Wind loads are the enemy of air barriers – whether air barrier only or air/vapour barrier – wind can displace loose attached membranes resulting in the loss of continuity and function.
Occupant comfort and thermal and energy efficiencies are mitigated by the reduction of the number of air changes per hour – the less air movement that an interior environment is subjected to – the more comfortable – stable temperatures and humidity and the less costly to condition – air conditioning in the warm months and heating is the cold months.
Air barriers must be installed very well, attached to the substrate that is the building structure and skin, must be continuous, must be completely sealed to windows, other openings and projections and most importantly must remain in place – with no breeches, detachments or displacements for the life of the wall system.
A well installed, high quality air barrier membrane is one of the most important systems of the building envelope. Building performance depends on a high quality air barrier – migration of moisture is either prevented entirely or allowed to disperse in a controlled fashion. The building structure, particularly with wood framed structures, is protected and preserved. The exterior finishes and claddings can provide the aesthetics for the building, the air barrier will provide the protection – from air and moisture – for the interior spaces.
Following is a Comparison Chart that summarizes the characteristics and performance issues associated with the two most used types of air barrier systems – the loose attached “Tyvek” polymer type of air barrier is less expensive, but provides considerably poorer performance, both long and short term. The self-adhered membrane or SAM is more costly, but provides superior performance in all categories.
The practical benefits and sales attributes of the SAM system – increased occupant comfort, long term protection of the building structure, reduced energy costs and overall superior performance of the wall system are worth the extra cost, if the project can afford the extra cost.
COMPARISON OF AIR/VAPOUR BARRIERS – LOOSE APPLIED VS. SELF-ADHERED
|COMPARATOR||TYVEK TYPE AIR BARRIER||SELF-ADHERED AIR BARRIER|
|Description of Composition||Woven polyolefin fabric with synthetic coating||Bituminous or synthetic membrane with robust reinforcement and adhesive|
|Typical Manufacturers or Tradenames||Tyvek, Typar||Bakor – Blueskin – bituminous Synthetic polymer – 3M|
|Type of Installation||Loose applied – fastened with staples (not rec`d) – nails and plates preferred||Fully adhered – self adhesive membrane – tenacious adhesive to substrate – no fasteners required|
|Ease of Installation||Large, wide sheets must be installed – as few the number of seams as possible||One meter wide rolls – SAM rolled out on substrate – number of seams not critical|
|Complexity of Installation||Controlling the size of sheets, taping all seams, fastening to substrate, taping around projections, detail strips of SAM required, attachment to windows by tape||Primer or no primer (3M), membrane adheres in place, no fasteners, use same membrane for details, windows etc. – ease of handling smaller sections of membrane|
|Typical Completeness of Installation||The installation is very fussy – fasten to substrate, install details strips of SAM, tape all seams and laps, tape to windows – 100% is difficult||Installation is much easier to complete – overlap and adheres seams, adhered to windows, use same material for detail strips – fewer steps|
|Effectiveness of Installation||Difficult to ensure continuity||Continuity much easier to attain|
|Long Term Ability to Remain in Place||Wind loads can displace and tear membrane, tape can fail, breeches can occur||SAM is permanently adhered to substrate, will not displace, no tape to fail|
|Positive Sealing of Seams||Fair – dependent on quality and amount of red tape applied||Excellent – self-adhering adhesive is extremely tenacious|
|Ability to Resist Wind Loads||Fair||Excellent|
|Long Term Performance||Fair to good||Excellent|
|Cost – psf – estimate for comparison only||$3.50-4.00||$7.00 – 7.50|