# Insulation choices

| Posted in Energy Efficiency and Durability on

Hi I am building a house in Michigan zone 5 and am deciding on either 2×4 walls 16″ oc with r13+5 or 2×6 walls 24 oc with r-23 mineral wool. Does the extra r value with the 2×6 wall make up for not having a continuous thermal break. Pricing wise they end up pretty close.  If the 2×6 wall has the same performance as r-13+5 I would go that route to save time installing exterior foam.  What are your thoughts. Thanks

## Join the leading community of building science experts

### Replies

1. | | #1

A simplistic calculation would be-if my math is correct-studs are r=1.25/in wood-at the studs 2x6 inch wall has 5.5x 1.25= 6.8, the four inch wall has 3.5x1.25=4.48, plus 5=9.48, but in a 24in oc wall studs are only ( 2.5/24=10.4 per cent of wall, whereas 16 ioc is 2.5/16=15.65%

2x6 wall 24 ioc sans ext insulation effective wall r value then is 6.8 x 0.104 =0.7 +23 x 0.896= 20.6;meffective r=21.3
2x4 wall is 9.48x0.156=1.47 + 13 x 0.944=12.27or effective wall r of 13.53

Ceteris parabis ie similar sheathing well sealed which would add say 7/16x 1.25=0.54 trivial and to both anyway
Also assume you will be same foundation dimensions and cantilevering the 2x6 _if not you either lose 8inches inside the house dimensionally !
Am assuming rock wool friction fit batts OR dense pack cellulose on so no sagging /ain’t voids even on the 24 ioc wall-

If I’ve done the math right from pure r value 2x6 without ext seems compelling choice, however these are average wall value- the 2x6 wall varies between 7-23, whereas the 2x4 with ext sheathing has less variability-r 9.3- 18 ;a range from also have to perhaps consider how how bad is risk of condensation on that 2.6 stud inside-ie is humidity rel high at any time fo year? Vapor barrier inside ?climate- I’d reckon most will vote for the wall without ext sheathing. And what about costs ? A 10 foot 2x4 stud runs , \$5, a 2x6 \$6; a strip of r23 mineral wool 2x10 would be \$3 @15 cents sq inch, when add in cost of foam
You are using 33% more lumber in the 2x4 wall, and the price of stud is only 20 per cent less AND you have to add the cost of the foam.

2. | | #2

Consider another alternative:
2x6 top & bottom plates, with 2x4's at 12" oc alternating every other one flush to outside/inside.
This gives you a complete thermal break between the top & bottom plates, and gives you the 6" of insulation. I found to be the same cost as 2x6 @24" oc because although it is slightly more board feet of lumber, the cost of 2x4's in my area is so much less per/bf than the cost of 2x6's, so the material cost was less. The upcharge in the labor was about equal to the material savings.

• |
• |
• |
• |

# Insulation Choices

| Posted in General Questions on
CHOOSING INSULATION ###Insulation cuts pollution and fossil-fuel demand Insulation is one of the most important components of any environmentally responsible building because it reduces energy consumption and therefore pollution. More insulation means less coal is burned at the power plant, and less gas or oil is burned in a furnace. In fact, with good design and appropriate levels of insulation, you can minimize (or even eliminate) the need for central heating and cooling in many buildings. This principle is demonstrated in all superinsulated buildings, including PassivHaus buildings. In this sense, any insulation material is a “green” product. Green builders need to focus on choosing which insulation is greener or better for a particular application. A few considerations include: What needs to be insulated Raw materials used for insulation manufacture The global warming potential of gases released during the manufacture or installation of the insulation Health and safety concerns for installers and occupants Whether scraps can be easily disposed of or recycled When choosing an insulation material, consider how it will work with the rest of the wall, roof, and floor system—and also consider what additional functions, such as air sealing, the material might serve. Some types of insulation stop air movement and reduce heat flow while shedding water and allowing drying (the four functions of the building enclosure). Here are a few issues to consider when pairing insulation materials and structural elements for maximum efficiency: Life-cycle considerations Because different types of insulation are made from different raw materials and are manufactured using different methods, their environmental impacts vary. These life-cycle impacts should be considered along with factors such as R-value, air-sealing ability, and cost. Installation—a 4% mistake yields a 50% penalty Poorly installed insulation will not achieve the energy savings that its rated R-value would suggest. A California study concluded…

## Join the leading community of building science experts

### Replies

1. | | #

toxicity?
Does anyone know which of the spray on insulation products off-gasses the least? I hear that Icynene needs 30 days to off gas once applied.

2. GBA Editor
| | #

Response to Jill Buffie
Jill,
Installers of spray polyurethane foam must wear protective equipment during foam installation, but I haven't seen any information to indicate that cured foam is toxic. Lingering odors have been reported in a small number of cases; if these cases worry you, you can always choose a different type of insulation.

3. | | #

Roxul
Under 'batts" the list of known insulations need to be updated to include Roxul don't you think? Now available in the United States mostly through special order.

4. GBA Editor
| | #

Response to Sacie Lambertson
Sacie,

5. | | #

Toxicity
Jill. If you are sensitive to smells, you might want to avoid spray foam. As far as I know, this isn't a toxicity issue but more a question of comfort. My properly installed open cell foam has an odor, but I don't notice it unless in an area where the foam is not encapsulated behind drywall. Your best option is likely to use rigid foam on the exterior, cellulose on the interior, or these two materials in combination.

6. | | #

Really need to find a non-toxic, no formaldehyde insulation. Any suggestions?

• |
• |
• |
• |