Husband wanted a sun porch. He had a pre-fab two wall room installed; third wall is an inside wall, fourth is taken up by oak French doors. Long story short I blocked off the vent he had hooked up because any heat caused huge condensation problems. There is nowhere to put more insulation and 3/4 of the porch walls are (supposedly) double pane glass with argon in between. What would happen if I covered the two porch walls and the ceiling with heavy duty plastic? Create more moisture problems? What about solid board insulation? Would love to just have it ripped off but that’s not an option.
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I would strongly advise not to install plastic. It can lead to more/worse moisture issues. The plastic will trap moisture and not allow for materials to dry out. You need to try to control the dew point in the room and in the walls. For that to work temp and moisture levels need to be adjusted. It’s a very complicated and hard task with out professional help. The prefab 3 season walls may never really allow for proper r value to achieve a no condensation room. Also, if 3 season room is on a slab or deck w/out a vapor barrier below moisture is coming up from the earth and it will not matter how well your walls perform. The 3 season companies don’t build walls according to the building requirements of a conditioned space. They aren’t required to as it’s not considered a living space of the home, but as a supplemental structure. It would be best if you can find a contractor that can help you with this. You may find yourself putting a bit of $ into a fix that won’t work if you do not have experience and know how. If it’s found that an improvement can’t be made within a feasible budget you may need to just focus on ventilation. If you can get a moisture controlled vent with proper CFM for the room, you may find that this helps a lot. This will probably be the most reasonable approach. Can get a through wall vent that will not require any piping to function off of a moisture controlled switch. Can find one off of the shelf at big box stores around me rated at 200 CFM and around $100.
Also, for heating and cooling install a basic window unit or through wall unit. They make window A/C with electric heat units. Not good to share the air of the rest of the home with the 3 season.
See this article against venting a crawl space. https://www.advancedenergy.org/2017/06/13/taking-the-vents-out-of-vented-crawl-spaces/