1. Welcome Hearth.com Guests and Visitors - Please enjoy our forums!
    Hearth.com GOLD Sponsors who help bring the site content to you:
    Hearthstone Soapstone and Cast-Iron stoves( Wood, Gas or Pellet Stoves and Inserts)

Insulating a wall with a lot of glass

Post in 'DIY and General non-hearth advice' started by SolarAndWood, Dec 12, 2009.

Thread Status:
Not open for further replies.
  1. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    I am siding a wall in the next couple of weeks that has 36 windows and 3 doors in it. The wall is 2x8 on the lower walkout and 2x6 on the main floor with 3/4 cdx sheathing. The windows sit right next to each other with 2 2xs in between. I am using rough cut pine ship lap siding and wondering if it is worth it to put a layer of foam on the outside of the sheathing. There is 2 feet below the windows on the walkout, 4 feet between the top of the walkout and bottom of main floor windows and then 4 feet above the main floor windows to the soffit.

    My guess is that because the windows make up more than half the area of the wall and the foam would make the siding job a lot more complicated that it isn't worth doing foam. This seem right?

    Helpful Sponsor Ads!





  2. EKLawton

    EKLawton Member

    Joined:
    Oct 18, 2009
    Messages:
    130
    Loc:
    central pa
    with that much glass id do what ever i could to raise the r-value of the rest
  3. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    I am foaming the stud bays but the struggle is all the shimming around the windows to accommodate the foam when the foam will cover less than half the wall anyway. The house is on an exposed windy ridge. The recent windstorm with single digit temps has me reconsidering this.
  4. Fi-Q

    Fi-Q Feeling the Heat

    Joined:
    Mar 5, 2009
    Messages:
    273
    Loc:
    Bonaventure, Quebec
    Mee too. I would insulate evrery suqare inch off the window that can be insulated.
  5. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    Is there any insulation in the wall right now? If so, you have "diminishing returns" by adding any more, especially if it complicates the new install of siding.

    Oh wait. I see later you said you are foaming the stud bays. If the walls would otherwise be uninsulated, I'd say definitely go ahead. But if the foam is an issue, why not use foamular or a similar XPS rigid board? Stays in place, and won't need to shim just to insulate.
  6. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    Diminishing returns is definitely the issue. Certainly, laying the pine directly over the sheating is the easiest thing to do. If I used a thin foam board, I could probably get away without shimming. But if I used the 2" board like I usually do, I think I would have to shim around the windows to keep the wood trim stable.
  7. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    One inch foamular is R=5, which is a lot better than nothing. Assuming your uninsulated wall would have R=1, by adding foamular 1 inch and going to a total of R=6, you cut back your heat loss through the wall portion by 83%. Using 2 inch foamular for R=11 would cut heat loss by 91%. So the bulk of the heat loss could greatly be slowed by one inch thick foamular or equivalent.

    Do you have room for one inch thick board?

    But is there already fiberglass or other insulation on the inside of the sheathing? If so, I'd skip the outside insulation.
  8. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    It is a new wall from the foundation to the top plate on the upper floor. I stuffed fiberglass in temporarily in the stud bays but am planning on foaming before I close the wall up. So, 5 1/2 inches of foam on the main floor and 7 1/4 on the lower walkout.
  9. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    Guess I'm still a little confused. Are you saying you are planning on putting insulation of those thicknesses inside the sheathing, between the studs, where now you have FG?

    If so, you will have a high R value already, so there'd be very little to gain from adding a thin layer of insulation outside the sheathing, beneath the siding.

    If it's wind-prone, a layer of Tyvek would sure help, beneath the sheathing, especially with all those doors and windows.
  10. Fi-Q

    Fi-Q Feeling the Heat

    Joined:
    Mar 5, 2009
    Messages:
    273
    Loc:
    Bonaventure, Quebec
    Me too..... Anyway, if you already have FG insulation bettween the studs, adding the extra foam outside won't make it worst, but it's not an absoluete necessity.
  11. dvellone

    dvellone Feeling the Heat

    Joined:
    Sep 21, 2006
    Messages:
    449
    If you're spraying in foam and sheathing with 3/4 cdx seems like you've got some heft in r-value. 2" foam board against your sheathing would make siding a chore and diminishing return would surely be a concern. If you were using batts I'd say the foam board would help, but you're going to be in great shape with foam in the studwalls. With that amount of glass and with the quality of insulating you're doing on your walls I'd think that your focus should be on the quality of the windows. As much as you insulate the walls, make them impregnable to heat loss, the windows are where you're going to feel the pain of heat loss. I know the expense of the glass is insane but they're the weak link that'll be there no matter how much you insulate your walls.
  12. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    I got a smokin deal a few years ago on 43 identical metal clad wood windows. They were meant for a brick building in Utica but the drafter goofed the print and they were too wide. Needless to say, they weren't going to rework the openings to match the windows. Worked fine for me as I had to reframe the exterior walls for the trusses anyway.

