Insulating my hot water pipes under the house...

This looked like the best place to ask this question.. I have ran a Watts recirc pump and ran a return line back to the inlet of the WH.. I want to insulate these lines as they are just open under the house strapped to the bottom of the floor joists.. The hot, cold, and hot return are all PEX and within a few inches of each other.. Pipe insulation seems to have such a poor R value because it is small and I guess they want to keep the cost down.. Thinking I could get some R-13 fiberglass and lightly enclose the pipes in a U shape of the insulation.. Thus having insulation between the floor joists and the insulation I'm installing covering all sides of the pipes..

Make sense?
Dumb idea?
Thanks!

I think the standard foam pipe insulation is the best way to go. It might be argued that your idea of fiberglass wrap might indeed have a slightly better R factor, but I really don't think the actual $$ saving would be enough to really determine. For example, my recirculation and hot water supply pipes are insulated with the foam strips and I run my pump 24/7 and have for several years. If this increases my water heating bill, it isn't significant. I could put the pump on a timer, but there are times I want hot water late at night. I just love having it virtually instantly when I turn the hot water faucet on.

I was thinking maybe taking out two birds with one stone and have a little warmth trapped for the cold water too to prevent freezing.. And the work would be a little more lazy for me as I could just staple down the facing.

Thank you for your reply...

P.S. I love the instant hot water too.. My master is at the total other end of the house than the WH.. So I'm in love with it!

If there's any air movement under there, fiberglass won't do all that much as it will act essentially like a big air filter. While pex itself probably wouldn't be harmed by freezing, any fittings would be, and while they do insulate better than copper, you will still lose energy from the pex pipes, so any insulation you apply will help, overall. The insulation available at the big box stores isn't the best stuff available; for that, you'd need to go to a supply house. Some of the Granger stores will sell retail, and they carry it.

The better foam pipe insulation has 3/4" wall thickness rather than the 3/8" stuff sold at blue or orange box stores. It's available through plumbing supply distributors or online. Grainger sells some too. ANY of it will be more worthwhile than low density fiberglass batting of any R value, but once have the pipe zipped up tight in the closed cell foam there is no harm in putting it between some insulation and the subfloor.

In Richmond's climate zone it's better to have no vapor retardent facer, and when it does have a facer it's better to place in on the conditioned-space (subfloor) side, not the crawlspace side. Fiberglass needs to have an AIR barrier on both sides in order to limit convective loss of R-value, which is acheiveable in a floor joist situation with a housewrap material (eg Tyvek). If this is in a fully or semi-conditioned basement, fuggedabouit, but if it's a crawlspace, there is some value to insulating the floor. (In many instances it's better to close the crawlspace vents and insulated the walls, not the joists, but that's a whole 'nuther topic.)

Just thought I would follow up...

Went ahead and ordered some 3/4" walled closed cell last night from drillspot.com...
Better pricing than Grainger direct and free shipping..

They had 1" walled as well, but I couldn't stomach the $$$ premium for it...

Thanks for your insight!