1. Introduction
2. Observations
3. Results
3.1. Source sample properties
3.2. Emission morphology
3.3. Size measurements
4. Analysis
4.1. The model
4.2. The testcase: IRAS4B
4.3. The other sources
5. Discussion
5.1. Uncertainties in our modeling
5.2. The CO binding energy
5.3. The CO freeze-out temperature
5.4. The $N_{2}$ binding energy
5.5. The $CH_{3}OH$ binding energy
5.6. CO abundances
5.7. Comparison with Jørgensen et al. (2015)
6. Conclusions