And propane has close enough properties to r21 that it can be used as a drop in replacement. Of course, HVAC guys flip their lid at the thought of it. Because t

And propane has close enough properties to r21 that it can be used as a drop in replacement. Of course, HVAC guys flip their lid at the thought of it. Because they want to get that sweet markup on the extremely expensive, discontinued r21 or a pricey alternative.