Replies

1. 
   shelternerd | Feb 25, 2008 01:11am | #1

   The problem here is that R-value is only a partial measure of wall performance. you also have to deal with bulk air leakage, convection, "air washing" and radiant heat transmittance all of which can be problematical with fiberglass and even cellulose to some extent.

   A wall with R-19 and an open window will not perform as well as a wall with r-10 and no open window foam closes all the little leaks that contribute to the open window effect. We've been using Joe Lstibureks "Air Tight Drywall Approach" for many years but our blower door test results really tightened up when we switched to foam. Our next project we'll be trying a combo called "flash & Batt" which is similar to applying an inch of styrofoam to the exterior sheathing of the house over the OSB but instead you frame the walls with 2x6's and spray an inch of foam in the walls before you fill them with fiberglass batts. The roof gets foamed to make a sealed attic and we do a sealed crawl per local code.

   At the International Building Show last week I asked Joe (actually got to sit and drink a beer with him) what he thought about spraying the foam on the underside of the roof deck vrs using a foil ply and vent channels to hold the foam down off the foil ply and prevent summer radiant heat transference and he indicated that this was a good idea for high performance homes. But then I don't think my homes are quite that high performance yet, we're standing at about 32% better than current code. But as we make more adjustments to our building systems to get closer to 50% better than code we'll probably get to the point where that starts to make more sense economically.

   <<The question in this post is, is it worth it in the long run to spend the extra money on 9 or 10 points of r-value over what is reccomended? Or is there a diminishing return and say r-20 is most effective. According to govt. list r-18 is suggested for walls in my zone. Not sure if thats for total wall system or in wall. R-19 total for me would be least expensive, r-24 would be about 2 1/3x, r-30 or so would be 5x the cost of r-19.In my other post the least expensive idea would give me approx r-19 total wall. >>

   For your situation I'd say first to talk with an Energy Star HERS rater about getting an overall evaluation of your plans and then I would say to look at Lstiburek's website http://www.BuildingScience.com and consider an airtight drywall approach on the walls with fiberglass insulation and spray foam on the underside of the roof deck with a sealed attic paying especial attention to sealing the rafter to wall top plate area to assure a sealed attic. Then I'd do a sealed crawl with foam board on the exterior wall per local code (it varies).

   That's my best recommendation, be well.

   M

   ------------------

   "We DON'T Build them like they used to."

2. 
   Riversong | Feb 25, 2008 02:36am | #2

   Ditto ShelterNerd.

   You have to consider the entire building envelope as a package:

   • R-values of foundation, floors, walls, windows, doors, ceiling/roof.
   • Air-tightness of envelope and provision for fresh air exchange.
   • Moisture management from the ground up to the roof.

   Do a heat-loss/heat-gain analysis and then compare all that to your current fuel consumption to determine payback.

   And don't forget that energy costs are going to continue to escalate.

   I've been building R-40+ walls and R-60+ ceilings for 20 years.  Even back then, it was cost-effective.  Today, it's foolish to NOT invest in the most efficient building envelope you can manage.

   But make sure it's a system that will work in all regards.

    

   Riversong HouseWright

   Design *  * Build *  * Renovate *  * Consult
   Solar & Super-Insulated Healthy Homes
3. 
   rez | Feb 25, 2008 02:38am | #3

   101362.1 is an extension of this thread topic.

    

    

   Peach full,
   easy feelin'.