HGC Launches Landmark CLT Acoustics Study to End Costly Panel Cuts

HGC is leading a multi-partner, full-scale research program to modernize how the industry handles acoustics in mass-timber buildings—specifically the flanking that occurs when CLT (cross-laminated timber) ceilings continue past demising walls. In close partnership with Woodworks, US Forest Service, University of Northern British Columbia, Canadian Wood Council, Softwood Lumber, Green Building Initiative, Forestry Innovative Investment, and many others, our aim is simple and high-impact: to deliver a practical, public database of tested configurations so design teams can avoid costly CLT cuts and dropped ceilings without sacrificing acoustic performance.

Why Does This Matter?

In a building, sound moves two ways: directly through the separating assembly, and indirectly around it via connected structures. In mass timber, when CLT floor/ceiling panels are continuous across a demising wall and left exposed below, the ceiling itself can become a path for sound transmission — ceiling-to-ceiling flanking. You can read more about structural flanking here: What’s the Difference Between Direct and Flanking Sound Transmission in Building Design?

Because this path hasn’t been well quantified, projects often default to cutting CLT at demising walls, increasing cost, complexity, and schedule risk. Our recent field experience suggests those cuts aren’t always necessary; the right toppings and details above the CLT can significantly reduce transmitted energy. This program has been developed to systematically prove when and why that’s true.

What Are We Testing?

At the Forest Products Laboratory (Madison, WI), we’re constructing a dedicated test facility built around three 5-ply CLT panels spanning wood-stud walls, with surrounding rooms for controlled measurements. From a neutral baseline, we’ll introduce many different variables to quantify their impact on flanking:

• Floor toppings (the big lever): wet and dry systems over acoustical underlayments—e.g., concrete vs. gypcrete vs. plywood; changes in topping material, topping thickness, underlayment type, and underlayment thickness. Target scope (subject to available funding) ~40–60 toppings, sequenced to manage install and curing/drying logistics.

  

• Structural effects: how energy dissipation into adjacent CLT panels (e.g., corridors) and mismatched resonances between rooms affect flanking; select connection and wall-loading variations (e.g., top floors vs. lower floors).

  

• Bulkheads: effect of none/one/two-sided bulkheads and size.

  

For each test configuration, we will be measuring:

• Flanking transmission: the vibration reduction index Kᵢⱼ of the ceiling-to-ceiling path per ISO 10848, then use ISO 12354-1 methodology to predict that path’s contribution to the overall ASTC. In parallel, perform ASTC tests of the demising wall to capture direct + all flanking paths (the mock-up is detailed to heavily attenuate non–ceiling-to-ceiling routes). You can read more about K_ij testing here: How to Measure ASTC ratings of Specific Sound Transmission Paths
• Direct transmission (supporting data): ASTC of the floor/ceiling for each topping set, plus select AIIC testing as funding allows. You can read more about ASTC and AIIC testing here: Sound Transmission Class (STC) and Impact Insulation Class (IIC) testing
• Structural floor vibration from walking above different toppings to support structural engineers with natural frequency and damping characteristics information of different toppings. You can read more about floor vibration here: Floor and Building Vibration Analysis, Control and Isolation

Logistics and Timeline

With major funding secured, we’ve shifted from concept to execution. Testing is slated to begin at FPL in late Q1 / early Q2 2026, starting with baseline measurements followed by progressive topping evaluations. Because wet systems can evolve during curing/drying, we’re currently validating how soon acoustical behavior stabilizes and exploring workflow options (e.g., pre-pouring toppings on CLT panels off the mock-up and swapping panels with spreader bars) to maximize the number of toppings we can test while keeping the lab clean and safe.

Research Deliverables

• A field-ready design database showing which combinations of toppings, underlayments, and details are preferred to control ceiling-to-ceiling flanking.
• Clear guidance on whether continuous 5-ply CLT across demising walls can meet typical ASTC targets without panel cuts, bulkheads or full drywall ceilings.
• Better economics for mass-timber multifamily, institutional, and commercial projects—less over-detailing, fewer change orders, and cleaner exposed timber.

An Exceptional Team of Partners and Collaborators

This work is possible thanks to an outstanding group of funders, collaborators, and in-kind contributors across wood, manufacturing, research, and construction:

WoodWorks • U.S. Forest Service / Forest Products Laboratory (FPL) • Canadian Wood Council (CWC) • Softwood Lumber Board • Forest Innovation Investment (FII) • Green Building Initiative (GBI) • University of Northern British Columbia (UNBC) • Element5 • Aspect Structural Engineers • National Council of Acoustical Consultants (NCAC) • Findorff

And manufacturers of isolation materials participating in the toppings program:

Pliteq • AcoustiTECH • Maxxon • Rothoblaas • USG • Kinetics • Regupol • Rockwool • Soprema • PAC International • Getzner • AcoustiGuard • DCC Solutions • Acousti Armour

Funding Momentum

The program is backed by a USDA Wood Innovations Grant (2025) and additional active grant efforts to scale testing breadth and speed. This support ensures we can move beyond a single baseline and build a robust, publicly accessible dataset for design teams.

Follow the research—and get involved

We’ll share milestones and early insights as testing proceeds and will publish results for open industry use. To receive updates, collaborate, or explore material participation, contact Simon Edwards via his email address below.