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Energy-efficient, custom home build in Oregon featuring 2.0″ polyiso continuous insulation, and 2.5″ ThermalBuck window bucks with strapping.


Performance. This is what matters.

While ThermalBuck is easier to install and flash than a wood buck, it’s the performance that interests most architects and builders. ThermalBuck is made of a type XIV EPS with a density of 3 pcf, and coated in a waterproof resin. It’s lightweight and flexible, yet surprisingly strong and durable, handling loads of 70 lbs. per linear inch. The compressive strength is 52 psi.

ThermalBuck supports the weight of heavy, triple-pane insulated frame windows, and prevents compression of the exterior insulation. The unique “L” shape is designed to transfer both wind & shear loads to the structure itself. Unlike a traditional wood buck, ThermalBuck will not warp or shrink over time, and helps to maintain the integrity of the window installation.

ThermalBuck insulates the mounting points of windows & doors with an R-value of 4.4 per inch. It prevents condensation by limiting thermal bridging around windows and doors. Both the waterproof coating and installation with approved air & water sealants helps to create an additional air & water barrier in the building envelope.

Technical Downloads & Testing Reports

The testing environment for ThermalBuck simulated a 600 lb. window, and ThermalBuck was installed with only the Dow Corning 758 Sealant. ThermalBuck didn’t fail – the wall failed. ThermalBuck outperformed the wall structure itself.  


ThermalBuck is classified as thermal insulation. To qualify as a flashing agent, we require installation with Dap Dynaflex 800 Premium Modified Polymer Sealant (or other approved sealants), then fasten directly into the structure with 2.0″ roofing nails every 10″ – 12″. The window nail flange (3 sides) and WRB transitions (all) must be covered with flashing tape. Consult the WRB manufacturers’ installation instructions for recommendations.



PHIUS Verified Window Installation:
Alpen 925 Zenith Nail Fin Window + 2.5″ ThermalBuck

Energy efficient windows are an essential component of any high-performance building. But the installation was always a challenge for nail flange windows, which have to be mounted on the same plane as the exterior insulation.

In 2017, ThermalBuck teamed up with North-American passive house window manufacturer Alpen High Performance Products to introduce their PHIUS Verified Window Installation Detail featuring the Alpen 925 Series Fiberglass window with 2.5″ ThermalBuck window buck.

PHIUS Verified Window Installation Data:

Alpen 925 Zenith Nail Fin Window + 2.5″ ThermalBuck 

ThermalBuck effectively limits thermal bridging at the rough opening, provides a superior air & water barrier than a wood buck, and also a strong support for the weight of the window.

Passive house designers and builders now have the data they need to confidently include ThermalBuck into their specifications.

The Alpen HPP Zenith Series ZR-9/ 925 window features up to R-9.1 insulation, 99.5% UV protection, insulated frames, and Alpen Glass, Alpen’s renowned Suspended Film Technology. Every Alpen window has 9 key design features that make these highly engineered windows some of the best in the industry. Read more about the technology and craftsmanship of Alpen windows here.

Alpen High Performance Products is a national distributor for ThermalBuck. thinkalpen.com

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