Construction Design-Randy’s Dream Design

Guess what?  I just won the lottery!  (Not really, but for the purpose of this blog, let’s pretend.)  I’m looking to build myself a new home.  I have choices.  I could build a McMansion with plenty of space I don’t need and will never use.  I could concentrate only on the interior finishes and how the house looks.  If you’ve ever read my blog, you already know what direction I’ll take.  The house won’t be big, a couple thousand square feet is plenty for me, maybe a rambler with a second story over part of it built on a slab, no basement or crawlspace.  Being a BS* guy, I would make an invest in the stuff that is hidden, those pesky control layers I often talk about.  It would be based on the Pretty Good House concept.

Lets get started with the foundation.  My climate of Minnesota requires foundations to be either frost protected or installed to a depth of 60 inches.  I’ve had success with the frost protected designs in the past, but I usually use those for a budget friendly option.  My preference is always a foundation built below the frost line.  I would choose an ICF or insulated concrete form.  For those who don’t know about ICF’s, they typically have 2.5 inches of expanded polystyrene insulation on two sides, concrete fills the center.  Some people in other climates don’t like having exterior insulation below grade.  In my area, it works.  We don’t have bugs that eat it and as long as it’s protected, it lasts.  One additional factor, in my area, it’s getting harder and harder to find tradespeople who lay block.  There’s a few around, but they are always busy.  The ICF is a carpenter friendly system.

I’m getting to the age where I can tell I won’t like stairs in the not so distant future so my design includes everything I need on one level, thus the decision for the slab on grade approach.  The upper level will be for the guests.  As much as I like the Concreteless Slab on Grade house,  I would build my home a little different.   I would pour a concrete slab.  I personally don’t care for living on such a hard surface so I would insulate above the concrete slab and add a wood subfloor.  (A design from Steven Baczek and Jake Bruton).  There would be 4 inches of EPS Type IX foam between the slab and wood subfloor.  A different approach to a slab.  (I’m probably going to take some heat for all the concrete being used.)

Next up, the walls.  2 x 6 standard construction with Zip Sheeting as both my water and air control layer.  The wall framing would be set back from the edge of the concrete slab 2 inches and I would use 4 inches of Rockwool ComfortBoard 80 for continuous exterior insulation.  (The 1/2 inch Zip and 4 inches of ComfortBoard would end up flush with the EPS foam of the ICF foundation.)  My climate requires a minimum of R-15 exterior for a 2 x 6 wall before the interior vapor control layer can be eliminated.  The ComfortBoard gives me R-16.  The wall cavity would be insulated using Rockwool ComfortBatt which is R-23.  I’d be nearing a R-40 wall assembly.  A couple advantages in using ComfortBoard over another rigid insulation, It’s hydrophobic, meaning it doesn’t care if it’s wet or dry and being manufactured from rock and steel slag, it’s naturally fire resistant.

To finish off the wall assembly, 1 x 4’s would be screwed through the Rockwool insulation and Zip sheeting and into the framing.  I’d have a vented rain screen where my solid wood siding would be attached..

With that much exterior insulation, I would have to make a decision on the windows.  Do I use ThermalBuck and a window with a nailing flange and install the window traditionally or do I go to a flangeless design and move the window into the center of the wall assembly?  Well, I won the lottery, remember.  I will be ordering windows from Europe and going flangeless.  I have a couple sources for European windows, Schuco or Glo.  Which ever I choose, it will be a triple pane with a very low U-value.

I’ve always questioned the way we air seal and insulate windows.  Back in the old days, (yes, I did use this method early in my career), fiberglass insulation was packed between the window and framing.  Not a good air seal and questionable insulation properties.  More recently we started filling the cavity with low expanding spray foam.  The issue with spray foam is you cannot see how far the foam is expanding.  Have you completely sealed the cavity or is some missing or thin?  Sealing the entire cavity may become problematic if there ever is a window leak from the outside.  We have taken care to install a pan flashing to protect the sill framing, but the insulation may trap water along the sides of the window.  I recently tried a newer method, at least in North America. window tape.  The photo below is Siga’s Fentrim 20 installed on a window in my home.  A little challenging to use, (I’ve only used it once), but I can see how well a product like this could work.  I think my window air sealing strategies will be changing to Fentrim and a backer rod whenever possible.

With the wall framing complete, it’s now on to the roof.  The biggest roof decision is how to extend the exterior air control layer to the roof.  I could build Joe Lstiburek’s perfect wall, make the exterior roof sheeting the air control layer and put insulation on top of the roof.  This design requires the roof eaves to be built later.  Personally, I like vented roof assemblies, especially in my climate, so my insulation will be in the unconditioned attic.  That still leaves how to get a continuous air control layer to the ceiling.  I would use tape.  From the wall Zip sheeting, up and over the top plate and tied to the interior ceiling air control layer.  I’ve done this in the past and it worked out well.  The attic would be insulated with blown cellulose to R-80.  I could use a few different products as the ceiling air control layer.  6 mil reinforced polyethylene sheeting like we used at the Code Minimum House or Intello like the Concreteless Slab on Grade Home.  Winning the lottery is allowing me to use Siga’s Majrex and all seams taped with Rissan.  (Rissan is my new favorite interior air sealing tape.)

The trusses would be designed so that I had a space to run utilities.  The plenum truss would be incorporated into the design and the ceiling strapped using 2x material.  This allows nearly all of the mechanical, electrical and HVAC to remain inside the building envelope.

Speaking of the mechanical systems, I would be using an air source heat pump (ASHP) mini split as my main heat source, this would provide both heating and cooling.  My area’s 99% design temperature, which is used for sizing heating systems is -23°F.  Most cold climate ASHP’s are rated to -10°F or so (though most I know of are still producing heat well below that temperature).  I’ll need a second source if the heat pump can’t keep up.  This second source of heat will simply be electric baseboard heaters.

This being Minnesota, we are required to install balanced mechanical ventilation in all new construction, I’d be using a HRV or heat recovery ventilator.

I would also install a Marathon electric water heater.  (In my opinion, the best all electric water heater on the market.)

The electrical service panel would be a Leviton using their smart breaker technology so I could track energy usage right through the breakers.

And lets not forget the grid tied solar system.  Being an all electric house, it would only make sense to use photovoltaic solar.  I would think about installing a battery system (such as a Power Wall) as an emergency back-up system.  This would provide an off-grid electrical system when the power is down, but the benefit of a grid-tied system during normal times.

Alright, I’ve covered the stuff that gets covered.  Finishing would be simple with a little character.  Air tight drywall on the lid and walls with a splash of wood paneling in areas.  Maybe a ceiling or feature wall.  Wood interior doors, case and base.  Wood cabinets with granite countertops, a combination of tile and wood floors, and of coarse, a custom shower or two.  I did win the lottery after all.

There you have it, my dream house.  Comfortable, healthy and efficient.  Ask me what my dream house is in a year to two, this will all change.  But for now…

*Building Science