Framing the sunroom - part 1.

The first major goal for the spring of 2012 was to frame the attached sunroom on the south wall of the house. There are a number of factors that led me to build the sunroom in the way that I have decided to proceed. This first part of a new series on the sunroom will set the stage by covering some of the important design considerations.

The attached sunroom in an early stage of construction during the spring. Spring was early this year so I had plenty of time to get'er done before we were due to leave on a trip to the west coast at the end of May.

Let me start with a little background information on this attached sunroom so you have an idea of where I'm coming from with my approach to building it. I plan on adding some construction details to the "details" section of this blog when time permits so bear with my verbal descriptions in the mean time. Some important design considerations are as follows:

• The sunroom is attached to the house but it is not part of the conditioned space of the house.
• The sunroom is to be primarily a place for raising plants and will often, I anticipate, have a much higher interior relative humidity than the main house and the R.H. is likely to be much higher than would normally be considered prudent for an interior environment. 
• The roof of the sunroom is to function as part of the deck that will be added to the east side of the house at some later date.
• The house and the sunroom share the same slab, leading to some challenging thermal bridging issues. Note that the ball got dropped during the design phase in this respect. Though I have devised a work-around, it is not an ideal solution and more thought should have been put into solving the problem on the drawing board.

The list of design considerations above will be addressed in the following ways:

• Since the sunroom is not to be conditioned space, the adjacent portion of the exterior wall of the house is treated the same way as it would be if there were no sunroom attached - insulation and housewrap to be applied before framing the sunroom against the house.
• To deal with the potential for a high R.H. environment in the sunroom, a high quality air barrier is required. In this case more "airtight sheathing" which will also function as the cladding for the exterior wall of the house. In addition, the portion of the "airtight sheathing" that functions as cladding for the exterior wall of the house is to be back-vented (like a rainscreen) with the gap coupled to the vented roof assembly of the sunroom.
• Since the roof over the sunroom also serves as a portion of the deck, it is by necessity very low slope (~1 1/4" over 14') - some minimal slope was provided to prevent water backing up against the house. The roof joist bays are to be ventilated to provide better drying potential.
• Thermal bridging through shared concrete structure (the slab and one small pony wall) will be treated with the application of rigid foam insulation - I made allowance for 1" XPS as a flooring "underlay" when framing the door openings.

So that is the plan. Much of it has already been executed and the results will be revealed over the course of the next few posts.

I will try to get some details drawn up and posted ASAP...

**UPDATE**
Details have been posted here, and some additional commentary on those details here and here.

Sundown - Gordon Lightfoot