If you’re trying to lower your energy bills, you have probably plugged many of your home’s air leaks and have added insulation to your attic floor. Now you may be wondering, “What should we do about our old windows?”
Unfortunately, there is no clear answer to this question. Sometimes it makes sense to leave old windows exactly the way they are. Sometimes it makes sense to repair the windows’ weatherstripping and add storm windows. And sometimes it makes sense to replace old windows with new energy-efficient windows.
Before providing a framework to help you decide what to do, it’s important to address a few basic questions about window replacement.
Will new windows save energy?
Here’s the good news: if you install new energy-efficient windows, your energy bills will go down.
Here’s the bad news: your bills won’t go down as much as the window salesman promised. In fact, your new replacement windows will save you so little money on your energy bills that the payback period for this investment may be more than 100 years — far longer than the new windows are likely to last.
Of all the data-crunchers who have looked closely at energy savings attributable to window replacement, none are more credible than Michael Blasnik, a Boston-based energy consultant with access to utility bill data for millions of U.S. homes. “I’ve looked at a lot of window replacement data,” Blasnik explained at the Building Energy 12 conference in Boston. “I’ve heard window salespeople say that you can save 50% on your heating bills if you replace all your windows. In fact, the amount of energy saved by replacing all of the windows in a home is generally on the order of 1% to 4% of the heating energy usage.”
Exaggerated marketing claims by companies…
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What a wonderful, fair and balanced article, I don't think I have ever read something of Martin's where I agreed with every single word. I have been saying this for years, but I am Past President of a big heritage conservation org so everybody thinks I have a vested interest. (I do) To see this here is just great.
The problem with the replacement window industry (like the solar panel industry) is that it is visible and in everyone's face; your friends don't notice when you seal your ducts or caulk. People like to see where their money goes and a lot of it goes into windows. And as Donovan Rypkema notes, they are called replacement windows for a reason: every 15 years you are going to have to replace them again.
Thank you Martin, for an important post.
Response to Lloyd Alter
Thanks very much for the feedback. I appreciate it.
We have done a fair amount of renovation and weatherization work to old (100 years+) windows. The gold standard is interlocking weatherstrip like Accurate Metal's. But it is indeed expensive work. I would guess it's actually pricier to put in interlock and a Low E storm window than to install a decent replacement unit. So as much as I love old windows and hate to see them ripped out, I have to admit the "full monty" doesn't make sense unless the windows and house are of an age and character to merit it.
There are cheaper options too. Q-lon, Conservation Technology and others make good quality bulb weatherstripping that can provide a decent air seal at much less cost.
And agreed, a really fantastic overview that I will share widely.
How about low-e window film? For example, "Gila LES481 Heat Control Residential Window Film, Platinum, 48-Inch by 15-Feet" on Amazon. It seems to be most popular in warm climates for rejecting solar heat gain (and has lots of reviews attesting to that), but it also claims to help in cold climates because it has a low-e coating. I could do all my house's windows for a couple hundred bucks. Seems like it's low hanging fruit.
Response to Nick Welch
Window films have a mixed reputation. The biggest downside to window films is that they aren't very durable. How long do they last? The length of time varies, and is hard to predict.
Another problem is that most window films affect the view through the window. Most window films significantly darken the glass.
The traditional use for window films is to reduce solar gain in hot climates. Used that way, they can be a good solution for specific windows that are hard to shade.
It's hard to say whether the low-e films make sense in cold climates, especially in light of the drawbacks I mentioned.
Finally, according to Patrick Stuart (see Comment #21 on this page), the use of window films can void some window warranties.
For more information, see New Low-e Glass or Window Film?
what part of the window gets hot?
185 degrees? The glass? which pane? both? The one with film?
Replacement of just the sliding sash on double-hung windows is an option oddly unmentioned above. It's most suitable for single-glazed double-hung windows with sprung, not counterweighted sash, the kind very common in the many brick ranch homes in our area built from the 1960's to the 1980's. Less expensive and less disruptive than replacing the entire window, and giving a substantial performance improvement at reasonable cost. The air-sealing benefits are as important as the glazing upgrade: not only are the sliding seals better on the new sash units, but as the interior trim is removed in the process it allows the remaining window surround to be properly caulked to the framing for the first time in its life.
Old wood windows reasonably restored will long outlast the low priced vinyl replacement windows .
When old wood windows are replaced by the typical installer, the installation is quite often going to cause wall moisture damage from poor installation details.
And yes, we do tune up and weatherstripping of windows and doors in New Orleans.
Detail For Eventual Replacement
One of the unanticipated consequences of building walls with a rain screen cavity is that you can easily detail your openings and trim so that the entire window can be replaced without compromising the flashing and air sealing. It's worth thinking through on new builds, making life a lot easier when the question of replacement inevitably comes up in the future.
Response to AJ Builder
Q. "What part of the window gets hot? 185 degrees? The glass? which pane? both? The one with film?"
A. I assume that your question refers to the modeling study performed by Thomas Culp regarding low-e storm windows installed on the exterior side of double-glazed low-e primary windows.
My understanding is that Culp is referring to the temperature of the air between the storm window and the primary window. Of course, if the air space between the two windows is at 185°F, then so is the exterior side of the primary window. The worry is that this high temperature can damage the seals at the perimeter of the IGU (the insulated glazing unit) on the primary window.
The air space between the two windows gets hot because low-e storm windows have high-solar-gain glazing. If an ordinary single-pane window has an ordinary storm window without low-e glazing, the heat that accumulates between the two windows can flow outward through the storm window and inward through the primary window. However, with a low-e storm window, the heat is trapped in the space between the two windows by the low-e glazing on both sides of the space.
Response to James Morgan (Comment #7)
There are several types of replacement window on the market. Homeowners in the market for replacement windows can choose new sash with double glazing, new insert windows (sash plus frame), or even new construction windows with flanges (if there is any reason to create a larger rough opening, for example).
When I was a builder, I installed Marvin's Tilt Pac system. This type of double-hung replacement window consists of new sliders and new sash.
There are advantages and disadvantages to all of the various types of replacement windows on the market. As far as I'm concerned, however, they are all replacement windows. Once the old sash are in the dumpster, you've replaced the window.
Response to Martin #9
Semantics and local usage I guess Martin. We generally differentiate between window replacement and sash replacement partly because almost all the total replacements we see are vinyl, substantially changing the character of the house. Sash replacements are not such a well-known option and I believe they offer substantial benefits. Not least, when I see a 1940's craftsman fitted with cheap vinyl replacements I know it's only a matter of time before a more sensitive owner takes them to the dump. New wood sash can be a seamless, durable upgrade to an older home for about the same cost as a cheapo full replacement.
Lear paint and LSWP
As Martin mentioned, homes and child occupied facilities built before 1978 potentially harbor lead in the paint.
Lead can cause neurological damage in children under 6 and in the unborn in pregnant women.
And in adults elevated BLL (blood lead level) can cause high blood pressure and other health effects.
Contractors working on all homes, not just the ones with kids, are required by the EPA to have a Certified Renovator on the job and the firm, sole proprietor or corporation, be a Certified Firm with the EPA.
The EPA has not done a good job of educating the public and many homeowners and contractors bypass the practice for cost reasons or a lack of understanding.
As Martin said, more can be learned on the EPA lead information site.
Follow up to Malcolm #9
I had installed good double pane sash kits in my 200+ colonial when we moved in. Unfortunately the original windows were to far gone. Some literally falling out of the opening. As I am now redoing the walls from the exterior side ( air sealing, Roxul, plywood replacing old boards, exterior foam board, rain screen, vertical grain claps, galvanized rose head nails etc...) Point being you are correct. I made PVC boxes for the sash kits to be installed in. Once you detail and flash these right; in the future if I want to replace the windows it does not effect the trim at all. Pull old out pop in new. Debated for a long time about PVC on 200 year old house but eventually broke down. Not painted yet but I think it is hard to tell all the trim is PVC.
Martin any thoughts on indoor storms in cold climates?
Looks great. When I'm doing renos I often spend quite a bit of the time cursing the original builder. But when I do come across some nice craftsmanship, or see how intelligently something was thought out I feel a real affinity with whoever did the job. A few decades from now some carpenter is going to go to work on your windows and smile.
Response to Terry Grube (Comment #14)
Q. "Any thoughts on indoor storms in cold climates?"
A. Interior storm windows can work well in some locations. The major disadvantages of interior storms are:
1. Most interior storms are not operable, so they are not appropriate for egress windows or windows that are occasionally opened for ventilation.
2. Interior storms are more likely to have problems with condensation and mold than exterior storms. Whenever an opening has two windows, it's better if the exterior window is a little leaky and the interior window is as airtight as possible. With interior storms, this ideal situation isn't always achieved. If the exterior window is relatively tight, and the interior storm is a little bit leaky, warm interior air can contact the cold window panes of the exterior (primary) window, leading to condensation and mold.
insulated window panels
The only way to seriously increase whole house r values is through closing window panels that are insulated to wall r value. For the past 4 years I have run an experiment on our 10,000sq ft house in Canada at 4200 foot elevation 50dgree latitude. zone 6 at least, Every window in the place ( 800 sq ft of windows has closing exterior or interior edge sealed r 25-30 polyiso filled wood panels. The difference is enormous. The window are all fiberglass double glaze argon e. But really windows at r 3.5? that is pitiful. Triple glaze at r10 for $100 sq ft. ? Absurd economics (but maybe nice aesthetics). So panels self built at $12 sq ft. The only warning...open during the day, direct sunlight on inner closing will crack double glaze. Outer closing are preferable if convenient access.
Great article - what about interior storms?
I agree with Lloyd, this is a very balanced and usefule article. I expecially like that the point is clearly made that ROI for new windows is not really there. What should also be considered is the improvement in comfort when you repair or replace old windows. This is something that is truly priceless, but never factored into discussions. I can sit near my repaired 100 year old double hung windows and be...comfortable, becasue I have instaleld weatehrstripping and also repaired the storms.
I also am currently exploring interior storms. A company in Hoosick Falls, NY makes a very tight double layer film product that is durable and very thin-framed, preserving the view range. They manufacture to specific sizes and have units that can be removed for summer or with "doors" to allow opening of the window even if the storm stays in place all year. Interior storms seem to avoid the condensation issues many exterior storms inadvertently create if the actual window is leaking moist air cosistently.
Hey - I just saw the comment post on interior storms. Funny, the reason they seem to work (where I've seen them) is that the interior storm was more airtight than the old poorly performing double-hung!
Response to Jodi Smits Anderson (Comment #18)
Thanks for your comments.
I addressed comfort in the final paragraph of my article.
I addressed indoor storms in Comment #16 of 7/28/14.
You wrote, "Interior storms seem to avoid the condensation issues many exterior storms inadvertently create." In fact, my observations are different from yours. In my experience, windows with interior storms are more likely to have condensation than windows with exterior storms, for the reasons explained in Comment #16.
[Postcript: It seems that you and I were writing simultaneously. While I was composing Comment #20, you were composing Comment #19. I see that you located and read my comment on interior storms.]
Another issue not mentioned about window films is they can void the window warranties. The manufacturers say the films can overheat in direct sunlight and break the glass. Whether this is true or more of a CYA clause is arguable, but a voided warranty on an expensive window can cause problems down the road.
Replacing toxic old windows makes a lot of sense
It looks like the lead in old windows might be a much bigger issue than energy inefficiency or historic beauty.
A recent Mother Jones article by Kevin Drum makes a number of eye-epening assertions about the toxicity of lead and about the benefits of removing it. ("America's Real Criminal Element: Lead. New research finds Pb is the hidden villain behind violent crime, lower IQs, and even the ADHD epidemic. And fixing the problem is a lot cheaper than doing nothing."
At least one study has looked specifically at the cost-benefit of replacing windows that have lead paint and found that,
"The benefit per resident child from improved lifetime earnings alone is $21,195 in pre-1940 housing and $8,685 in 1940-59 housing (in 2005 dollars). Annual energy savings are $130 to $486 per housing unit, with or without young resident children, with an associated increase in housing market value of $5,900 to $14,300 per housing unit, depending on home size and number of windows replaced."
The fact that today approximately 35% of American housing has lead paint is at least partially due to the scorched earth resistance strategy paint companies employed to resist regulation before 1978 and escape responsibility afterwards. Amazingly Benjamin Franklin wrote that he saw neurotoxic effects from lead exposure 200 years before we finally took it out of paint and gasoline.
I've done window rehab, and I don't think I'll ever do it again. I had planned to completely rehab my own 15 double hung divided light historic wood windows until I read this stuff. Children are more beautiful than old windows.
