Complex sentence: Generally most materials will not remained sealed over the long term and this system is very limited, but ordinary residential construction often treats walls as sealed-surface systems relying on the siding and an underlayment layer sometimes called housewrap.
Split into simpler sentences: Generally most materials will not remained sealed over the long term and this system is very limited. Ordinary residential construction often treats walls as surface-sealed systems relying on the siding and an underlayment layer sometimes called housewrap.