Is chambray just lightweight denim?

No. Weight is not the difference; structure is. Chambray is a plain weave and denim is a twill. A true lightweight denim still shows a diagonal wale and a pale back, while chambray is flat and identical on both faces.

Why does denim fade but chambray barely does?

Denim's indigo warp floats over three weft picks, so the dye sits exposed on long float crowns that abrade with wear, revealing the white core of ring-dyed yarn. Chambray binds every thread at every intersection, so there are no long floats to wear down.

Which is cooler to wear?

Chambray. The plain weave is lighter and more open, which is why chambray reads as a summer cloth while denim holds heat.

Side by side

Chambray vs Denim

Chambray and denim are woven from the same yarns, a dyed indigo warp and a white weft, but in different structures. Chambray is a plain weave, one over and one under, so both yarns show equally and the cloth is light, flat, and the same on both faces. Denim is a 3/1 warp-faced twill, so the indigo floats dominate the face, the white shows on the back, and the cloth is heavier and tougher.

Chambray

No. 022

plain weave · first documented c. 1500s

Denim

No. 023

3/1 warp-faced twill · first documented 1600s

The differences

Aspect	Chambray	Denim
Weave	Plain weave: each weft passes over one warp end and under the next.	3/1 warp-faced twill: the indigo warp floats over three picks at a time.
Faces	Identical on both sides, with an even, frosted color.	Blue face, pale underside, with a visible diagonal wale.
Weight and hand	Light, smooth, breathable. A shirting cloth.	Heavy, dense, stiff at first. A trouser and jacket cloth.
Fading	Fades evenly and subtly, since both yarns are bound at every intersection.	Fades with high contrast: abrasion wears indigo off the float crowns and exposes the white core.
Best for	Warm-weather shirts, dresses, linings.	Jeans, jackets, workwear, anything that must outlast abrasion.

Which to choose

If you want the look of denim at shirt weight and softness, chambray is the answer; that is what it is for. If the garment has to survive wear and show character as it ages, the twill floats of denim are the entire point.

Common questions

Is chambray just lightweight denim?: No. Weight is not the difference; structure is. Chambray is a plain weave and denim is a twill. A true lightweight denim still shows a diagonal wale and a pale back, while chambray is flat and identical on both faces.
Why does denim fade but chambray barely does?: Denim's indigo warp floats over three weft picks, so the dye sits exposed on long float crowns that abrade with wear, revealing the white core of ring-dyed yarn. Chambray binds every thread at every intersection, so there are no long floats to wear down.
Which is cooler to wear?: Chambray. The plain weave is lighter and more open, which is why chambray reads as a summer cloth while denim holds heat.

Full entry: Chambray Full entry: Denim

Sources & References

1.Cambric and chambray, Wikipedia
2.chambray, Wiktionary
3.Denim, Wikipedia
4.Jeans, Encyclopaedia Britannica