• Seal foundation wall penetrations, especially near and above grade

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Walkout basements

• Airtight sheetrock on framed sections of basement

wall

• Stapled polyethelyne NOT effective as air barrier

• Seal bottom edge of sheetrock to slab or stepped

foundation

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Air Sealing at Band Joist

• Sill seal works well under mudsill

• Seal electrical service penetrations,

fuel lines, central vacuum, and any

holes in band joist with foam or caulk

• Seal top and bottom edges of rim and

band joist to mud sill and subfloor

• Same practices apply to rim joists



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Another Band Joist Solution:

Two-part polyurethane

foam

•High R-value per inch,

•effective as an air barrier,

•and vapor retarder

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Two-part polyurethane foam

• Cost is high compared to other

insulation

• But two-part foam is also an

effective air barrier, and

•An effective vapor retarder, and

•It can be applied very quickly.

•Best to apply thinly for air sealing,

use bulk insulation to get desired R-

value

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Best places to use two-part foam

• Open band joist/sill areas

• Chases with multiple mechanical services - use

fiberglass as “backer” to spray against

• Places where foam gun makes access difficult

• Where mish-mash of penetrations means tedious

foam gun use

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Air Sealing in the middle of the building

• Fireplace inserts

• Tubs and Showers on exterior walls

• Sheetrock to subfloor

• Electric boxes

• Windows and doors

• First floor bath fans

• Tuck-under garages

• Cantilevered floors and overhangs

• Kneewall Spaces

• Common Walls / Fire Walls

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Gas fireplaces Right ->

• Hang sheetrock, mud and

tape before enclosing

fireplace

• Seal “pipe-within-a-pipe” to

sheetrock with fire caulk

Wrong ->

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Bottom edge of sheetrock on exterior walls

Seal bottom edge of sheetrock to subfloor on exterior

walls with foam gun

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Electric boxes

• Use gasketed boxes or

seal box edges to

sheetrock with minimally

expanding foam, caulk or

mastic

• foam wire entry into box

with caulk or mastic

From: http://www.lessco-airtight.com/

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Windows and Doors

• Use minimally expanding foam around window

and door frames

• Don’t try to fill cavity with foam, just span the

width of the gap and fill the depth with fiberglass

• Use oversized rough opening – leave room for

the foam gun

• Seal jamb extensions to sheetrock with caulk

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First floor bath fans and dryer vents

• Fan housing sealed to sheetrock

• Where ducting penetrates band joist, seal with minimally

expanding foam, or insulate rim joist with two-part foam

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Tubs and showers on exterior walls

• Rigid sheathing or air barrier sealed “sheetrock-style” before tub is

installed

Good: GWB->

Good: OSB->

Bad: No Air barrier

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Tuck-under and attached garages

• If garage is filled with water and house turned

upside down, where does water leak into house?

• Seal common walls and floors on both sides

• Seal band joist, or block and seal joist bays at

garage/house common wall

• Caulk or foam all penetrations of surfaces that

adjoin living space

• Seal bottom edge of sheetrock or sheathing to

slab with caulk or minimally expanding foam

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Cantilevered floors and Overhangs

• Potential problem where exterior walls do not align

• Floor system is connected to outside – air barrier is

discontinuous

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How to seal cantilevered floors

• Blocking in floor bays

• Seal edges and joints of panel under cantilever with caulk or

minimally expanding foam

• Make sure insulation is in contact with subfloor and exterior

sheathing/soffit



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Cantilevered floors after air sealing

• Seal underside sheathing - seal joints and edges with caulk

or minimally expanding foam

• Block floor joist bay and foam blocking in place

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Cape-style framing: Visual Clues

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Capes: The kneewall problem

• Floor system open and connected to

outside

• Backside of kneewall has no sheathing

• No air dam at outside edge of kneewall

floor – windwashing results

• This contributes to ice dams and

distinctive snow melt patterns on

capes



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Cape kneewall: One solution

• Install and seal blocking in floor

joist bays below kneewall

• Seal bottom edge of sheetrock

on kneewall with minimally

expanding foam or caulk

• Rigid sheathing on backside of

kneewall

• Air dam at end of joist bay



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Cape kneewall: Another solution

• Install sheetrock or rigid foam

on underside of slant as air

barrier

• Seal top, bottom and both sides

of air barrier

• Don’t forget end wall!



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Common Walls & Fire Walls

• Seal all gaps at top plates

• Seal all gaps and penetrations

through end studs

�Sealed top plate

Un-sealed gap

at end stud -->

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Common Walls & Fire Walls

• Seal the gap between top

plates and end studs of a

double stud wall

• For end studs, consider

running continuous

sheetrock behind end stud

�Un-sealed gap

between top plates

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When to do air-sealing

• During framing and sheathing:

• box sills,

• rim and band joists

• After plumbing and electrical rough-in

• cantilevered floors, overhangs,

• cape details, kneewall spaces

• end studs at exterior walls

• After sheetrock and before attic insulation:

• top floor ceiling electrical and plumbing penetrations

• fan housings and recessed cans

• Everything else can be sealed during finish stage

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How do I know I did it right?

• Test air leakage with blower

door during construction

• Helps to have experienced

operator to “interpret” leaks

• Efficiency Vermont does

pre-drywall inspections,

using the Thermal Bypass

Inspection Checklist

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Another tool: Infrared imaging

Dark areas are cold spots – could be due to:

• air leakage, or

• air movement through insulation, or

• insulation voids, or

• thermal bridging,

• Or even drying mud!

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Infrared in Combination with a Blower Door

•IR images while blower

door is running can show air

leakage routes

•Very powerful if interpreted

correctly

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Air Sealing Goals: A New Metric

• Old metric Natural Air Changes per hour (ACHnat) from the LBL

model. Old goal: 0.35 ACHnat

• Only really applies to small residential buildings

• Implies that natural ventilation is the same year round

• Larger homes with smaller surface area/volume ratios get lower

numbers – doesn’t scale well

New Metric is more broadly applicable:

• New metric: CFM50 per sq. ft. of external surface area

(CFM50/SFESA). New goal: 0.25 CFM50/SFESA

• Allows comparison of a wide range of building types and sizes

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Air Sealing Goals

Qualifying as a Vermont Energy Star Home requires

meeting the requirements of the Thermal Bypass

Inspection Checklist.

Two basic principles:

1. Air barrier is continuous and rigid

2. Insulation is in contact with the air barrier

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Success Story

Before Air Sealing

Before air-sealing…

… and after

Documents

Air Sealing in New ConstructionKL rev1.ppt · • box sills, • rim and band joists • After plumbing and electrical rough-in • cantilevered floors, overhangs, • cape details,