Blow-in-Fiberglass Between Airtight Pro Clima Membranes is High Performance Insulation

Building Science Corp is clear on the fact that airsealing trumps insulation values. For truly high performance insulation to achieve its rated R-value and prevent thermal bypass, it should be air sealed on both the interior and exterior. The concept is akin to a Patagonia jacket - designed with air sealed compartments that optimize the insulating capacity of its down filling. We find that dense insulation alone is not enough for a high performance/passive house project.  This is one of the reasons High Performance Insulation Professionals (HPIP - formerly known as Blow-In Blanket Contractors Association) has endorsed Pro Clima's INTELLO, DB+, and SOLITEX MENTO membranes as vapor control systems that air seal blown-in fiberglass insulation on the interior and exterior.

BIBS behind MENTO PLUS membrane-WRB

We cannot overstate: air sealing is the driving force towards more efficient construction  - designers, architects, and builders take note.*   It drops the energy use at every step. Try modeling it in PHPP: go from 3.0ACH50 to 1.5 to 1.0 to 0.6 to 0.4 then 0.3ACH50. Each change drives the heating demand down significantly. We want well-insulated assemblies to go down below 1.5ACH50, which means we need to design/specify the air barrier properly and assure it is continuous/durable. These walls need to be designed so they stay dry, even in the unforeseen occasion that they get damp (or wet). The most resilient solution in a cold/mixed climate is an interior airbarrier and a vapor-open exterior WRB/material (see photo) with sufficient insulation between the two.

What else is crucial for insulation to perform as intended?

  • Install insulation evenly, not compressed, free of voids, and with every nook and cranny filled. Blown insulation is able to do this very well, as the force of the blower packs the entire bay.
  • Make sure to fill all bays.  When in doubt, or if blowing behind OSB or other non-transparent materials, verify with infrared.
  • Maintain a density that can retard airflow. This prevents thermal bypass in the form of convective loops.
  • Check density by counting bags per bay and testing installed insulation (see here, here or here).


Regardless of insulation thickness, it won't work as advertised/modelled/lab tested when not properly air sealed on all 6 sides. Even very densely installed fiberglass (or cellulose) will leak significant amounts of air. This becomes obvious when you consider that insulation installed at manufacturer's recommended densities allows airflows of 3.5CFM/SF to 2.3CFM/SF at 50 Pascal (see references below).  For example: imagine a test house - 20'x40'x10', without windows, pipes, and construction errors. Conventionally leaky insulation would lead the envelope of 2800 SF to leak at a minimum 9800-6440 SF or 1.2-0.8ACH50. The projects leak even more through seams, cracks, and openings. Additionally, that insulation cannot meet its potential R-value. Most importantly the house performance will be disappointing - not meeting energy models or expected comfort.

Smart vapor retarder, vapor open membrane and BIBS spray in fiberglass insulation working together

Leonard Farm - left installation of spider BIBS insulation over SOLITEX MENTO Plus - not strip of DB+ pre-installed behind rafter - right photo DB+ installed over shaved back spider as airtight/smart vapor retarder

The more insulation contractors, GCs, design-builders, and architects know how to get the result - the better their projects perform, which should result in happy customers with low bills, referrals, and comfort for all.


Related blogposts:

References: HPIP prospectus 2014 including ProClima INTELLO,DB+ and SOLITEX MENTO endorsement on page 7 Johns Mansville - reports 2.3CFM/SF for its SPIDER insulation (ASTM C522) Owens Corning reports 3.5CFM/SF can be met with PROPINK L77 at 2.1lbs/CF

"Air sealing installations must be installed to be permanent improvements to the structure. Products with an expected lifespan of less than 20 years shall not be used. Pressure differential analysis shall be used to assist in identifying leakage paths and prioritizing air sealing measures. Using Insulation for Air Sealing for leakage paths through enclosed cavities which cannot be accessed or reasonably sealed using conventional air sealing techniques, BPI requires installation of high density pneumatically applied insulation - which complies with BPI - 102 “Standard for Air Resistance of Thermal Insulation Used in Retrofit Cavity Applications – Material Specification” - or air impermeable foam insulation, to reduce airflow through the building shell.

Cellulose insulation used in an enclosed cavity shall be installed at 3.5 pounds per cubic foot or greater density. Blown fiber glass, mineral fiber, rock and slag wool, or spray foam used in an enclosed cavity shall be installed at or above the manufacturer’s recommended density to limit airflow that corresponds to an air permeance value of ≤ 3.5 cfm/sq ft at 50 Pascals, as measured using BPI-102 “Standard for Air Resistance of Thermal Insulation Used in Retrofit Cavity Applications Material Specification”

From BPI report 104:

Continue the conversation on social media