Animal urine smells are worse when it gets hot. Sounds like your cat liked to sleep in the windows. Other threads on here have discussed how to neutralize the odor. Don't paint until you put the neutralizer on.

I could also be an outside animal. When I lived in the city we had a mysterious urine smell coming from the corner of our living room by our patio door. We were blaming our cats (we had 2 at the time), until one night hubby and I were sitting there and actually saw the offending stray cat come right up and spray our patio door. Hubby said he was marking "his" territory and harassing our cats. We eventually scared him off enough that he must have gone off to harass some other poor house cat somewhere, but it was months of smelling it before we figured out what it really was.

I was going to spray fox urine (like around my garden in the country) to scare him away, but living in the city with hot summers and an enclosed courtyard I shared with other's, it didn't seem like a neighborly thing to do. If it turns out it is an outside animal, some sort of animal repellent may work for you.

This will sound crazy, but do you by chance have any boxwood bushes outside the house where you're smelling the cat urine?

We just bought an old house last year and were completely dismayed when it appeared that every cat in the neighborhood was using our front garden as a litter box. It was always worse when it was warmer and we had the windows open - to the point where I feared opening them.

Come to find out, it was the darned boxwoods. They smell like cat urine when the sun warms them. We had the bushes taken out and have never smelled a whiff of that awful smell since. Apparently some folks are much more sensitive to the odor than others. Google 'boxwood' and 'cat pee' to learn more. Good luck!

This post was edited by circuspeanut on Wed, Jul 17, 13 at 21:26

Good response circuspeanut.......... and I was just getting on to suggest something similar (gardener by profession) and have worked with plantings of evergreens with distinct cat urine aromas. Warmth and humidity definitely increase the potency of any smell. Even benches of petunias in a greenhouse can smell like cat urine to some people.

There are lights that can show urine or bodily fluids in the dark. Maybe you can rent or purchase one of these and then at night check out your house top to bottom, inside and out.

I copied this from a wikihow.

uy a decent-sized UV or "fluorescent black" light. Look for a long bulb (preferably longer than 12 inches) so you can cover more ground at once. Cheap bulbs with the housing included can be found at most hardware stores. Although pet stores sell the lights as well, they're usually smaller and more expensive. They can also be purchased at a reasonable cost online, depending on how much time you have to wait around living with the odor.
2 Search at night or in complete darkness. Cat urine can be difficult to see, especially when old, so maximize your search efforts by taking advantage of darkness. Either wait for night, or make the room as dark as possible.
3 Use a sweeping motion and gradually move farther away from the generally smelly location. The cat urine that you're looking for should show up as a yellow/greenish color. You may be surprised to find a stain farther away than you thought. Don't just check the floor; cats are inventive! Try the following areas:
Bookshelves
Furniture
Cloth decorations
Inside vents
Objects that appear to have "holes," such as inside portable heaters
Clothing that your cat may be able to access
Other small areas your cat could squeeze into
Anywhere else you can think of - if you can see it, it could be your source.

Thanks for all your ideas. I do have the nature's miracle neutralizer which I tried to pour in at the baseboard level so it would go where the cat has peed (it's not my cat... a previous owner's cat I'm thinking... I don't have cats). I also bought a black light, but no luck with that so far. I think it's gone under the floorboards. I'm still baffled how it has gone down to the rooms on the main floor after putting the kilz on. I'm also confused why it is only when the window is open. There are no boxwood plants, the smell is definitely coming from inside, but only when the window is open.... very puzzling.

Are there any sinks or drains in the vicinity? An open window may cause a pressure drop or venturi and suck gas from the sewer system.