{
 "id": "running-base-layers",
 "name": "Running base layers & tops",
 "description": "The layer against your skin: merino and synthetic thermals, half-zips and light fleece.",
 "department": {
  "id": "running",
  "name": "Running",
  "url": "https://agenticrecs.com/running/"
 },
 "count": 5,
 "msrp_range_usd": [
  69.95,
  200
 ],
 "updated": "2026-10-26",
 "schema": {
  "id": "running-apparel",
  "name": "Running apparel",
  "description": "Clothing and accessories for running. Ranges and scales assume steady running, not standing around.",
  "attributes": [
   {
    "key": "fits",
    "label": "Fits",
    "type": "set",
    "filter": true,
    "compare": true,
    "values": {
     "men": {
      "label": "Men's",
      "definition": "Cut for men; sized in men's sizes."
     },
     "women": {
      "label": "Women's",
      "definition": "Cut for women; sized in women's sizes."
     },
     "unisex": {
      "label": "Unisex",
      "definition": "One cut sold for everyone."
     }
    }
   },
   {
    "key": "warmth",
    "label": "Warmth",
    "type": "enum",
    "ordered": true,
    "filter": true,
    "compare": true,
    "sort": "desc",
    "sort_label": "Warmest first",
    "values": {
     "light": {
      "label": "Light",
      "definition": "Thin single-layer knit or shell. Roughly 40 to 60°F on its own."
     },
     "mid": {
      "label": "Midweight",
      "definition": "Brushed or grid-fleece interior, or midweight merino. Roughly 25 to 45°F."
     },
     "thermal": {
      "label": "Thermal",
      "definition": "Heavy brushed fleece, heavyweight merino, or wind panels over fleece. Roughly 10 to 30°F."
     },
     "insulated": {
      "label": "Insulated",
      "definition": "Synthetic or down insulation, or a windproof membrane over fleece. Below about 20°F."
     }
    },
    "about": true
   },
   {
    "key": "wind",
    "label": "Wind",
    "type": "enum",
    "ordered": true,
    "filter": true,
    "compare": true,
    "values": {
     "none": {
      "label": "None",
      "definition": "No dedicated wind protection."
     },
     "panels": {
      "label": "Wind panels",
      "definition": "Windproof panels in key areas (usually the front)."
     },
     "full": {
      "label": "Windproof",
      "definition": "Windproof fabric throughout."
     }
    }
   },
   {
    "key": "water",
    "label": "Water",
    "type": "enum",
    "ordered": true,
    "filter": true,
    "compare": true,
    "values": {
     "none": {
      "label": "None",
      "definition": "No water treatment."
     },
     "dwr": {
      "label": "Water-resistant (DWR)",
      "definition": "Water-repellent finish or water-resistant fabric. Sheds light rain and snow."
     },
     "waterproof": {
      "label": "Waterproof",
      "definition": "Waterproof membrane with taped seams."
     }
    }
   },
   {
    "key": "visibility",
    "label": "Visibility",
    "type": "enum",
    "ordered": true,
    "filter": true,
    "compare": true,
    "values": {
     "none": {
      "label": "None",
      "definition": "No reflective features."
     },
     "reflective-details": {
      "label": "Reflective details",
      "definition": "Small reflective logos, strips or prints."
     },
     "high-vis": {
      "label": "High visibility",
      "definition": "Fluorescent colorway and/or large reflective areas."
     }
    }
   },
   {
    "key": "temp_range_f",
    "label": "Temperature range",
    "short_label": "Temp range",
    "type": "range",
    "unit": "°F",
    "alt_key": "temp_range_c",
    "alt_unit": "°C",
    "basis_key": "temp_basis",
    "basis": {
     "brand": {
      "label": "The brand's stated range",
      "definition": "The brand states this range."
     },
     "reviewer": {
      "label": "From published reviews",
      "definition": "Taken from the conditions reported in the cited reviews."
     },
     "estimate": {
      "label": "Our estimate from the construction",
      "definition": "Our estimate from the construction, using the warmth scale. Not measured."
