Before we get into Armstrong's fourth quarter and full year 2020 financials, I want to take a moment to recognize what we've all been through, on a personal level, these last 12 months.
Our 2,700 Armstrong employees, our communities, our partners, suppliers, our customers and our shareholders have, no doubt, been impacted by the multiple crisis we've all faced.
For the full year, 2020 sales of $937 million were down 10% from 2019.
Adjusted EBITDA of $330 million was down 18% from 2019, coming in at the high end of our guidance range.
Adjusted free cash flow for the year was a strong $212 million or 23% of sales, once again demonstrating the strength of Armstrong's best-in-class value creation model even in the face of a pandemic-driven sales declines.
Core product mix, the underlying driver of AUV improvement for the past 10 years has continued to be positive in 2020 as our higher-end solutions, including the Total Acoustics and Sustain families, outperformed the lower price range of our portfolio, reflecting the continued desire of architects and building owners to improve the performance and aesthetics of their spaces.
We identified and acted on $40 million of temporary cost savings that would not impact our growth and value creation opportunities.
We launched 35 new products in 2020.
That's a 50% increase from our normal pace of activity.
Even with enhanced access protocols for our labs and test chambers, our team brought to market at record speed the 24/7 Defend portfolio to address the need for healthier, safer spaces and to specifically improve indoor air quality.
As you likely know, we spend 90% of our time indoors.
To help us learn more and bring healthier spaces to life faster, we are installing our 24/7 Defend solutions in our own facilities, and we are retrofitting one of the existing buildings on our corporate campus to be a Healthy Spaces living lab.
92% of architects and designers surveyed said they are having conversations with their clients on how to make their spaces healthier and safer.
Sales of $239 million were down 3% versus prior year, a continued sequential improvement from the third quarter, when year-over-year sales were down 11%.
Adjusted EBITDA fell 19% and margins contracted 580 basis points.
Adjusted diluted earnings per share of $0.77 fell 31% as our 2019 fourth quarter tax rate benefited from stock-based compensation deductions.
Adjusted free cash flow declined by $3 million versus the prior year.
Our cash balance at quarter end was $137 million and, coupled with $275 million of availability on our revolving credit facility, positions us with $412 million of available liquidity, down $42 million from last quarter as we completed the Arktura acquisition during this past quarter and down $18 million from the fourth quarter of 2019.
Net debt of $578 million is $12 million higher than last year as a result of our acquisitions, partially offset by cash earnings.
As of the quarter end, our net debt-to-EBITDA ratio is 1.8 times versus 1.5 times last year as calculated under the terms of our credit agreement.
Our covenant threshold is 3.75 times.
In the quarter, we repurchased a 126,523 shares for $10 million for an average price of $79.04 per share.
Since the inception of our repurchase program, we have bought back 9.7 million shares at a cost of $606 million for an average price of $62.35.
We currently have $594 million remaining under our share repurchase program, which expires in December 2023.
In the quarter, sales were down 7% versus prior year but improved sequentially from the third quarter, when year-over-year sales were down 14%.
Adjusted EBITDA was down $15 million or 19% as the volume decline in channel-driven AUV weakness fell through to the bottom line.
Sales were up $6 million or 13% as the 2020 acquisitions of Turf, Moz and just recently, Arktura, contributed $11 million in the quarter and offset COVID-driven organic sales decline of 9%.
AS organic sales also improved sequentially from the third quarter when year-over-year sales were down 14%.
Our 2020 acquisitions added a net $3 million adjusted EBITDA benefit and delivered a 27% adjusted EBITDA margin.
Cash flow from operations was down $8 million on lower sales and partially offset by lower capital expenditures of $5 million.
First, we received $13 million related to environmental insurance recoveries in the quarter.
Second, we made a $10 million one-time endowment-level contribution to the Armstrong World Industries Foundation with funds earmarked from a portion of the $22 million of proceeds from the sale of our Qingpu, China facility that we received earlier in the year.
Versus prior year, sales were down 10% and adjusted EBITDA was down 18%.
Adjusted earnings per share was down 24%, driven by a lower 2019 base period tax rate due primarily to deferred state tax adjustments and, to a lesser degree, the stock-based compensation deduction that impacted Q4 in 2019.
Again, COVID-related volume declines are the main driver as they impacted EBITDA by $65 million and were partially offset by volume-driven contributions of $14 million from our 2020 acquisitions.
slide 11 reflects full year adjusted free cash flow of $212 million.
In a year significantly impacted by the pandemic, we delivered a 23% adjusted free cash flow margin.
We anticipate revenue in the range of $1.03 billion to $1.06 billion, or up 10% to 13% versus prior year.
Driving this growth is a return to positive mix starting in the second quarter, continuation of like-for-like pricing with the backdrop of rising inflation, which will result in the resumption of our historic 4% to 6% AUV growth rate.
We expect adjusted EBITDA to grow 9% to 13% as the benefits of sales growth falls through and we continue to drive productivity in our plants and benefit from improved results at WAVE.
At the midpoint, our EBITDA margin of 35% is slightly down in 2021, driven by the impact of 2020 acquisitions on a full year basis.
Adjusted free cash flow will be 19% of sales as we resume our historic levels of capital spending and as working capital expands to support sales growth.
We expect to return to our greater than 20% historical average in the short term.
page 13 is not something we typically share as our seasonality across the quarters is usually very consistent year-to-year.
We launched 35 new products, many industry-leading, to serve today's most pressing needs for improving indoor air quality, including our AirAssure and VidaShield products.
In the past 18 months, there's been a good deal of effort behind the scenes on ESG-related matters.
And that said, I also believe we can improve.

Summary:
Sales of $239 million were down 3% versus prior year, a continued sequential improvement from the third quarter, when year-over-year sales were down 11%.
Adjusted free cash flow declined by $3 million versus the prior year.
Sales were up $6 million or 13% as the 2020 acquisitions of Turf, Moz and just recently, Arktura, contributed $11 million in the quarter and offset COVID-driven organic sales decline of 9%.
Our 2020 acquisitions added a net $3 million adjusted EBITDA benefit and delivered a 27% adjusted EBITDA margin.
We anticipate revenue in the range of $1.03 billion to $1.06 billion, or up 10% to 13% versus prior year.
We expect adjusted EBITDA to grow 9% to 13% as the benefits of sales growth falls through and we continue to drive productivity in our plants and benefit from improved results at WAVE.
And that said, I also believe we can improve.