Horse chestnuts are inedible, which may support the idea that they contain some chemicals noxious to spiders. Some have suggested you need to open the chestnut up or poke holes in it to take effect. I have bags of chestnuts for 4 dollars each and they work great to keep spiders away in your house. Just put a hole in each one and place them randomly around and it keeps the spiders away. I have done this for years and rarely see a spider in my house now.