Insulating a wall with a lot of glass

[SolarAndWood]

Just curious...where did you read that R=11 would cut heat loss by 91%?

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.

Thanks for the education. So, the highest return solution for me is a fresh layer of Tyvek over the cdx, enough foam in the stud bays to make it airtight and then whatever is cheapest to fill the bays? Probably something like 2" of foam and then R-11 batts?

That is likely going to be true for my attic floor as well? I have 14" steel on 2' centers underneath the old flat roof. Then, as much room on top as I choose to use on top of the old roof deck. I could do 2" of foam to seal the old deck. 12" batts underneath and then 12" of blown-in on top? Gets me somewhere in the R70 range?

This is a long term investment for me, so it can be amortized over 50 years even if it does not improve the value of the house at all. The dollars saved are a bit hard to measure because my wood is currently largely free. But I imagine that gathering and processing firewood isn't going to get any easier as time goes on and someday we may be prohibited from burning and forced to buy from an energy company anyway.

 

[Fi-Q]

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.

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."

Well, I just build my house, I'm 28 and planning to keep this house untill I die. I have R36 Walls, R-60 roof, both the basement to fisrt floor crown & the first to second floor crow are 4 in thick Soya base urethane, and the whole basement is 2.5 in of that same stuff (Airmetic Soya). And no I didn't make any calculation on what I'm really going to save.

When I'm not home, I heat with electricity, and we all know that energy price will just keep on going higher and higher. As this was a new construction the price to get it well insulated was not a very big diffrence in the price so I'm sure that I won't regret it and that good insulation will certainly pay fot itself over the next 40-50 year.

 

[rmcfall]

I understand what you are saying in that the science doesn't lie. However, on the face of it it seems something must be missing. It appears that the more one can reduce heat loss, the greater their savings are going to be on gas/electric over the long haul. Also, it isn't just companies producing insulation that suggest higher R-Values than what the formula cited seems to recommend. Maybe the writers of the Fine Homebuilding article get a payback from companies...I don't know... There is also a great book by Bruce Harley, Insulate and Weatherize, where higher R values are recommended. The Department of Energy also recommends certain figures, although not as high. Based on the formula, however, why bother insulating an attic to greater than R-30?

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."

 

[SolarAndWood]

Well, I just build my house, I'm 28 and planning to keep this house untill I die. I have R36 Walls, R-60 roof, both the basement to fisrt floor crown & the first to second floor crow are 4 in thick Soya base urethane, and the whole basement is 2.5 in of that same stuff (Airmetic Soya). And no I didn't make any calculation on what I'm really going to save.

I'm in roughly the same place thinking about this, the big difference is rebuild vs new construction. Did you do the Airmetic Soya installation yourself? From their website, it looks like it needs pro installation?

 

[dave11]

If you're not actually making a numerical comparison, using R values, the areas of your walls/ceilings, and the average daily winter temperature in your area, then you can't approximate how much R value you need. Also, if you aren't figuring out the true cost of installing insulation, or adding to it, you can't know with any certainty whether it makes sense to spend any more money on insulation.

As I said before, the more insulation, the better, as long as someone else is paying for it.

If YOU'RE paying for it, you should start with a basic heat-loss analysis of your home, which you can do yourself, and see where the money should be spent.

 

[SolarAndWood]

If YOU'RE paying for it, you should start with a basic heat-loss analysis of your home, which you can do yourself, and see where the money should be spent.

I didn't find this easy to do because of the irregular shape of the house, multiple phases of construction and exposed windy ridge. The results were like 1/3 of what the previous owner's propane consumption suggested. I have just gone with the I'm going to be here for 50 years so find the best deal on better and more that I can approach. Patience and flexibility with the project has saved me a ton of money, the windows were 15 cents on the dollar still shrink wrapped in the warehouse. I'd like to do that with insulation as well but it seems I would have to go into the spraying business to avoid paying retail.

 

[rmcfall]

I hear what you are saying, but the formula you cited only calculates heat loss, not savings.

R-20 cuts loss by 95%

R-40 cuts loss by 99.975%

So it will cost the same amount as it cost to cut loss by 95%, as it will to cut that additional 4.975%. Clearly, cutting out the 4.975% of heat loss costs more to insulate. The upfront cost is relatively high, compared to the cost to cut the first 95% of heat loss. However, if it is not done, how much is that heat loss of roughly 5% going to cost over the years? That will depend on the cost of utilities, which will likely continue to rise. Short-term, this doesn't seem like a big deal. But long-term seems like a different story altogether.

 

[dave11]

I hear what you are saying, but the formula you cited only calculates heat loss, not savings.

R-20 cuts loss by 95%

R-40 cuts loss by 99.975%

So it will cost the same amount as it cost to cut loss by 95%, as it will to cut that additional 4.975%. Clearly, cutting out the 4.975% of heat loss costs more to insulate. The upfront cost is relatively high, compared to the cost to cut the first 95% of heat loss. However, if it is not done, how much is that heat loss of roughly 5% going to cost over the years? That will depend on the cost of utilities, which will likely continue to rise. Short-term, this doesn't seem like a big deal. But long-term seems like a different story altogether.

