Yes, the high efficiency furnace PVC vent / exhaust pipes can be insulated. There are certain installations and location conditions (climate related) in which they have to be insulated (always check with the appliance manufacturer for details. Insulating the PVC air intake pipe is not required under any conditions because its only purpose is to provide combustion air for the appliance’s burning process.
You can find the following paragraph in the PVC vent pipe installation manual of various high efficiency / condensing type furnace manufacturers:
When the vent pipe is exposed to temperatures below freezing, i.e., when it passes through unheated spaces, etc., the pipe must be insulated with 1/2 inch (22.7 mm) thick Armaflex-type insulation or equal. If the space is heated sufficiently to prevent freezing, then the insulation would not be required.
Some other examples of areas where the PVC vent pipe insulation might be required (cold and extreme cold climates of course) are unconditioned attics / garages / crawlspaces, extended runs on the outside walls of the house, or sections above the roof.
The reason behind the insulation requirement is to prevent freezing of the condensate flowing inside the PVC vent pipe towards the furnace and its drainage system. Therefore, insulating vent pipes installed / running inside the conditioned (heated) area does not increase the efficiency of your appliance. It is actually better to have the vent pipe exposed (e.g. in your basement) so it radiates some heat into the surrounding space.
Armaflex R value (1/2” thick foam) is slightly above 2 and on a longer runs it gets quite expensive to insulate a large diameter vent pipe, therefore, you may want to consider using some fiberglass insulation instead.
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