The Flat Head Long Sleeve Indigo Knit Heavy Thermal

This Heavy Indigo Thermal from The Flat Head is built using a truly exceptional 23oz fabric, far heavier than conventional thermal knits. The yarns are rope-dyed with natural indigo at a historic Japanese dye house with over 130 years of expertise, leaving a white core that gradually reveals itself through wear and washing, creating dramatic, high-contrast aging unique to indigo garments.

Unlike standard thermal fabrics, this piece is knitted on dedicated machines capable of handling extremely thick yarns, resulting in a low-gauge, knit-like structure that feels substantial yet flexible. The texture is bold and three-dimensional, offering excellent stretch, moisture absorption, and breathability—making it versatile enough to wear as a spring knit or a cold-weather base layer.

Indigo-dyed ribbing is used throughout, ensuring uniform fading across the garment rather than isolated color loss. The neckline is finished with The Flat Head’s signature three-needle, one-off stitch construction, using thick cotton thread for long-term durability and a distinctive vintage look.

This is a statement piece rooted in traditional Japanese craftsmanship, designed to age beautifully and last for years.

Fit Notes: All Flat Head tees run smaller than your regular American size, and this ultra-heavy model fits even smaller than standard Flat Head tees. If you usually wear a size 42 in Flat Head tees, you'll need to size up to a 44 in this one. If you're not familiar with Flat Head or other Japanese brands, a size 44 in this model is between a US Medium and Large. In any case, as always, we recommend checking our size chart to ensure the perfect fit.