Insulating a porch roof to create a four-season living space
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I live in southeast Nebraska and I would like to turn an east-facing 19 by 8 ft. already enclosed porch with a concrete floor into a 4 season living space. there is no structure above the roof.
I don’t see a way to vent the roof and would like to know if I used R-30 insulation and Reflex-it vapor barrier , with the proper dead air spacing and sealed it so very little if any water vapor could escape to get to the under side of the R-30 insulation , if I would be creating a problem with condensation.
If that type of system would not work can You Folks give me some ideas to avoid any moisture issues.
Thank you very much for your time
Rick
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Replies
Rick,
In your climate zone (Zone 5), the 2012 International Residential Code requires that ceilings and roofs have a minimum R-value of R-49. That's a good target to aim for -- so your plan to install R-30 insulation isn't a great plan.
It's perfectly OK to design this roof to be unvented. There are several ways to do this:
1. You can install R-49 of rigid foam above the roof sheathing, followed by a layer of sheathing and new roofing.
2. You can install a combination of R-20 rigid foam above the roof sheathing, and R-29 or more of air-permeable insulation (fiberglass batts or cellulose, for example) between the rafters (and in direct contact with the roof sheathing above).
3. You can install R-49 of spray foam between the rafters.
4. You can install a combination of R-20 closed-cell spray foam against the underside of the roof sheathing, along with at least R-29 of air-permeable insulation underneath the cured spray foam and in direct contact with the spray foam.
Details on all of these approaches can be found here: How to Build an Insulated Cathedral Ceiling.
Step back and ask yourself why you want to turn your porch into a 4-season conditioned living space. Is it lack of space in the house? Is the porch so uncomfortable that you don't use it right now? I feel that this question is important to ask because it can focus your thinking and help you get the best bang for your buck. Turning an enclosed porch (a sunroom) into a pleasurable living space can be a surprisingly difficult task and yield surprisingly poor results because it's a space that was fundamentally never designed to be a conditioned interior living space. Are you planning to replace all the windows with typical solid insulated walls? If not, the room will have enormous heat gain and loss and be difficult to keep comfortable. How are you going to heat and cool it? Will you add a dedicated mini-split or something?