Applying the Lstiburek wall to convert a 3-season room into a conditioned room
I currently have a 3-season room on the back of my house that I would like to convert into a conditioned portion of the house to create a sunroom. For insulation and element control, I plan on following a Lstiburek wall construction for the walls – using exterior rigid foam insulation with sealed seams for the WRB. On the walls and ceiling, I plan on using an internal barrier like MemBrain for vapor control.
My question concerns how to best apply the Lstiburek principles to the floor of the enclosure. The floor has open air space below it and we live in Iowa, climate zone 5, but almost zone 6. I would like to use radiant floor heating covered by either porcelain tile or slate. What are some recommendations on how to insulate, air seal, and vapor seal the floor? There is a lot of differing views here. Anyone have experience with this application and able to suggest a layer scheme for me to ensure adequate moisture control in humid summers and cold winters?
Thanks,
Gary
Replies
In the "open air space" under the floor is there any room to work?
Yes. There is plenty of room to work in. Attached is an image of the underside.
Thanks,
Gary
I’m assuming the floor is an exterior deck or is it a vented crawlspace?
It is the underside of an exterior deck.
Building a "sun room" conjures up visions of lots of glazing area, I'd factor that in big time in Iowa. It could easily add a heat load to the rest of the house no matter how efficient the envelope, less glazing. But if you can isolate the space with doors as needed that can be minimized.
Is there currently a step down into this area? If so, how much room do you have above the current finish floor to the house threshold? And how do the two ceilings line up?
I’ve built a lot in Western IA and Eastern NE (mostly in Omaha area). Assuming code enforcement is the same for your area. You will not be able to have the new room w/in the existing homes building envelop. That means you will need to keep them thermally separated. Also, the heating and cooling can not be shared w/ existing home. If you want a room that is an addition vs a 3 season room you will need to hire architect/engineer for the building permit. Most code jurisdictions in the area will not allow deck type construction in the same fashion as typical home construction, so you will need a licensed engineer to design, stamp the plan and take on liability of the design.
I’ve done similar improvements as you are describing. The most important first step was to reassure the city that I was not building an extension and that it was a 3 season room (they will not except the term 4 season room). I made sure the house was isolated from the 3 season room. There are no code regulations in regards to 3 season room environment, so can basically insulate any way you want, just need to abide by the safety and structural standards of code. For heating and cooling, you will need plug in type of units (like window unit type) that only require an electrical permit. If you get into heating and cooling that requires a licensed Hvac installer the city will not approve. Also, I’m sure it goes w/out saying, but definitely want electrical in floor heat vs water. Water will freeze.
For insulating refer to the building code for thickness/r value minimums with use of foam. The green building council states that they are good with how the code is written for foam. You will need to insulate your floor very well as your in floor heat will struggle to keep the chill off. If you have the budget can spray the entire floor cavity with closed cell. If budget is a concern, I would install a layer of 4” foam (or two layers of 2” foam) to the underside of the joist. I would install 1x3 strapping to the underside of that which will hold the foam very tightly and nicely to underside of the deck. You will attach soffit board over that to finish it off. Fill the joist cavity with mineral wool or fiberglass batts between the floor joist to beef up the r value. Just make sure you do not use another vapor barrier anywhere foam is used as you will trap moisture. Only one vapor barrier in all assemblies, foam is the vapor barrier. So if you use fiberglass batts need unfaced type.
The magazine has a pretty good article about it. https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
That about covers it. But I can't emphasize enough how bad fiberglass is in this application. I've come across more than enough proof.