and VA3 properties, where we have placed into service large contiguous data center NRSF within the last two years, has increased $11.0 million, $3.0 million and $2.7 million, respectively, compared to the six months ended June 30, 2019. These increases are primarily due to the commencement of large scale customer leases throughout the past twelve months, which generate variable revenue growth as customers deploy their IT equipment and increase their power consumption. This activity was offset by a customer move-out of 28,456 NRSF at SV2 and a customer move-out of 17,231 NRSF at BO1, as well as, other customer move-outs across various properties.
In addition, interconnection revenue increased $3.8 million, or 10.2%, during the six months ended June 30, 2020, compared to the 2019 period. The increase is primarily due to the net increase in volume of cross connects from new and existing customers during the twelve months ended June 30, 2020, and revenue increases resulting from customers migrating to our higher priced fiber and logical cross connect products.
Operating Expenses
Operating expenses during the six months ended June 30, 2020, and 2019, were as follows (in thousands):
| | | | | | | | | | | | |
| | Six Months Ended June 30, | | | | | | |
| | 2020 | | 2019 | | $ Change | | % Change | |
Property operating and maintenance | | $ | 81,220 | | $ | 76,177 | | $ | 5,043 | | 6.6 | % |
Real estate taxes and insurance | | | 11,789 | | | 12,184 | | | (395) | | (3.2) | |
Depreciation and amortization | | | 82,770 | | | 72,642 | | | 10,128 | | 13.9 | |
Sales and marketing | | | 11,981 | | | 11,436 | | | 545 | | 4.8 | |
General and administrative | | | 22,870 | | | 22,452 | | | 418 | | 1.9 | |
Rent | | | 17,394 | | | 15,421 | | | 1,973 | | 12.8 | |
Total operating expenses | | $ | 228,024 | | $ | 210,312 | | $ | 17,712 | | 8.4 | % |
Property operating and maintenance expense increased $5.0 million, or 6.6%, primarily as a result of an increase in salary and benefits expenses related to new operations at SV8 and VA3 and an increase in power expense due to increased customer power utilization related to the commencement of new and expansion leases, net of customer move-outs, and decreased contract and repair expenses across multiple sites.
Real estate taxes and insurance expense decreased $0.4 million, or 3.2%, during the six months ended June 30, 2020 primarily due to real property tax adjustments at SV2 and CH1.
Depreciation and amortization expense increased $10.1 million, or 13.9%, during the six months ended June 30, 2020, compared to the 2019 period, primarily as a result of an increase in depreciation expense from approximately 214,000 NRSF of new data center expansion projects placed into service during the twelve months ended June 30, 2020 with a cost basis of approximately $234.9 million.
Sales and marketing expense increased $0.5 million, or 4.8%, during the six months ended June 30, 2020, compared to the 2019 period, primarily due to an increased headcount, resulting in higher salaries and non-cash compensation.
General and administrative expense increased $0.4 million, or 1.9%, during the six months ended June 30, 2020, compared to the 2019 period, primarily as a result of an increased bad debt expense of $1.0 million related to customers in bankruptcy proceedings and increased customer financial distress related to the COVID-19 outbreak, higher salaries and non-cash compensation. The increase is partially offset by decreased legal expenses compared to the six months ended June 30, 2019.
Rent expense increased by $2.0 million, or 12.8%, during the six months ended June 30, 2020, compared to the 2019 period. The increase was primarily due to additional rent expense incurred from our leasehold interest properties during the six months ended June 30, 2020, related to the completion and placing into service of an additional computer room at LA1 during the six months ended June 30, 2019.