Dark stain on insulation

[emmaamme]

Any ideas what this dark staining could be on the top of my loft insulation? It's just on a small section near the eaves which are ventilated. It doesn't feel damp and no signs of a leak around it.

We do get cluster flies at that side of the house which tend to try to hibernate in the loft (I zap them with fly spray).

The bungalow is 7 yrs old.

I have seen similar on a recent roll of insulation that I bought although just a small amount so wondering if it could have been like that from installation 7 yrs ago?

 

[Wayners]

Mice.

Will smell if it is from mice

 

[emmaamme]

Really? Is it from their urine and should I remove that bit?

 

[Wayners]

I've seen similar and it stunk when i disturbed it.
I dumped what I found.
Go have a look

 

[emmaamme]

Just been back up and it doesn't smell. There's a couple of other small similar bits dotted around up there too. I've removed the bit in the photo. Couldn't see any droppings around. Might stick some traps up there and see I catch any.