....assuming you have a SIM or something else on top before your sleeping bag?
Remember, all you are doing here is "reflecting" cold air back....not warmth up. Any reflective blanket per se, to work in reflecting body heat, need to be as close to that body as possible.. and insulated below/outermost.
Quality SIMS also have a reflective capability and insulation to a degree, built in.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets