--- July 12, 2010 Quadomated Blog Post: Construction Specifications - Part 1
This was a no-brainer for me, but a steady and constant struggle between me and one of my parents. Now, I won't name any names, but she certainly feels that a house in Northern Maine should have a basement! My thoughts: I can't get down there, and it's far more effective to have that large thermal mass right under your feet in a passive solar home. Fortunately for me, the additional price for the basement was much higher than a certain somebody had hoped, so it put that thought to rest.
I then spent time reading up on concrete slabs, talking to some people in the trades, and to our mechanical guy to figure out what would be best way to pour our slab. The method we decided is a little extra money, but I think well worth it considering the cold climate and amount of labor/materials we'll have poured into the concrete. This slab will be:
- 6" monolithic slab with perimeter edges thickened to 12"
- 3 rows of 1/2" rebar in the thickened parameter with 1' squares in the interior
- 2" Dow Styrofoam and a 6 mil vapor barrier underneath with the foam extended out 2' beyond the outer perimeter as a frost barrier. A 2" layer of Styrofoam will enclose the outer perimeter of the concrete with a 1" layer extended down from the wall section for a total of 3" on the parameter.
- Pressure-treated lumber will skirt the foam and encircle the perimeter for added protection.
I did a lot of research on different methods of construction to maximize energy efficiency and comfort while staying in a reasonable budget. The alternatives I evaluated included insulated concrete forms (ICF), structurally insulated panels (SIP), double stagger studded 2" x 4" walls, and 2" x 6" studded walls with XPS Styrofoam on the outside. Each method has its own positives and set of followers, but after taking into consideration that the house would be built on a concrete slab (no access for wires and plumbing underneath), have two areas of cathedral ceilings (less access for wires overhead), and the requirement for a significant amount of wiring within the walls to operate the home automation and media distribution equipment it was obvious that both ICF and SIP were out. As much as I like the idea of both, they make it more challenging to run wires in the walls. Add in the high cost of the foam/concrete combination and the large expense to ship the SIP panels way up here and the decision was even easier. We would build a stick framed house. Now it was just figuring out how we would frame it.
I'm still torn on this. I really like the idea of a double studded wall and how it eliminates the problem of thermal bridging across the wood member by creating an insulated space between the inside and outside wall. This method does have the drawback of using more material, labor, and sacrifices interior square footage (the wall is 1 foot deep after all), but at least the extra wood is a renewable material.. The 2" x 6" method does somewhat address the thermal bridging across the wood member by adding an additional R5 of 1" foam on the exterior, but even so using 16" o.c. framing you still have an area that is only approximately R11 every 16". If advanced framing techniques and 24" o.c spacing are used this is improved, but most builders in this area prefer the standard 16" o.c.. This is something I'd definitely like to research more, but for right now it makes the most sense to build our home the way our builder prefers (after all... he's the one that's actually done this before!). The main construction of the house will be:
- Pressure-treated sill plate
- Exterior walls: 2" x 6" framing at 16" o.c. with 1/2" Advantech sheeting and housewrap
- Interior walls: 2" x 4" framing at 16" o.c.
- Windows and door headers" 2" x 10" framing
Insulation is definitely one of the most important aspects of an energy efficient home, and yet another one of those decisions with a plethora of possibilities with dramatically different prices. The three possibilities I've looked into are fiberglass bats/blown in fiberglass, dense pack blown in cellulose, and closed cell spray foam. From a materials/sustainability standpoint I really like the idea of blown in cellulose insulation. It is inexpensive, easy/safer to work with, and has a high percentage of postconsumer recycled content. Overall, it seems like the "greenest" solution, but has the drawback of a lower per inch R value, especially when compared to closed cell foam, and I have concerns that it might settle over time. Closed cell foam on the other hand, has by far the highest per inch R-value (almost double that of fiberglass and cellulose), and has the added benefits of tightly sealing the house and adding structural integrity to the walls, but is almost 3 times more expensive than the alternatives. For this project, which is already stretching the budget, we have a hard time justifying the added expense of spray foam when it seems that the alternatives will provide close to as good or at the very least "good enough" performance. The final option, fiberglass bats/blown in, is the choice of our contractors, and one that seems reasonable enough to me, but is for whatever reason not well embraced by the green building community. I don't really understand why either, because fiberglass in certain products is very well-liked (take high-performance, triple pane fiberglass windows for instance), but not well received for insulation. Maybe someone can enlighten me on the reasons.