    Sounds like the consensus is Tyvek over the cdx and foam in the stud bays. That is where I was but single digits and 50 mph gusts for a few days with just plywood and my temporary fiberglass had me second guessing it. The King was running wide open to keep up...kinda.
  13. Danno77

    Danno77 Minister of Fire

    Joined:
    Oct 27, 2008
    Messages:
    5,009
    Loc:
    Hamilton, IL
    shoot, that sounds like it would get the same kinda comments my house gets

    "wow, that's cool, but I sure wouldn't wanna live there"

    that's a lot of windows, and I bet they let in a lot of light, but I bet nobody sits by them in the middle of winter!
  14. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    Not sure I've heard the that's cool part of that. Most just think I'm nuts. I'm OK with that.

    The windows are all on the south and sw facing walls with a handful thrown in the other walls to make my wife happy. There is a 20 mile view to the sw which was the primary attraction of the house in the first place. This is the wall that is getting sided and it is hard to avoid the windows as they extend the whole length of the house.

    Before pic and a pic after the first half of the roof went up last fall:

    Attached Files:

  15. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    If you've got decent insulation on the room side of the sheathing, then I'd definitely vote to skip putting any insulation beneath the siding. "Diminishing returns" for sure.

    You'll be losing far, far more heat through those windows, but of course there's the view to consider. Insulated curtains, right down to the floor, might make a huge difference for night time.
  16. rmcfall

    rmcfall Feeling the Heat

    Joined:
    Nov 28, 2005
    Messages:
    308
    Where did you get these figures? I've read articles that suggest up to R-40 in walls, which would be 8 inches of extruded polystyrene. Seems that if 2" cut heat loss by 91% at R-11, then there wouldn't be much need to go much past that...


  17. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    I've looked at the insulated blinds but the cost is out of hand. I'm thinking of building a box above the windows and building a 50' wide insulated Roman blind that would go right down to the floor at night and retract out of sight during the day. Have a few design challenges to work out on that one...

    The wind is definitely the issue. If it weren't for the zoning restrictions, a wind generator would be going up instead of solar panels on the south roof. I almost wonder if I could get away without shimming if I used the 1/2" foam board over a fresh layer of Tyvek. Given that I am using pine for siding, the trim should still be supported adequately and I shouldn't have to hand nail the siding as I probably would with 2". It certainly is a lot easier to put it on now than later and I will get the benefits for the rest of my life. Maybe I'll test it out on one of the smaller walls and see how it goes.
  18. SolarAndWood

    SolarAndWood Minister of Fire

    Joined:
    Feb 3, 2008
    Messages:
    6,770
    Loc:
    Syracuse NY
    There seems to be a lot of discussion on this. I have 3 ft allocated for insulation in the attic and my architect says I am nuts and wasting money. He thinks that R38 is overkill.
  19. rmcfall

    rmcfall Feeling the Heat

    Joined:
    Nov 28, 2005
    Messages:
    308
    Just curious...where did you read that R=11 would cut heat loss by 91%?
  20. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    It's all based on the formula for heat loss via any surface, namely Q=(SurfaceArea)(temp difference)/(total R Value).

    Surface area and the temp difference can be considered constants at any point in time, allowing the heat loss to vary inversely with the R-Value.

    If an uninsulated wall has an R=1, and you add insulation to make a new R=11, you have cut the rate of heat loss to 1/11, or 9% of what it was at R=1. This is a reduction of 91%.

    If you add more insulation to make R=21, you have cut the rate of heat loss to 1/21, or approx 5%. So there is a "diminishing return" from adding insulation. If you paid $1000 for the first additional R10 material, you lowered your heat loss by 91% for your money. If you also paid $1000 for the second R10, you only got an additional 4% reduction in heat loss for your money.

    So while more insulation is better up to a point, there comes a time where the extra cost outweighs any potential savings.

    The codes and mandates and recommendations regarding insulation are mostly at the urging of the companies that make insulation. Yes, more insulation is always better, as long as someone else is paying for it.
  21. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    There are some disadvantages of foam under the siding. Bugs and rodents can burrow and make nests. And make sure, if you do it, to get panels with the right moisture permeability for your type of construction and climate.
  22. rmcfall

    rmcfall Feeling the Heat

    Joined:
    Nov 28, 2005
    Messages:
    308
    I am just trying to understand.... how are you calculating the Surface Area X Temp Difference?

  23. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    You don't need to calculate it. At any given time, the surface area and temp difference are constant, and can be dropped from the equation. This leaves "Q varies with 1/R."
  24. rmcfall

    rmcfall Feeling the Heat

    Joined:
    Nov 28, 2005
    Messages:
    308
    Ah-ha. I see.... thanks.

    It's interesting because many do suggest super insulating to save $$. I tried to attached a Fine Homebuilding article that shows increased savings with increased insulation values...R-40 in walls, R-60 in the roof. The article is too large, but I can email it to you if you like. It is titled Remodeling for Energy Efficiency. Unfortunately, the only houses studied were in northern states. It would have been nice if they had recommended R-values for homes in mixed climates.
  25. dave11

    dave11 Minister of Fire

    Joined:
    May 25, 2008
    Messages:
    604
    Loc:
    Western PA
    Well, they can recommend whatever they like, but the science doesn't lie. The general public is happy to buy into the idea that "more insulation is better," but that is, in fact, very misleading. I don't doubt they can show increased savings, but they certainly can't show cost-effectiveness, which is completely different, and a much better indicator of the usefulness of insulation.

    The key number is "BTUs saved per dollar spent on insulation."
Thread Status:
Not open for further replies.

Share This Page