How can a duct tape block waves?

All matter attenuates (blocks to some degree) some frequencies of the electromagnetic spectrum. Many duct tape brands have a very thin layer of aluminium incorporated in the backing, that can attenuate some frequencies slightly (Water would attenuate many frequencies even more). read more

Duct tape, packing tape, or masking tape to hang up the sheets. RF / MW (radio frequency / microwave) signal detector, optional but highly recommended. Two that work well are the Cornet RF Meters on eBay and 3-Axis EMF Meter (orange ball) on eBay. Detectors are important. read more