For now, the insulation specifications for the home are:
- Exterior walls: 1" extruded polystyrene foam (XPS) on the outside of the sheathing (R-5), 5 1/2" kraft-faced fiberglass insulation (R-21) for a total wall value of around R-20 (if you take into consideration the bridging losses of the 2" x 6" at 16 o.c.)
- Ceiling: R-19 fibergass bats topped with blown-in fiberglass insulation for a total of R-49. I may discuss using blown in cellulose insulation instead with the contractor and loose filling it to 18" for a total ceiling value of around R-60.
- Interior walls: 3 1/2" fiberglass insulation for sound. Is there a better way to sound dampen between rooms?
- Concrete slab: 2" Dow Styrofoam and a 6 mil vapor barrier underneath with the foam extended out 2' beyond the outer perimeter as a frost barrier. A 2" layer of Styrofoam will enclose the outer perimeter of the concrete with a 1" layer extended down from the wall section for a total of 3" on the parameter.
I also plan to discuss increasing the 1" XPS foam on the exterior to a thicker value, and/or possibly use advanced framing techniques with 24" o.c. with the contractor.
If anyone has any good suggestions/reasons to consider another or different insulation technology please chime in below in the comments because we'd definitely like to hear your thoughts.
Excuse the rant! The way windows are manufactured in this country and the Energy Star criteria are so stupid! The National Fenestration Rating Council (NFRC) certifies U-Factor, Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) values for windows to be considered Energy Star certified. To be considered "Energy Star" eligible a window must have:
- a U-value of less than or equal to 0.30.
- a Solar Heat Gain Coefficient (SHGC) of less than or equal to 0.30.
THIS IS SO STUPID! AND YES I AM SHOUTING, BECAUSE I'M SO FRUSTRATED THAT NONE OF THE AMERICAN WINDOW MANUFACTURERS GET THIS! Let's pick a criteria that averages what works best for a geographic area that spans from Northern Maine to Southern Texas and then force everyone to adopt it. Do people need to build homes in Texas to the same energy standards that they build them in Northern Maine? HECK NO! So why the heck should our windows be based on criteria that work for Texas!?!
Number 2 above says the Solar Heat Gain Coefficient (SHGC) must be less than 0.30. That says windows should only let in 30% of the heat generated by the sun. Now why would anyone in Northern Maine want to limit the heat that they can get from the sun to only 30% of what's available. MAKES NO SENSE AT ALL! I've even called representatives at a very well known/well respected window manufacturer in Maine, that sells primarily to New England customers, and still, their supposed technical specialists didn't understand a dang thing about solar heat gain. He said I should just match my SHGC to my U-value and be done with it. NO WAY! I want my solar heat gain, and think it is a hugely important/underutilized asset of a good home.
So the windows... we want as low of a U-value as possible on all of the windows, and a high SHGC for the windows on the south side of the house and a low SHGC for the windows on the north/east/west sides of the house. No way to do this with your big name American window manufacturers so I've had to look north to the smart guys in Canada to provide me with a solution that is far superior to the American solution. If the budget will allow, what we absolutely want to get it is a fiberglass encased triple glazed window with a solar heat gain coefficient that is tuned depending on the location of the house. With these windows we should get an insulation value that is at least double that of one of the big name American manufacturers, and a solar heat gain coefficient that should provide a good portion of our home's heat. A real quick back of the envelope calculation shows that we should save at least 250-300 gallons of oil per year!
I could keep going on and on, but this quick post is quickly approaching 2000 words so I'll have to save more for part 2. In Part 2 expect an explanation of the roof, siding, mechanical, and electrical systems.