Additional Cost of Double Stud Wall

[Jesse Thomas]

I've been trying to find decent assumptions on price increase per inch of thickness for double-stud or Larsen Truss walls for cost effectiveness scenarios in the design/planning phase.

Firstly, is it naive of me to think that as the wall thickness increases, the price per inch of thickness remains relatively constant? That, although one more inch barely adds to the cost of the wall, it does add area to the roof and slab.

Any good assumptions?
Dan, Rob....come on, give it up.

http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/double-stud-wall-construction-path-efficiency-budget

Jesse Thomas

[Adam Cohen]

Jesse, it depends as always.  

As a builder / designer I have spent alot of time trying to VE my construction and what I have found in rural Virginia may be different from your area, but in general the cost delta depends alot on what your building with.  We tend towards floor and roof trusses so adding an inch here and there can really throw costs off.  We pay alot of attention to truss spacing centers to VE our projects.  We find that extending the length is not as critical.  When we work with dimensional lumber we pay more attention to the length of material (make sure we are not cutting of 1'-10" of a 14 footer) more than the center spacing.  

When I am designing it is a feel for what is the most effective that is informed by experience.  Your calculator won't be linear.  If you are adding an inch or two but that changes the depth of framing member or length and result in higher cost or larger % waste factor then if you are adding an inch or two for the same member.

Hope that helps

Jesse Thomas

--
You received this message because you are subscribed to the Google Groups "Passive House Northwest" group.
To view this discussion on the web visit https://groups.google.com/d/msg/PassiveHouseNW/-/H9CUhN6DpQoJ.
To post to this group, send email to Passive...@googlegroups.com.
To unsubscribe from this group, send email to PassiveHouseN...@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/PassiveHouseNW?hl=en.

--
Adam Cohen
Certified Passivhaus Consultant - North America and Europe

Registered Architect, LEED AP, NAHB Green Professional

Design/Builder of the First US Passivhaus Public School Building

Structures Design/Build, LLC
5104 Bernard Drive
Roanoke, VA 24018

Web site: www.structuresdb.com
Passivhaus information: http://www.passivehousedesign.us/

More Passivhaus info: http://www.viking-house.us/

540.774.4800 (office)
540.989.7062 (fax)

[Sam Hagerman]

Jesse-

I think it's folly to calculate cost per inch of thickness.  A double stud wall is roughly 75% more than a conventional wall for the framing members L&M, holding constant for the structural sheathing, but increasing from a 12" DSW to at 16" DSW is basically only the additional cost of the cellulose.  The same principle holds true for a Larsen Truss.  A 12" Larsen truss is only marginally more expensive than an 8" Larsen truss wall. 

Some basics assumptions:

Framing lumber material for conventional construction:  $9.11 sq ft. 
2x4 per lf: $.26
1.65 framing member length for Advanced frame wall: 1.65 per 1.
$.24 per board foot for DP cellulose.
200 lf. of exterior wall (avg.) in a 2000 sq ft house= $858 for framing materials for second layer of double stud wall.
Labor for this piece: $2,600 (WAG factor)
1600 sq feet of exterior wall, at $.24 per inch of thickness, $1,536 to add 4" to the exterior insulation system.

So, it's about $3,458 to $4,000 to go to a DSW from a conventional for a 2,000 sq ft house for the framing.  PLUS $396 per inch of insulation.

Anyone else?

Sam

[Jesse Thompson]

The modular builder we're working with on projects in the northeast (http://www.kaplanthompson.com/attainable_now.php) priced out several different methods to increase their standard production R values to the R-40 range we were looking for.  The least expensive method that suited their production system was a double stud 2x4 wall.

They priced the cost increase of going from their standard 2x6 Energy Star wall to a 12" double stud to be ~$12.00 per linear foot of 8' tall wall back in 2009 (~$1.50 / SF). Deeper drywall returns were ~$30 - $150 per window or door opening depending on size.

Jesse Thompson

Kaplan Thompson Architects

Portland, ME

[Jesse Thomas]

Wow! Thanks for the thoughtful responses Adam, Sam, and Jesse.

Basically the answer is..Yes, it is foolish, and lazy, to assume that a linear calculation holds the secret to a cost-effective wall assembly. I'll review you numbers in depth.

I've been using $0.50 per square foot for each additional inch of wall, which generated 10" as the most cost-effective insulation thickness over a 30-year mortgage @ 5.0%. But this is for a house with a low-quality HRV and slightly above standard windows, near Seattle.

Further fine-tuning would require coefficient values for upgrading to double-studs, one for the wall assembly area, another for the perimeter of wall openings, then another per inch of thickness beyond.

Something else that might be useful for cost-estimating when adding thickness to walls in PHPP is a coefficient based on the envelope/floor ratio. For my project it worked out to about a 1% increase in envelope area for every 2" of insulation added to the wall. Again, this may be lazy, but I just multiply all areas by 1% for a house with 3.8 envelope/floor ratio. Any thoughts on this?

The reason I'm searching for these types of solutions is to quickly (with about four buttons pressed) arrive at basic assumptions for cost-effectiveness that help steer the preliminary design phase.

Thanks,
Jesse Thomas

[Adam Cohen]

Jesse, I feel as if you are trying to get to a precision (1" vs 3")  level that will be hard, too fine grained at the conceptual design phase.  I think as a designer it is better to roughly harmonize basic ratios (floor to wall to roof. . .  my climate is about 1:2:4) than to worry if it pays back for an extra inch make sense.  Besides changing from a hip to a gable or adding an offset can have more effect that a little extra umph in a wall cavity.

For my clients it is simple, we start by shooting to deliver PH at cost parity (Monthly Cost of mortgage + Energy for a PH is equal to are less than a std house).  Then we design the project, using our knowledge and expertise of synergies and cost effective design, then we run both std and PH versions through the energy model to show the monthly energy savings and equate that to an additional amount in the mortgage, which most times is net positive in the PH scenario.  

We have honed a construction system that seems to work in our climate, should be easy peasy in Seattle i would think, the land of mild climes and cheap wood!

--
You received this message because you are subscribed to the Google Groups "Passive House Northwest" group.

To view this discussion on the web visit https://groups.google.com/d/msg/PassiveHouseNW/-/z-xHdEbeEQIJ.

To post to this group, send email to Passive...@googlegroups.com.
To unsubscribe from this group, send email to PassiveHouseN...@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/PassiveHouseNW?hl=en.

[Jesse Thomas]

Adam,
I understand exactly what you are saying. There is a sensible path, based on knowledge, common experience, and good design sense.
For now, I consider the cost-effective design approach an experiment.

Jesse Thomas