In this first quarter of 2021, adjusted revenue of $253 million increased 2% from prior year, driven by sales of our 2020 acquisitions, which more than offset COVID-driven volume reductions in our organic business.
Adjusted EBITDA of $85 million declined 12% from the prior year driven by COVID-related volume declines, continuing investments in our growth initiatives and the resumption of spending that was deferred when the pandemic hit.
Our architectural specialty business delivered solid top-line growth of 25% versus prior year quarter driven again by our 2020 acquisitions of Turf, Moz and Arktura.
Given our strong backlog, we remain confident in delivering our 2021 sales outlook of more than 30% growth.
As a result, beginning back in December, we have implemented five price increases totaling more than 40%.
As a result, we have announced a heavier-than-normal 10% price increase on Mineral Fiber products and pulled the effective date up to May, earlier than normal.
This is on top of the implemented February increase of 7%.
Adjusted sales of $253 million were up 2% versus prior year.
These adjusted sales include approximately $700,000 of purchase accounting adjustments related to our 2020 acquisitions.
Adjusted EBITDA fell 12% and EBITDA margins contracted 520 basis points.
Adjusted diluted earnings per share of $0.84 were 23% below prior year results.
This result includes $6 million or $0.09 of amortization expense related to our 2020 acquisitions.
Adjusted free cash flow declined by $13 million versus the prior year.
Our balance sheet remains in a strong position as we ended the quarter with $397 million of available liquidity, including a cash balance of $122 million and $275 million of availability on our revolving credit facility.
While net debt of $587 million was $43 million above Q1 2020 results, our net debt-to-EBITDA ratio of 1.9 times as calculated under the terms of our credit agreement remains well below our covenant threshold of 3.75 times.
In the second quarter, we repurchased 126,000 shares for $10 million or an average price of $79.60 per share.
Since the inception of our repurchase program in 2016, we have bought back 9.9 million shares at a cost of $616 million for an average price of $62.57 per share.
We currently have $584 million remaining under our repurchase program, which expires in December 2023.
In the quarter, sales declined 5% versus prior year due to the impact of COVID.
Through Friday, April's month-to-date daily ship rate is up 58% versus prior year and higher than 2019.
Mineral Fiber segment adjusted EBITDA was down 10% as a result of the COVID-driven volume declines, SG&A spending to support our growth of investments and the reinstitution of the 2020 temporary cost reductions.
Adjusted sales grew 25% versus prior year were $13 million as the 2020 acquisitions of Turf, Moz and Arktura contributed $17 million in the quarter and more than offset COVID-driven organic sales decline of $4 million.
EBITDA for our AS segment declined $3 million as EBITDA contributions from the 2020 acquisitions were more than offset by AS organic performance.
Cash flow from operations was down $13 million driven by lower earnings.
We remain confident that we will deliver the 19% free cash flow margin that we have guided too for the year.
We are reiterating our overall expectations to grow sales 10% to 13%; adjusted EBITDA 9% to 13%; adjusted earnings per share 5% to 15%; and deliver free cash flow yield of 19%.
As many of you have seen earlier this month, we launched a redesigned sustainability section of our website that reflects our three pillars of focus: People, planet and product and establishes our 2030 goals in these areas.
Our 24/7 defend family of products including AirAssure, CleanAssure and VidaShield ceiling solutions are designed specifically to help improve air quality and ventilation sustainably.
What's encouraging is that despite being launched just five months ago, 24/7 defend products have been sold into all of our core sectors: office, education, healthcare, retail and transportation.

Summary:
We are reiterating our overall expectations to grow sales 10% to 13%; adjusted EBITDA 9% to 13%; adjusted earnings per share 5% to 15%; and deliver free cash flow yield of 19%.