Therm-a-Rest Boost 650 20 vs Mountain Hardwear Bishop Pass 15
In short: The Mountain Hardwear Bishop Pass 15 costs $5 less; the Mountain Hardwear Bishop Pass 15 is rated 6 °F colder on the figure each maker publishes; the Mountain Hardwear Bishop Pass 15 saves 5.6 oz.
| Spec | Therm-a-Rest Boost 650 20 | Mountain Hardwear Bishop Pass 15 |
|---|---|---|
| Approx. price | ~$340 | ~$335 |
| ISO comfort | 32 °F | 26 °F |
| ISO limit | 20 °F | not published |
| Rating on the label | 20 °F | 15 °F |
| Honesty gap | 12 °F | 11 °F |
| Rating standard | EN 13537 | ISO 23537 |
| Weight | 2 lb 11 oz | 2 lb 5.4 oz |
| Warmth per ounce | 0.65 °F/oz | 0.91 °F/oz |
| Fill | 650 fill power down | 650 fill power down |
| Fill weight | 23 oz | not published |
| Loft budget | 14.9k in³ | — |
| Shape | mummy | mummy |
| Shoulder girth | 64 in | not published |
| Zipper | full | full |
| Shell fabric | 20D recycled nylon ripstop | 20D ripstop nylon |
| Hydrophobic down | Yes | No |
| Packed size | 12.5 × 7.5 in | not published |
Shaded cells mark the row where one has a measurable advantage. Weight, thickness, girth and shape are left unshaded on purpose: heavier is sometimes tougher, wider is sometimes warmer, and calling a winner on those would be an opinion dressed as a measurement.
What each one does well, and costs you
Therm-a-Rest Boost 650 20
- Six sizes across three lengths and two widths — a 76-inch shoulder girth in Regular Wide is a full foot more than a standard mummy.
- Publishes EN comfort (32 °F), limit (20 °F) and extreme (-9 °F) figures, and the per-size girth numbers alongside.
- 650-fill duck down is RDS-certified, PFAS-free and hydrophobic-treated.
- $340 for the Regular is mid-market for a down bag with a real lab rating behind it.
- 43 ounces in the Regular and 61 in the Long Wide — that is three and three-quarter pounds for the biggest size.
- Packs to 12.5 × 7.5 inches, which is a lot of a weekend pack's volume.
- 650-fill down buys loft by bulk, so the Wide sizes get heavy fast.
- As with every 20-degree bag here, the comfort rating is 32 °F; treat 20 as the survival end of the scale.
Mountain Hardwear Bishop Pass 15
- ISO comfort of 26 °F published, on a bag that costs $335.
- Five and a half inches of measured loft — as much as bags costing twice as much.
- The Performance Plus cut adds room at shoulder, hip and knee without going to a semi-rectangular shape.
- RDS-certified 650-fill down and a 20-denier shell that is tougher than the ultralight bags'.
- 37.4 ounces, and 650-fill down means it packs large for its warmth.
- The down is not hydrophobic-treated.
- Mountain Hardwear publishes the comfort figure but not the lower limit, so half the ISO picture is missing.
- Only the Regular's figures were verifiable from published sources; the size run is not tabulated here.