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I already insulated the exterior outlets and wall switches, but Mom went to plug in a space heater today, and felt a cold draft coming from an INTERIOR outlet, while the furnace was running. Huh?!?

I insulated it, but there's still a little coolness coming out. I plan to get some of those outlet plugs you use for childproofing homes, but still, I wonder why air is coming from an interior wall at all.

Can someone explain to me how this could be happening?

WildIrish...funny you should mention this. When I remodeled the bathroom I noticed a lot of cold air blowing up around the bathroom floor register as I was installing it. Took me a while to figger this out but here's what I found. My old home is layed out with the master bedroom at one end..the next room is the bathroom. That wall that seperates the master bedroom from the bathroom has been a source of problems from day 1. The supply plumbing for the washing machine and toilet goes thru this wall. Then it drops down into the floor-joists and runs alongside the heat-duct to feed the kitchen sink. All this plumbing is concealed inside this wall..and this is where it always froze up. Temp. dropped below freezing..no toilet..no washing machine..no kitchen sink! What I found out was the underbelly of the home had been gnawed thru right on that wall. Go figger? Don't know how critters know just where to gnaw thru to make life miserable..but they do. Check the underbelly in the area below that wall. Hope this helps...Audie..the Oldfart..

Hi, Audie, and thanks for the reply!

Our MH is on a foundation, and nothing's been gnawed through. The problem with the cold air coming from the outlets is all over the house, not just in one place. Weird...

You say that you've insulated the outlets/switches already. But can you feel a draft when the furnace is not running? And you should see if the air is actually coming out of the outlet, and not being pushed into. One test is to strike a match and hold it to the outlet and see if the flame is drawn towards or away from it. Since you notice it with the furnace running, you should do it then.

Air coming in through outlets is a common symptom of a supply duct leak. What happens is as the furnace is running and creating positive pressure in the trunklines, it's also creating negative pressure inside the house because the trunklines have gaps/holes that allow the pressurized air to escape. When the air escapes, it gets drawn in (through the negative side of the [closed] system) through holes in the walls (outlets) and from door/window seals. The only solution is to seal up the leaks in the trunkline, or add an outside air intake to the furnace to essentially pressurize the house. Which is an intentional leak, but at least the air is conditioned or heated.

Also remember that outdoor air is good for the inside environment, but more at a controlled rate.

http://www.epa.gov/iaq/ia-intro.html