Expert: Ted Barnhill (Principal, Design45 LLC) - 3/12/2008

Question
Ted, I am about to add a modern 2 storey with basement addition onto my home. Smooth and split face architectural blocks are the look we are trying to achieve. I also understand that if I were to use a hollow core architectural block - both the exterior and structure could be completed by one trade. No need for wood framing and then a 4" masonry veneer. I understand that single wythe construction to be used primarily in commercial settings (factories and units for the most part).  My motivation is to eliminate the need and cost associated with 2x6 wood framing.  This represents a large cost savings if a masonry crew could assemble the structure and then rely on carpenters soley for the floor joists and flat roof deck.  The issue I am encountering is that there would not exist a secondary wythe as a back up in the event water penetrated through the block.  As well in the Toronto climate I must achieve an R20 insulation value and provide a cavity for drainage and ventilation between the exterior and interior warm side of the house. My thought at overcoming this is as follows.

1.  Proper flashing and weep holes above grade at floor joist and below windows.
2.  Installation of a drainage mat on the interior of the block wall with flashing to direct any moisture to the center  or weep hole of the block.
3. Followed by 1" EPS rigid insulation to act and provide a solid and sturdy drainage path along with the drainage mat.
4.  Spray foam insulation applied next to achieve desired R20 value.
5.  Metal stud of interior wall for drywall.

Again the idea is to save money by not utilizing a framing trade and the associated lumber and to take advantage of the aesthetic appeal the split face and polished block on the interior along with the structural benefits of hollow core block.

Am I out to lunch or can this actually work?


Answer
Paul,

You are an innovator!  While you have brought a lot of workable concepts together in your scheme, I have a couple of concerns.

1. Cost.  I would expect to see an increase, not a decrease, in costs with your method.  You will already be using rough carpentry in your floor and roof systems, so the additional cost of wall framing is limited.  In fact, you are proposing metal studs which on their own would likely offset the wood framing.  The EPS would more than offset plywood or OSB sheathing; etc.  You could of course get bids both ways to prove this.

2. Performance.  The brick cavity wall is very hard to improve upon for its handling of moisture.  It provides a continuous drainage cavity, whereas your system would interrupt the cavity with the floor structure, requiring perfect flashing.  Moreover, you'd essentially be assembling your wall in reverse order, and would have to invent ways to fasten the components in non-standard ways.  This is worrisome.

3. Finishes.  You mentioned having polished block on the interior.  This would be a durable and unique (for residential construction) look, but only achievable with a system having in insulation on the interior, such as a standard double-wythe cavity wall.  If this is the look you prefer, I think you would save more money by using a burnished 8" structural wall with rigid foam on the interior and a 4" veneer block.  This would omit the metal framing, spray foam, and drywall.  Any interior walls would still be drywall so you could have some color/texture contrast to enliven your modern look.

If somehow your idea does price out as cost-competitive, and you have the patience to try and resolve all of the unique issues of assembly, I'd be very interested to hear how it goes for you.  Good luck!  -Ted