     }
    },
    "estimate_value": "estimate",
    "scale": [
     -20,
     60
    ],
    "ticks": [
     -20,
     -10,
     0,
     10,
     20,
     30,
     40,
     50,
     60
    ],
    "visual": "thermometer",
    "compare": true,
    "sort": "asc",
    "sort_label": "Coldest rated first",
    "definition": "The temperatures a product suits as the main piece of its kind, for steady running.",
    "about": true
   }
  ]
 },
 "guides": [
  {
   "guide": "cold-weather-running",
   "chapter": "base-layers",
   "title": "Winter running base layers and long-sleeve tops",
   "url": "https://agenticrecs.com/guides/cold-weather-running/base-layers/",
   "picks": [
    {
     "label": "Best overall",
     "product": "smartwool-classic-all-season-merino-1-4-zip"
    },
    {
     "label": "Warmest",
     "product": "smartwool-classic-thermal-merino-1-4-zip"
    },
    {
     "label": "Best budget",
     "product": "rei-coop-midweight-half-zip-base-layer"
    },
    {
     "label": "Best midlayer",
     "product": "janji-rover-merino-hoodie"
    }
   ]
  }
 ],
 "products": [
  {
   "id": "smartwool-classic-all-season-merino-1-4-zip",
   "name": "Smartwool Classic All-Season Merino Base Layer 1/4 Zip",
   "brand": "Smartwool",
   "subtype": "lightweight merino quarter-zip",
   "msrp_usd": 105,
   "price_tier": "Mid-range",
   "attributes": {
    "fits": [
     "men",
     "women"
    ],
    "warmth": "light",
    "wind": "none",
    "water": "none",
    "visibility": "none",
    "temp_range_f": [
     35,
     55
    ],
    "temp_range_c": [
     2,
     13
    ],
    "temp_basis": "estimate"
   },
   "awards": [
    "Best overall"
   ],
   "summary": "The most consistently recommended merino base layer for winter running right now: thin enough to wear alone when it's merely chilly and easy to vent through the zip, but it needs a shell or midlayer once it gets truly cold.",
   "last_verified": "2026-10-26",
   "url": "https://agenticrecs.com/products/smartwool-classic-all-season-merino-1-4-zip/",
   "json_url": "https://agenticrecs.com/products/smartwool-classic-all-season-merino-1-4-zip/index.json"
  },
  {
   "id": "smartwool-classic-thermal-merino-1-4-zip",
   "name": "Smartwool Classic Thermal Merino Base Layer 1/4 Zip",
   "brand": "Smartwool",
   "subtype": "midweight merino quarter-zip",
   "msrp_usd": 130,
   "price_tier": "Mid-range",
   "attributes": {
    "fits": [
     "men"
    ],
    "warmth": "mid",
    "wind": "none",
    "water": "none",
    "visibility": "none",
    "temp_range_f": [
     20,
     40
    ],
    "temp_range_c": [
     -7,
     4
    ],
    "temp_basis": "estimate"
   },
   "awards": [
    "Warmest"
   ],
   "summary": "The warmer sibling of the All-Season: a 250 g/m² all-merino quarter-zip that reviewers rank among the best base layers for serious cold, though it costs more and needs gentler care than synthetics.",
   "last_verified": "2026-10-26",
   "url": "https://agenticrecs.com/products/smartwool-classic-thermal-merino-1-4-zip/",
   "json_url": "https://agenticrecs.com/products/smartwool-classic-thermal-merino-1-4-zip/index.json"
  },
  {
   "id": "rei-coop-midweight-half-zip-base-layer",
   "name": "REI Co-op Midweight Half-Zip Base Layer Top",
   "brand": "REI Co-op",
   "subtype": "budget synthetic thermal half-zip",
   "msrp_usd": 69.95,
   "price_tier": "Budget",
   "attributes": {
    "fits": [
     "men"
    ],
    "warmth": "mid",
    "wind": "none",
    "water": "none",
    "visibility": "none",
    "temp_range_f": [
     25,
     45
    ],
    "temp_range_c": [
     -4,
     7
    ],
    "temp_basis": "estimate"
   },
   "awards": [
    "Best budget"
   ],
   "summary": "The value pick: a soft, fast-drying synthetic half-zip that OutdoorGearLab rates a Best Buy, giving up some warmth compared with wool in exchange for a much lower price.",
   "last_verified": "2026-10-26",
   "url": "https://agenticrecs.com/products/rei-coop-midweight-half-zip-base-layer/",
   "json_url": "https://agenticrecs.com/products/rei-coop-midweight-half-zip-base-layer/index.json"
  },
  {
   "id": "janji-rover-merino-hoodie",
   "name": "Janji Rover Merino Hoodie",
   "brand": "Janji",
   "subtype": "merino-blend running hoodie (midlayer)",
   "msrp_usd": 136,
   "price_tier": "Mid-range",
   "attributes": {
    "fits": [
     "men",
     "women"
    ],
    "warmth": "mid",
    "wind": "none",
    "water": "none",
    "visibility": "none",
    "temp_range_f": [
     25,
     60
    ],
    "temp_range_c": [
     -4,
     16
    ],
    "temp_basis": "brand"
   },
   "awards": [
    "Best midlayer"
   ],
   "summary": "A warm, stretchy merino-blend hoodie that works as a standalone top on cool days or a midlayer when it's cold; the snug hood is the highlight, but testers found it picks up odor fast when worn next to skin.",
   "last_verified": "2026-10-26",
   "url": "https://agenticrecs.com/products/janji-rover-merino-hoodie/",
   "json_url": "https://agenticrecs.com/products/janji-rover-merino-hoodie/index.json"
  },
  {
   "id": "tracksmith-franklin-fleece",
   "name": "Tracksmith Franklin Fleece",
   "brand": "Tracksmith",
   "subtype": "running fleece midlayer",
   "msrp_usd": 200,
   "price_tier": "Premium",
   "attributes": {
    "fits": [
     "men",
     "women"
    ],
    "warmth": "mid",
    "wind": "none",
    "water": "none",
    "visibility": "none",
    "temp_range_f": [
     25,
     40
    ],
    "temp_range_c": [
     -4,
     4
    ],
    "temp_basis": "reviewer"
   },
   "awards": [],
   "summary": "A premium, very breathable running fleece that Treeline chose as its best fleece: cozier and airier than a wind shell in dry cold, but a narrow-use piece that needs a shell over it when it's windy or wet.",
   "last_verified": "2026-10-26",
   "url": "https://agenticrecs.com/products/tracksmith-franklin-fleece/",
   "json_url": "https://agenticrecs.com/products/tracksmith-franklin-fleece/index.json"
  }
 ],
 "url": "https://agenticrecs.com/running/base-layers/",
 "markdown_url": "https://agenticrecs.com/running/base-layers/index.md",
 "json_url": "https://agenticrecs.com/running/base-layers/index.json",
 "range_temp_range_f": [
  20,
  60
 ]
}