We delivered strong third quarter topline growth, up 19% versus 2020 results with Mineral Fiber sales increasing 15% and Architectural Specialties sales improving 31%.
Adjusted EBITDA of $99 million was 8% ahead of prior year results.
Specifically within our Mineral Fiber segment, we reported third quarter AUV growth of 14% which is the highest level we've achieved since we separated from the flooring business in 2016.
And I can share with you with great satisfaction that this measure not only remained above our 90% threshold throughout 2021 for the Mineral Fiber segment in particular but it's improved in the third quarter.
Because of their design, our SimpleSoffit systems can be installed up to 3 times faster than traditional methods, with less material and labor hours.
New construction and major renovation activity improved but was uneven due to project delays, even with those challenges and our continued growth investments in this segment, Architectural Specialties' EBITDA margin improved 350 basis points sequentially and I expect these improvements to continue back above the 20% level.
GDP forecast remained above 5%, the Architectural Billing Index ended September well into expansionary territory at 56.6, up from August reading of 55.6, similar to the second quarter of Dodge data for both bidding and construction starts improved double-digits.
Adjusted net sales of $292 million were up 19% versus prior year.
Adjusted EBITDA grew 8% and EBITDA margins contracted 320 basis points.
Adjusted diluted earnings per share of $1.17 was 9% above prior year results.
Adjusted free cash flow was 28% above prior year results.
Our balance sheet remains healthy as we ended the quarter with $439 million of available liquidity, including a cash balance of $94 million and $345 million of availability on our revolving credit facility.
Net debt at the end of the quarter was $533 million and our net debt to EBITDA ratio of 1.5, as calculated under the terms of our credit agreement, remains well below our covenant threshold of 3.75.
In the quarter, we repurchased 187,000 shares for $20 million, for an average price of about $107 per share.
As of September 30, we had $544 million remaining under our repurchase program, which expires in December 2023.
Last week, we announced a 10% increase in our quarterly dividend, this is our third increase in the last three years and when paired with our share repurchases, is a reflection of our commitment to our balanced and disciplined capital allocation priorities that continue to be investing in the business, expanding into adjacencies through acquisitions, and returning capital to shareholders.
The $8 million adjusted EBITDA gain was primarily due to favorable AUV driven by positive like-for-like pricing and favorable channel mix, increased volume driven by the 2020 acquisitions, and contributions from WAVE equity earnings.
We expect inflationary pressure to continue into the fourth quarter, we now see cost of goods sold inflation somewhere in the 4.5% to 5% range for the full year 2021.
In the quarter, sales increased 15%, mostly due to favorable AUV previously mentioned.
Mineral Fiber segment adjusted EBITDA increased 10%, driven by the AUV gains and another strong quarter of equity earnings from the WAVE joint venture.
In addition, we experienced a $3 million headwind due to unplanned maintenance activities at two of our larger plants.
Third quarter adjusted net sales grew 31% or $19 million with the 2020 acquisitions in terms of Turf, Moz, Arktura, contributing $16 million and organic sales increasing $3 million.
AS segment adjusted EBITDA increased 1% as improved sales from the 2020 acquisitions and the organic business more than offset project push outs, higher SG&A, and increased manufacturing costs.
The adjusted EBITDA margin for the segment improved 350 basis points sequentially from the second quarter but contracted 500 basis points when compared to the third quarter 2020 results.
Sales for the first nine months of the year were up 18% and adjusted EBITDA increased 10%.
Adjusted diluted earnings per share increased 12% to $3.28.
We are narrowing our guidance ranges for all key metrics and now we expect year-over-year revenue growth of 17% to 18%, adjusted EBITDA growth of 13% to 15%, adjusted earnings per share growth of 14% to 16%, and adjusted free cash flow of down 7% to 2%.
You'll notice the increase in Mineral Fiber AUV range from 9% to 11% as our teams continue to do a great job of realizing price from our three Mineral Fiber increases this year.
We're bringing down the range of our Mineral Fiber volume to 1% to 2% as near-term choppiness remains and projects are delayed into the out months and 2022.
We now expect the 2020 acquisitions to contribute about 30% growth and AS organic in the mid-to-high single-digit range.
On a year-to-date basis, sales of these products have increased 38% versus 2020 and over 20% versus 2019 levels.
What's most encouraging is that approximately 60% of these sales are now coming from outside of the healthcare vertical.

Summary:
Adjusted diluted earnings per share of $1.17 was 9% above prior year results.