Many of you have heard the story about the thermal shortcomings of Hardy (sic) acker, a cement-based product sold as an alternative backer for bathroom ceramic tile, etc. A similar product is used as house siding. It looks much like the proper Healey insulating panels. I had rather thoroughly insulated the underside of my '60 BN7 with the 1/4"-thick Hardy product. Finally convinced by several forum posters who said the maker of that product disavowed, I followed up on another poster's recommendation to use Duraboard 500ES, a product sold (but not made) by Thermal Products Company, Inc., Norcross, GA. (See their Web page.) Since sheets of 1/4"-thick Duraboard are expensive to ship, I paused on an Atlanta trip and visited that firm, purchasing two 25x13x38.6" sheets for $55.95. I have since re-insulated the underside of my car with Duraboard, completely covering the area above the muffler, etc. In a couple places, I laminated the Duraboard between the flooring and Hardy pieces since I didn't think it would do any harm. In addition to the obvious locations--my car was purchased with zero underside insulation--I covered a number of areas around the firewall. Not satisfied, I even covered the insulated backer plate between the carbs and manifolds, etc. This product cuts easily with a shop knife, and, unlike the Hardy product, could be damaged by impact, etc. Might add that the interior of this car was already insulated with a Dynimat (sic) type product plus felt insulation. I have now driven the car in hot Florida weather and can say that interior heat is greatly diminished, something I couldn't say before I installed the Duraboard. I have no connection with Thermal Products, which offers a rather extensive line of insulating products.