Practical ‘super’ insulated walls
After reading old posts on ICFs, truss walls and double-walls, I’m still confused on the best approach for getting the most insulation ‘bang for the buck,’ especially when I try to factor in labor costs. My basic question is, why not just use a ‘rock wool’ batt product like Roxul on the outside over the exterior plywood sheathing? When used with vertical furing on spacers over a ‘conventional’ 2×6 wall, the non-absorbitve (if that’s a word), non-rot, non-sag quality of the rock wool batt behind fiber-cement siding would seem to provide a thermal break that the conventional wall lacks while providing not only and additonal R-15 but also path for moisture to escape behind the siding. It seems like the simpler approach would have greater ‘pay-back’ for the increased cost.
Thanks.
Jeff
Replies
Well, in the grand scheme of things, an additional R15 is chicken feed. Yeah, it's insulation, but nowhere near "super".
And there's nothing magical about mineral wool. Technically fiberglass is a form of mineral wool, with most of the same characteristics of the other available varieties.
Well, R15 + R23 is 38...not too bad for walls. And if you wanted more put some rockwool on the inside, but that gets back to the basic question of being practical.
Keep in mind that somewhere around R20 losses through windows will exceed losses through the insulated walls.
Sqft per sqft windows exceed wall loss at wall R-value of R-4 or less. Typical window area heat loss often exceeds typical wall area heat loss for the entire house when walls are insulated to R-19. Sorry ... mistyped that statement.
Practical superinsulated walls
Jeff,
Mineral wool is too squishy to be substituted for rigid foam sheathing, so you need to come up with some type of stand-off or sleeve for the screws that hold the vertical rainscreen strapping. It's possible, but fussy.
I don't consider ICF super insulated in any fashion. Lots of sales hype about 'effective' R-value w/ ICF, but it's largely BS. Depending on the system you might get maybe what an R-20 at best out of an ICF system (i.e. 2 inches of expanded polystyrene of both sides continuously).
Your approach is reasonable. I don't disagree w/ the comment about FG vs rock wool.
Frankly, I think its amazing the efforts and expense people put into super insulation while they ignore the heat loss of windows and other design considerations. Super insulating walls is like having a collander with lots of tiny holes and one big hole (the windows). You think of plugging the little holes while leaving the big hole alone.
Why not take the money and effort and buy better windows? Why not check the design of the house (e.g. orientation and shear house size)?
Windows and superinsulation.
I replace my double panes with tripple panes with krypton gas. I would never do it again. There are just to many problems that crop up with tripple pans and the diference was minimal if at all. You can save a lot of money with interior storms that cost less and do a better job. Once you get the hang of it you can make your own (I use two layers of mylar film for a double pane interior window and . If your walls are thick enough you can add a second or third interior storm window. Follow this up with closed interior shutters in the evening and when the room is not in use. Go nuts and add exterior insulating shutters. My point is that you can do a lot with cheaper and better alternatives than high performance windows.
Bob's Builders
Upholstery Foam
When I was living in Vegas and working nights, I went to an upholstery shop and had them make me a 4-inch foam that was an inch bigger than the window in both directions.
I spray glued some of the mylar bubble insualtion they sold at the pool supply for a floating insulator blanket to that, and it made big difference in how hot the room got. My orignal intent was to get the room dark enough to sleep in. The fact that it rejected heat was just icing.