That building is a "structure." Which is a technical term used in vanilla minecraft. The ocean monument, witch hut, and nether fortress are all structures. Special spawning rules apply to these area's.
The structures are defined somewhere in the saves data. There are tools to edit them, as some CTM map makers have done so, but I don't know how, and I don't know if it works on modded stuff. (it is possible Roots uses custom methods and doesn't hook into the vanilla code for structures)
EDIT: It seems the above information was incorrect, read post below.
From my experience with these Roots buildings, Mobs don't spawn in daylight, but torchlight does nothing. I haven't tested modded items that prevent spawning such as the the powered lamp from Immersive Engineering, or the Interdiction torch from Reliquary, or the Giant torch from Torch Master. If you do test them (just cheat them in to test) please let us know your findings.
If all you want to do is loot the chest in peace, then just do so in the day, and make sure enough daylight enters the building.
If all else fails; You could just fill the structure with blocks to stop the spawning.