See my prior post before this one.

Also, spending more than you need to on insulation can't be defended by saying "It will pay back in the long run." While this might be true, you will have lost $$ overall by not having spent it on other areas more needy, like windows or doors. Upgrading your windows from R2 to R3 will save you much more money over time than increasing your attic from R20 to R40. You will also have lost by not putting it into your retirement account or another safe investment, where over 20 or more years you would have made much more than the little you saved in heating bills.

 

[Fi-Q]

Well, I just build my house, I'm 28 and planning to keep this house untill I die. I have R36 Walls, R-60 roof, both the basement to fisrt floor crown & the first to second floor crow are 4 in thick Soya base urethane, and the whole basement is 2.5 in of that same stuff (Airmetic Soya). And no I didn't make any calculation on what I'm really going to save.

I'm in roughly the same place thinking about this, the big difference is rebuild vs new construction. Did you do the Airmetic Soya installation yourself? From their website, it looks like it needs pro installation?

I had it done by a professional. The house is 28x36. The 2 crown and the full basement cost about 4500$. Real nice job. I'm not sure if it'S really worth for the basement, but definatlly worth it for the crown. It ''Seals everything. My house is very tight... too tight, I will need a air exchanger.

 

[dave11]

If YOU'RE paying for it, you should start with a basic heat-loss analysis of your home, which you can do yourself, and see where the money should be spent.

I didn't find this easy to do because of the irregular shape of the house, multiple phases of construction and exposed windy ridge. The results were like 1/3 of what the previous owner's propane consumption suggested. I have just gone with the I'm going to be here for 50 years so find the best deal on better and more that I can approach. Patience and flexibility with the project has saved me a ton of money, the windows were 15 cents on the dollar still shrink wrapped in the warehouse. I'd like to do that with insulation as well but it seems I would have to go into the spraying business to avoid paying retail.

Sorry, Solarandwood, I think I'm letting your post get hijacked. Other folks should start another thread if they want to debate this insulation issue.

Hope the project turns out well for you.

 

[rmcfall]

My apologies if I have hijacked this thread. I guess I did not see this as an entirely different topic, as I thought it was relevant to the original poster's questions.

 

[dvellone]

One thing to consider SolarAndWood is air infiltration. I may be stating the obvious but, if your exposure is subject to consistent high winds more insulation won't make much difference if you have much air infiltration. Caulking and taping all seams goes a long way to increasing r-value and is an economic approach as well. We probably all know that the listed r-value of fiberglass batts is an ideal that's measured in a controlled environment. In other words once you introduce air leaks and compression the r-value drops considerably.
Are you going off-grid with your solar installation, or adding panels as a supplement? I'm in the west-central adks and power up my house with a solar system. I'm doubling my solar rack soon and between the federal and state credits it's a 55% rebate!

 

[SolarAndWood]

Hope the project turns out well for you.

Thanks for your input Dave.

 

[SolarAndWood]

My apologies if I have hijacked this thread. I guess I did not see this as an entirely different topic, as I thought it was relevant to the original poster's questions.

No apology required.

 

[SolarAndWood]

One thing to consider SolarAndWood is air infiltration. I may be stating the obvious but, if your exposure is subject to consistent high winds more insulation won't make much difference if you have much air infiltration. Caulking and taping all seams goes a long way to increasing r-value and is an economic approach as well. We probably all know that the listed r-value of fiberglass batts is an ideal that's measured in a controlled environment. In other words once you introduce air leaks and compression the r-value drops considerably.
Are you going off-grid with your solar installation, or adding panels as a supplement? I'm in the west-central adks and power up my house with a solar system. I'm doubling my solar rack soon and between the federal and state credits it's a 55% rebate!

Air infiltration is brutal. We are on one of those red spots on the wind maps. Anytime it clocks up like that it has a westerly component. So, I will probably do all foam on that side of the house and just do fiberglass after sealing the seams with foam on the rest. Definitely going to go through some caulk on that west wall.

 

[Gooserider]

I don't know if it would apply in this case or not, but I have seen some people argue in favor of 1" or so foamboard on the outside as a way to prevent thermal bridging on the part of the framing... At least w/ a 2x4 stud wall, the claim is that this gives major advantages in that it breaks the thermal path through the wall offered by the framing members... Essentially its the same idea as the wall designs I've seen where they make a 6" thick wall by staggering the 2x4 studs so that there isn't any stud that touches both the inside and outside wall surfaces...

Gooserider

 

[SolarAndWood]

There was a recent series on This Old House where they did a factory built house I think in VT. As I recall the walls were triple sheathed to achieve the higher performance and eliminate the bridging. That is another option I considered. Shim the outside horizontally, run 2" foam board, sheath with OSB, then Tyvek outside that. The high winds are really the variable I haven't found consensus on. I'm thinking the extra expense might be worth it given the long horizon in the house.