Dave's Quips - What's behind the drywall?

By now, I’ve written a few entries on this blog about wall construction and the methods I’m using to construct the building envelope. I can’t stress enough how important choosing a solid wall construction method is to the overall energy efficiency of your home. 

Say you move into a brand new house. It may look nice on the outside, with a fresh green lawn and ornate door fixtures that greet you each time you pull into the driveway. Open the front door, and the first thing you see is a stone tile floor illuminated by an ornate lighting fixture. Maybe you sunk some money into things like marble countertops in the bathroom, modern plumbing fixtures, a large kitchen island. Cosmetically, this hypothetical house looks fantastic.

But the real question is, does the hypothetical home of your dreams feel as nice as it looks? Do the marble countertops seem to make you shiver when you’re in the kitchen? Do you need to wear a sweater when you go upstairs? Is the basement a little bit too warm? You can engineer these problems away when building a new home.

There are lots of home construction blogs out there, and many of them have articles about what features and trends are in demand when it comes to building new homes or renovating existing ones. I challenge you to find a single blog that mentions advanced wall construction techniques as a feature that’s in demand.

Homes have layers to them. You can usually find out the truth about the quality of a home by peeling back the layers - behind that tastefully decorated and appropriately painted living room wall, you might find the cheapest insulation possible with a lower quality of wood used for studs and framing. It’s what’s on the inside that counts - your first time homebuyer newlywed couple might not be thinking about the mechanical systems inside their new home, but they’ll certainly feel the consequences down the road.

Whether you’re buying a stock home or building from scratch, a simple investigation into what’s behind the drywall or inside the mechanical closet can help you make a better decision - an important decision, especially when comfort is at stake.


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  • Paul, thanks for your kind words! As far as SIP systems go, I understand how they work and I’ve done two projects with them. For this project though I wanted to use locally sourced material and because of our proximity to the shoreline (a source of hurricanes and strong winds), I felt that a conventional frame blocked and braced was the way to go.
  • Lawrence, this is a great comment. In my opinion, there’s no perfect way to build a super insulated wall – if there were, everybody would be using the exact same method!

    Each way of building an envelope has its pros and cons – I chose my method based on the fact that we’re in Connecticut, a heating climate. If my home were in Florida, I might have chosen a different system.

    Keep following along to find out how all of these systems are going to come together in the end!
  • I applaud your concern with moisture movement through your walls, and your PERSIST type construction techniques. I think you need to seriously consider vapor movement from and into the inside of your house and bulk moisture outside the poly iso insulation layer.
    You say that the poly iscocyanurate insulation is “permeable”, though I would guess it’s less permeable than many would believe. 4" of poly-iso with 4 layers of the organic facing and you have a permeability much less than 1 perm, probably more like 0.25 perms or less. (https://buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing/view). The poly iso in the middle of the wall is almost a vapor barrier. So vapor moving out from the inside of the house will be mostly stopping at the poly iso insulation layers and hopefully not condensing on that layer. Come warmer weather vapor will need to move toward the inside of the house to allow that wall to dry after the winter. Similarly, there will be vapor movement from the outside and stopping at the poly iso insulation. Vapor and bulk water will need to move out again, through the plywood sheathing and through your air barrier.
    I think others considering your design of wall might want their second air barrier (if used) directly against the poly iso on the outside, then some 1X battens between that and the plywood layer (the plywood layer to which the shingles are nailed). That would allow better drying between the plywood and the low permeable insulation layer – especially for any bulk moisture that gets in there, and allow drying from the back of the shingle siding inward toward this air gap through the vapor permeable plywood.
  • Great attention to detail, and that will be a warm envelope. Did you you ever consider using SIP system (Structural Insulated Panel) construction, using plywood as a stressed skin? It would be faster and a lot less labor. The only thing is to use caution on using SIPs for the roof, as you could get condensation in the roof, which would be bad for the panels.
  • Cedar shake siding. Yikes! That stuff is kindling.
  • Really good information on how to build a tight home and save money