Alford, MA: GUTEX Roof Makes Sustainable High Performance

GUTEX UltraTherm - a vapor open alternative to foam

UltrathermWe've blogged about the many benefits of GUTEX wood fiberboard insulation for truly green construction - see this video introduction and our GUTEX primer. To begin with, it's one of the most sustainable insulation products, created from wood scraps and manufactured with a super efficient dry process for minimal environmental impact. It is also very practical, delivering continuous insulation and weather protection in one step. The tongue and groove edges make installation a snap and result in a windtight and waterproof exterior layer that can be exposed for up to three months. And unlike rigid foam, this insulation is vapor open - with 44 perm/in - to ensure that moisture in your walls can dry out if unforeseen humidity or moisture enters the enclosure.

These features are on display on a recent project in Western Massachusetts. The new construction home followed EcoNest principles for a "holistically designed and hand-crafted natural home" - using timber-frame, straw-clay walls, earth plasters and natural and non-toxic finishes. The walls were constructed with foam-free Durisol ICF blocks. For the roof, the project planned to use closed cell foam to insulate the cathedral ceilings. While healthy and green are two words not typically associated with foam (see here for more on this topic), this project seemed resigned to using closed cell foam to reach the R-49 residential code requirement that had gone into effect in July 2014 for climate zone 5.

Foam-free solution for R-49 roof:
R-41 cellulose plus R-8 GUTEX UltraTherm 60 (2-3/8") wood fiberboard

Things took a different turn after 475 proposed GUTEX UltraTherm over densepack cellulose for an R-49 foam-free roof insulation system. Evan Hardcastle from 377 Builders crunched the numbers, and it didn't take long for the team to jump on board. The switch to foam-free didn't require modifying the 2x12 structure - especially important for a project already well underway - and just needed a few inches of exterior insulation over cellulose to meet the new code.  For everyone in climate zones 5 and 6 who needs to hit R-49, here's the math:

  • 2x12 rafters densepacked with cellulose (R-3.7/in) yields R-41.6. (11-7/8" I-joists get you to R-43.9)
  • A layer of UltraTherm 60 (60 mm thick, or 2-3/8"), at R-3.4/in, adds R-8
  • Total nominal R-49.6
  • For those going for Passive House or just want to factor in thermal bridges (assuming rafters 16" o.c.), the total effective R-value comes in around R-44
  • For comparison, the effective insulation of 11.25" closed cell foam is R-50, or R-4.5/in - about 25% less than nominal

The cellulose-GUTEX combination offers a vapor open assembly with superior drying potential. For interior vapor control, code requires a Class I or II vapor retarder. The solution: INTELLO Plus fastened to the inside of the rafters to provide airtightness, intelligent vapor control (Class II vapor retarder in winter and vapor open in summer) and netting/support for the blown in cellulose. With its superb airtightness, INTELLO is the perfect accompaniment to exterior windtight GUTEX.


Installation is easy (especially with a cherry picker). UltraTherm boards are 23.6″W x 70.1″L (1780 x 600 mm) - and at 39 lbs are easy for one person to handle. They come factory tongue & grooved to span multiple rafters, so no need to worry about lining up with rafters! Just cut the last piece in the row for a nice press fit.

A tap here and there - from above or below - gets the GUTEX windtight and secure. After the boards are installed, 2x4 strapping is added for structure and back-venting. A rainscreen on your vapor-open roof is essential for optimal drying potential.

In the project shown, a waterproof roof offers a belt-and-suspenders approach, but in rainy Europe GUTEX is commonly installed under nothing but clay roof tiles - and GUTEX provides all the necessary drainage.  The GUTEX is the WRB.  However, while a secondary WRB isn't required over all of the inter-locking wood fiberboard, valleys need to be addressed with Pro Clima Primer RP and TESCON Vana tapes, and we recommend wrapping SOLITEX Mento 1000 (or similar waterproof membrane) around the first (lowest) GUTEX course to provide drainage to a gutter. And the roof cladding can wait - GUTEX has 3 month exposure.

With the roof exterior windtight and weathered in, the Alford project continues to make the house airtight at the interior. Interior airtightness is our best practice recommendation for a number of reasons - see this blogpost. The interior airtight plastered Durisol walls connect to windows, doors and INTELLO Plus air/vapor on adjoining wood-framed walls and roofs using CONTEGA FC- a special fleece tape ideal for these types of airtight connections.

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