Jaket ɗin ƙasa shine jaket wanda aka keɓe tare da taushi da dumi a ƙarƙashin gashin tsuntsu daga duck ko geese. Down wani babban insulator ne mai ban sha’awa kamar yadda soro (ko fulffiness) na ƙasa ke haifar da dubban ƙananan aljihunan iska waɗanda ke kama iska mai zafi kuma suna riƙe da zafi, don haka yana taimakawa mai sawa dumi sosai a cikin yanayin sanyi.