Best wall construction for long term efficiency & durability: Zone 6A
First off, thank you GBA for creating this fantastic resource for all to use. My wife and I built our current home 5 years ago and we were considered cutting edge for our area with the first home to use SIPs (PU 4") and a Superior Walls Basement. Now we are looking to build a new home for our growing family and move even further north in Michigan (Zone 6A per your map) so we were hoping for even better insulation in our new home. We currently have R24 walls and R60 attic with energy heel trusses. Our initial thought was to go with PU SIPs again but their difficulty in future modifications (i.e. adding/changing windows or door openings) made me start looking at using prebuilt wall panels from our truss company and then simply spraying the cavity full of SPF. Needless to say, you guys have really gone crazy with this technology in the last 5 years (or maybe the information is now more accessible on sites like this) and my head is spinning with concerns about OSB and moisture with PU foam (inside, outside, both sides). I thought I had found the trick by using a product such as Dow SIS or ZIP R for the exterior sheathing but still it seems that I was not fulling informed. Am I correct that even the thin structural layer of the SIS if sandwiched between PSF and the 1" of built in Dow foam will be at risk for water issues and future strength & durability concerns? Why isn't there a simple solution for sheathing yet (i.e. steel) to do away with these concerns while allowing for great insulation in the wall cavity and a layer of foam as an thermal break on the exterior.
We are trying to build as green as possible and have a system that is very quick to install similar to the SIPs to help keep construction costs in check while also acheiving a high r-value to help keep our heating and cooling costs down. That is why I was leaning towards a wall panel approach in conjunction with SPF after the electrical is run. We are planning on using several exterior finishes including hardi-plank, cedar planks, cedar shakes, & stone veneers and to be honest I am clueless on what extra I will need to do if we do need to add 2-4" of extra blue foam to the exterior in order to allow for all these exterior finishes to work while creating the proper insulation on the exterior to protect the sheathing.
I should also add that we liked that the 4" SIPs of our current home did not require expensive jambs for our windows. We will have even more windows in our next home since it will be in the middle of the woods and we want to bring all the natural light and nature into the house. This raises some cost concerns for the jambs with all these super think wall structures that I am reading about (i.e. 4" exterior foam or double walls). We plan on having large energy heels on the trusses to get the insulation in the attic up to near R70 (Foam seal & blown in cellulose) and also using a radiant barrier on the roof sheathing to help control loss of heat in winter and keep it cooler in the summer.
We hope that this will be our last home and it will stay in our family forever so we want to build it right. Durable and efficient as the title suggests. Yet we are also trying to do it as cost effectively as possible. I know I am probably going to hear 1000 different suggests but I am open to all at this point. I appreciate everyone's input and I appologize for being such a "beginner" on this.
Posted Mon, 03/05/2012 - 02:26
Other Questions in General questions
IRC 316.4 - Code interpretation on the use of rigid foam as air barrier on backside of attic knee wall