You could start by leaving the access panel open and see if that solves the problem.

It it works you could replace the access panel with a decorative metal panel with some holes to allow air movement.

You could also try to get some insulation between the pipes and the outside wall,and stop any infiltration by filling any gaps to the exterior.

You could install a recirculating pump under the sink in that upstairs master bath. It will require an electrical receptacle in the vicinity.

Watts and Laing both have a good product. They circulate the water from the hot supply into the cold supply line. This will keep both lines from freezing, and save you water at the same time. An added bonus will be instant hot water at all the points of use between the water heater and the master bath.

This can be bought and installed for a few hundred $

No additional plumbing work is necessary.

Thank you for the suggestions. I have considered blowing insulation in from the access panel as far around and in back of the whirlpool tub as I can get. The access panel already is a metal grate -- made to look like a return for the HVAC. I like the idea of the recirc pump, but I am unclear how that works. The part of the line that freezes is probably 15' upstream of the faucets.

I have also considered pulling out the tub to see what I can see. Have either of you done that? Do I need 3 men and a small boy to lift it?

Here's one of the retro-fit recirc pumps... http://lainginc.itt.com/pages/autocirc.htm

You can download the spec sheet, brochure, and instruction manual at that site to see how it installs and how it works.

I've had one for mnay years and it has proved reliable and does what they say it does.

Don't pull the tub unless you're gutting the bathroom.

The recirculating pump creates a continuous loop with the hot and cold supply lines. The pipes are already connected at the hot water heater. The kit installed under the sink completes the loop. When the pump runs, water moves throughout the entire system.

All, thank you very much for the information. Excellent stuff. I read up on the recirc pumps. It looks like the biggest problem there would be getting power in the base cabinet and having a solid mounting surface. However, DW is not thrilled about the idea of luke warm water coming out of the cold water tap, even for a few seconds, so I am probably going to try to blow in insulation. Any code against doing that when it will get up and around the workings of the whirlpool tub?

Why not get some heat tape and wrap the pipes? Use the outlet/power for the tub to power the heat tape. Get a thermostatic outlet that turns on the heat tape only when the temps get cold.

The access panel for the whirlpool tub is on the opposite end of where the plumbing runs, so I have no access to the lines.

If there is a light above the cabinet it's usually an EZ job for an electrician to drop power to the inside of the cabinet.

The pump mount is quite simple and will mount to sheet rock or the back or side of the inside of the cabinet

justalurker -- thanks. I have an outlet I can drop from for power. I read that these pumps can be noisy if not firmly mounted, so I would need to at worst go through the cab and into the drywall and at best hit a stud. It's making sure the holes in the cab are in the right spot that causes me trepidation, especially for the outlet, since I'd have to cut a big hole.

"I read that these pumps can be noisy if not firmly mounted..."

Recirc pumps in general or the Laing Autocirc?

My Laing Autocirc is silent. With the cabinet open and my head 12" from the pump I have to strain to hear it. My pump is mounted with two wood screws in common (expanding) plastic anchors to the back of the kitchen cabinet on what I can only describe as 1/4" press-board with woodgrain contact paper covering.

AFAIK the pump sold now is unchanged from mine.

If dropping the power to the cabinet gives you trepidation then a professional electrician is the answer. As Clint Eastwood said "a man (woman) has got to know his (her) limitations" and it is to your benefit that you do. Electricians drop outlets all day long.

No one should rule out a worthwhile project because they lack the skills or the tools to do the job. That's what there are professionals.

If you don't have good access blowing insulation into the wrong spots might make the problem worse!