Bit late to this thread but yeah, had this exact nightmare with my van build a couple of years back so I reckon the same principles apply to garden offices too.
The culprit for me was relying on just a gland nut without any secondary sealing. Water finds its way in eventually, especially with our lovely British weather doing its freeze-thaw thing all winter.
What actually sorted it:
- Sikaflex 512 around the outside of every entry point — proper marine-grade stuff, doesn't harden and crack like silicone tends to after a year or two
- IP68 cable glands rather than the cheap IP65 ones (massive difference in practice)
- Running cables in a slight downward curve outside before they enter — stops water tracking along the cable itself into the penetration
For a garden office specifically I'd also be thinking about whether your entry points are on a sheltered wall or exposed to the prevailing wind and rain. Worth relocating them if not — so much easier than fighting damp long term.
Anyone tried the Spartan gland plates for multiple cables through one entry? Seen them recommended elsewhere but not used them myself. Wondering if they're worth it for a cleaner install or just another thing to potentially leak.
What's the construction of the wall itself — SIP panels or timber frame? That might change what sealing approach works best.
