Crawl-space and attic work eats pants. The fabric meets a roofing nail, a screw point, an insulation staple, or just the raw edge of a 2x4. The knee meets a joist for the eighth time that hour. By the end of the week, the pant has a hole in it that wasn't there Monday.
What you want is duck canvas, double-knee, and a gusseted crotch. Duck canvas — the heavy, tightly-woven cotton Carhartt built its name on — resists abrasion better than denim. Double-knee adds a second layer of fabric where the wear actually happens. The gusseted crotch is the diamond-shaped panel sewn into the inseam that lets the pant move with you instead of ripping when you squat.
Stretch matters more than people give it credit for. The Carhartt FLEX line, the Dickies FLEX line, and a couple of Wolverine pants build mechanical stretch into the canvas — the fibers themselves don't stretch, but the weave allows give. For crawl work where you're constantly squatting, kneeling, and contorting, that stretch saves the seams.
Avoid pure synthetic blends for attic work. Polyester pants are great for the office trades but melt against hot pipes, which attics have, and tear against splinters in a way canvas doesn't.
Knee pad pockets are nice if you use insertable pads. The pocket sits inside the knee panel and holds a foam or gel pad in place. If you don't use them, the pocket adds weight for nothing — most guys skip the pocketed version.
Color is mostly preference, but tan or brown shows insulation fibers worse than black does. Black pants come out of an attic looking like you rolled in cotton candy. Brown forgives.
We carry the 105475 Carhartt double-knee in regular and FLEX, the Dickies FLEX double-knee at a lower price point, and a Wolverine option in between. If you crawl two days a week, the FLEX is worth the upcharge. If you crawl twice a year, save the money.