You know by now that we’re all about finding the best bang for our buck when it comes to energy efficient construction. That’s how this whole thing started. It’s been a while since we’ve posted on advancements in our building philosophy, so why not talk about our next super-insulated wall assembly that we think could just possibly be the most affordable wall assembly that achieves maximum R-value and minimum thermal bridging with the least impact on your budget.
This is a bold statement, so I expect some comments and rebuttals. Here is my rationale.
THE Wall Assembly
- Double stud wall, 2×4′s on 24″ centers.
- Dense-packed cellulose in the wall cavity.
- OSB exterior sheathing.
- R-10 continuous exterior insulation, taped at all seams.
Here’s why this wall assembly is chalk full of awesomeness:
- Double stud walls on 24″ centers is the easiest and cheapest built thick wall cavity. Both material costs and labor costs are extremely low compared to alternatives (TJI’s for instance).
- Cellulose is economical while also being the best green insulation product known to man (see future post on cellulose superiority). It’s cheap. It barely has a carbon footprint. It fills all voids. It retards air flow like a champ. It has high thermal mass compared to alternatives like foam and fiberglass. Did I mention it’s affordable? This is important.
- OSB is cheap and makes it easier to attach cladding and laterally brace your structure.
- Continuous exterior insulation is the icing on the cake. It’s not uber cheap or low in the carbon footprint category, but used sparingly, it can be quite effective. It eliminates any thermal bridging worries that might have you up at night. When taped, it also can virtually eliminate air penetration and double as a water resistive barrier allowing one product to serve three important roles.
The next Postgreen Home, the 2 point 5 beta, will incorporate a 9″ thick double stud wall filled with dense packed cellulose (R-30) and sheathed in OSB and capped off with 1.55″ (R-10) of exterior Poly-iso rigid insulation. This will result in a wall with an R value of 40 and absolutely no thermal bridging. This will be our best wall assembly to date and most likely the wall we will stick with going forward. [Please forgive discrepancies from this description and the image above]
One of my hopes in posting this hypothesis is that one of you guys out there will prove us wrong and give us a better wall assembly that packs more bang for the buck. Here are some of the other options I will shoot down in advance:
- Excessive exterior insulation – As we stated above, rigid insulation is not cheap and has a larger carbon footprint than many other insulations. Therefore it should be used as sparingly as possible. R-10 is the minimum amount needed in most US climates to mitigate dew point issues that could cause moisture in your walls. Lastly, going over 2″ in thickness in exterior insulation will drive up the cost of your fasteners and labor required to attach your exterior cladding to your framing underneath.
- Spray foam – While spray foam could be argued to be one of the more effective insulation options out there, it’s by far the most expensive and one of the most damaging to the environment with it’s ozone killing component and high carbon footprint.
- Any type of batt insulation – Seriously?
- ICF’s - Expensive, built of harmful foam, hard to get a high r-value and filled with concrete.
- Big ass SIPs – These have the same problems as excessive amounts of foam as that is what the core is comprised of. Also, have you ever tried to lift a 10″ – 12″ roof SIP into place as a wall? They’re really heavy and unwieldy, adding to your install costs.
- Blown-in Fiberglass – Substituting fiberglass for cellulose would decrease your costs slightly while achieving the same R-value. The big downsides are that it has a much higher carbon footprint, virtually no thermal mass and does not resist airflow, crippling it’s advertised R-value in the face of any air movement through your walls.
Let the comments roll.