Screened-in porches provide protection from the rain during the warmer months and prevent larger bugs and insects from invading the living space. Greenhouses are more typically freestanding and are often intended more for plants than for living space and are therefore unlikely to require the quality of materials one ought to use in the construction of a sunroom that is attached to the home
Screened porches and patios typically fall into Category I sunrooms (click here for building codes) and, while less expensive to install, may not increase the value of your home as much as a true four-season sunroom.
Our things to consider page provides information that may be helpful to you as choose a sunroom manufacturer.
SUNROOMS New England