Basements, Attics and Crawl Spaces - Help! Basement Odor

We live in lower Michigan and now that the weather has changed, we have a strange odor in our basement (and throughout the house as well).

The problem stems from the slab for our addition. In the basement, you can see the floor joists where the addition connects to the house. Addition is about ten years old. Previous owner stuffed insulation where basement ceiling and addition's floor joists meet. The insulation is badly deteriorated.

This is the third year I've noticed the odor which occurs only at this time of year. It seems to happen when the temps drop enough that you don't need a/c but don't need heat either. In the past, the smell has gone away after a few days. This year, it is persisting. Has been a week or so.

Anyone with simliar problem? Do you think it will help to replace insulation and then seal off with visqueen, etc.? I seriously doubt this is a problem with mold/mildew, since I only smell this odor at this time of year. Could it be something to do with the ground beneath the slab?

The smell is very annoying since I'm a very neat housekeeper with a strong sense of smell! Can't stand this! Any tips?

Thanks!

You don't describe the nature of the odor. E.g., rotten eggs (gas leak), methane or sulfur (sewer gas), damp (mold or fungi), or...?

Tha fact that the odor is strongest when the recirc fan is not operating makes sense. With the HVAC fan off, you will have an easier time locating the source of the odor. Find the source if you can.

And why is the insulation badly deteriorated? Insulation should last for decades, unless there is moisture infiltration. With moisture, you get mold, rot, insects, vermin, and other odor sources. If the source is the room addition, I would look at flashing between the addition and the main house.