TL;DR
Wool shrinks by felting, and felting cannot be undone. Wash wool and cashmere in cool water -- never hot -- with a wool-safe detergent, never agitate or wring, and dry flat away from heat. Never use chlorine bleach or oxygen bleach on wool, and never put a wool sweater in a hot dryer.
Why does wool shrink so badly?
Wool fibers are covered in microscopic overlapping scales. Heat, moisture, and mechanical agitation together make those scales open and then interlock permanently, pulling the fabric tighter and denser. That is felting, and unlike a stretched-out cotton shirt, it cannot be reversed by any home method.
The three things that felt wool
You need all three at once for serious damage, which is why the standard advice removes each one.
Heat. Hot water, and especially a rapid temperature change between wash and rinse. Keep wash and rinse water the same cool temperature.
Agitation. Rubbing, scrubbing, wringing, and machine tumbling. A wool cycle on a modern washer is designed to rock rather than tumble, which is why it is safe on machine-washable wool and a normal cycle is not. Picking the right washer cycle covers the difference.
Alkalinity. Wool is a protein fiber and it is damaged by high pH. Ordinary laundry detergent is alkaline. This is why wool detergents exist -- they are pH-neutral or mildly acidic.
Never use bleach on wool
Both kinds do damage, for different reasons.
Chlorine bleach dissolves protein fiber outright. It will yellow, weaken, and eventually destroy wool.
Oxygen bleach, which is safe on most cottons, is also unsafe on wool. It attacks the protein structure and causes yellowing and fiber loss, and it is one of the most common ways a well-intentioned stain treatment ruins a sweater.
The same applies to enzyme detergents. Protease enzymes exist to digest protein soils, and wool is protein. Read the detergent label and choose one that says wool-safe or is marked as containing no enzymes and no bleach.
The hand-wash method, step by step
Check the label first. Some wool is treated to be machine washable, and some is strictly hand wash or dry clean. How to read a care label decodes the symbols, and dry clean only versus dry clean recommended covers how much force that label carries.
Fill a clean basin or sink with cool water. Cool means genuinely cool to the touch, in the range of 65 to 85 degrees F. Not warm.
Add a small amount of wool-safe detergent and swirl it in before the garment goes in, so it never sits on the fabric undiluted.
Submerge the sweater and press it gently. Push it under, squeeze the water through it a few times, and then leave it. Ten to twenty minutes of soaking does more than any amount of rubbing. Do not scrub, twist, stretch, or rub the fabric against itself.
Drain and rinse in water of the same temperature. Refill, press the water through, drain, and repeat until no suds appear. A sudden switch from warm wash to cold rinse is a classic felting mistake.
Do not wring. Press the water out with flat hands against the side of the basin, then lift the garment supporting its full weight so it does not stretch. Lay it on a dry towel, roll the towel up like a jelly roll, and press down to transfer the water.
Dry flat, out of the sun and away from heat. Reshape the sweater to its correct dimensions on a dry towel or a mesh drying rack, and let it air dry. Turn it once partway through. Never hang a wet wool sweater -- the weight will stretch the shoulders and body permanently, and folding versus hanging clothes covers why knits should be folded for storage too.
How often should wool actually be washed?
Far less often than cotton. Wool resists odor naturally because of its structure and its moisture-handling, and every wash carries some risk.
For most sweaters, two or three washes per season is plenty. Between wears, air the garment out for a day before putting it away, and spot clean small marks with a damp cloth and a drop of wool detergent rather than washing the whole piece.
Source: The Woolmark Company fiber care guidance
Pilling, moths, and storage
Pilling is normal on wool, especially cashmere, and it is worst in the first few wears as short fibers work loose. Remove pills with a fabric shaver or a sweater comb, working in one direction. Do not pull them off by hand, since that drags fiber out of the knit.
Moths eat protein fiber, and they target soiled wool because body oil and food residue attract them. That is the practical argument for washing wool before storing it for the season -- a clean sweater is a much less attractive target than a worn one.
Store folded, in a breathable container, in a dry place. Cedar and lavender are mild deterrents at best, not treatments. If you already have an active moth infestation in a closet, that is a pest control matter rather than a laundry one, and it needs to be handled as such.
Can a shrunken sweater be saved?
Sometimes, partially, and only if the shrinkage is mild.
Soak the garment in cool water with a capful of hair conditioner for about thirty minutes. The conditioner relaxes the fiber slightly. Then press the water out with a towel, lay the sweater flat, and gently stretch it back toward its original shape, pinning or weighting the edges while it dries.
This can recover some size. It cannot undo true felting, where the knit has become dense and the stitch definition has disappeared. At that point the fiber structure has physically changed and the garment is what it is.
FAQ
Can you wash wool in a washing machine?
Only if the care label permits it, and only on a wool or hand wash cycle with cool water and the lowest spin. A normal or delicate cycle still tumbles too much. When in doubt, hand wash -- machine washing an untreated wool sweater on a regular cycle will felt it in one load.
What detergent is safe for cashmere?
A pH-neutral wool detergent with no enzymes, no bleach, and no brighteners. Ordinary laundry detergent is alkaline and enzymatic, and both properties damage protein fiber. A small amount of gentle liquid designed for delicates is an acceptable substitute.
Why did my wool sweater get smaller and thicker?
That is felting, not ordinary shrinkage. Heat and agitation caused the fiber scales to interlock, permanently compacting the knit. There is no reliable way to reverse it. Mild cases sometimes respond to a conditioner soak and gentle reshaping.
Is oxygen bleach safe on wool?
No. Oxygen bleach is safe on many cottons but attacks protein fibers, causing yellowing and weakening. Neither chlorine nor oxygen bleach belongs anywhere near wool, cashmere, silk, or alpaca.
How do I get a stain out of a wool sweater?
Blot immediately with cool water and a clean cloth, working from the outside of the stain inward, and never rub. For anything beyond a light mark, a professional cleaner is the safer choice, because home stain treatments are generally alkaline or enzymatic and both damage wool.
If the wider house needs the same attention, that is what house cleaning services cover.
Wool care is mostly a matter of restraint: cool water, no scrubbing, no heat, and patience while it dries. If keeping up with the household laundry has become the thing that never gets done, help is available. Every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.