A spring insulator in the form of a specified flexible polyethylene plastic foam sheet is provided for wire spring cushion structures such as are employed in mattresses for preventing the soft cushioning material from dimpling down into the springs and for preventing the objectionable feel of the wire springs from penetrating through the cushioning material. The flexible polyethylene foam sheet spring insulator may be used alone or in combination with other materials such as plastic netting. The polyethylene foam sheet may be provided with means for dampening the sound which may be generated by contact between insulators and wire spring cushioning.