Don't forget the history part of it
One point that none of these GBA articles have made is the role that the actual historic value of the windows plays, and how to assess this. This, in my view, is by far the most important starting point for all the other issues discussed in these articles. The age of the window is the first and most important criterium in deciding how to proceed. For example, if you're looking at a pre-1850 window, then preserving it--maintaining its structural and aesthetic integrity, while doing as little as possible to disturb the historical integrity--becomes by far the most important strategy in assessing how to proceed. Dealing with a pre-1850 window means maintaining that fine balance between restoration (structural integrity) and preservation (historical integrity). Do what you need to do maintain a high level of structural integrity while at the same time fanatically retaining historical integrity.
Only when this balance of restoration and preservation are dealt with should one bring in energy efficiency as an additional element: "Now that we've got our approach for dealing with restoration/preservation, how can we bring in efficiency here without jeopardizing or disrupting the first two values?
As you move into the 20th century this dynamic, or tension, of course changes.
Response to Norm Farwell (Comment #22)
Like you, I take lead safety seriously. Back in the early 1990s, when I was involved with rehab work on multifamily projects, I went through the training and examination process to be certified in lead abatement in Vermont.
Lead dust is dangerous to children. Double-hung windows are especially dangerous, because opening and closing the sashes tends to create lead dust that accumulates on the sill and stool.
Lead paint hazards should be taken seriously. If readers have older windows that test positive for lead paint, window replacement by a contractor familiar with lead-safe work practices is one option -- but it's not the only option. It's also possible to pay a lead abatement contractor to safely remove lead paint from the windows.
Response to Roger Brisson (Comment #23)
Your stated opinion -- that "if you're looking at a pre-1850 window, then preserving it ... becomes by far the most important strategy in assessing how to proceed" -- is shared by many preservationists. It makes sense in areas of the country where pre-1850 houses are rare.
In areas of the country where pre-1850s buildings are a dime a dozen, not all homeowners can afford such a meticulous approach to preservation. Buildings are part of the fabric of our community, and they change all the time. Not every older building can be preserved in amber, as if it were a museum exhibit.
response to Martin and Roger
I agree with Martin that not every old building needs to be preserved. Here in Maine, there are many thousands of old houses, with little or no historical significance. My own house was built around 1790. Around 1890 or so, someone removed the hip roof and replaced it with a steeper gable roof. I'd be crazy to restore it. It's just an old house.
Two hundred years from now, will someone want to preserve every McMansion that managed to survive?
By far the largest source of lead in the bloodstream was auto exhaust. The end of the production of leaded fuel marked the swift decline in atmospheric lead, and lead in the bloodstream. Lead paint is certainly an issue, but not one that I think is part of this particular decision making process.
Response to Keith Gustafson
I guess that we will just have to agree to disagree on this one.
If a homeowner is pregnant, or if the family includes preschool children, it absolutely makes sense, in my opinion, to test the paint on old windows for lead content before deciding how to proceed with any planned renovation work.
and another thing
"overspending" on windows can have side benefits. When finishing my own remodel[not primarily an energy retrofit] I realized how close I was to not needing a boiler. Unfortunately the old boiler needed to be one of the first things to go. Had I known from the beginning that we would be able to do as much as we did, I could have gone a bit further and depended on the minisplits entirely for heat. That would have saved over 10k even after buying a water heater.
Building America Report on Window Replacement & Repair
Great column Martin! (and comments too) FYI, if anyone is interested, my colleague Peter Baker spent some time writing a report for DOE (Building America) on window replacement and repair. The focus of the analysis was *not* energy savings and cost-effectiveness: it was more, "If you're going to do a repair or replacement, here are recommendations on how to maximize airtightness/energy efficiency, and avoid moisture problems." In the bibliography, it should reference the Canadian papers that spent a lot of time talking about condensation risks when using interior storms. They're your tax dollars at work--please make use of them!
Building America Report 1203: Measure Guideline—Wood Window Repair, Rehabilitation, and Replacement
Response to Kohta Ueno
Thanks for the link to a valuable resource. I have added a link to the document in the "Related Articles" sidebar on this page.
And thanks for seconding my advice about condensation risks when using interior storms. I'm guessing that the Canadian paper you are talking about is A. G. Wilson's 1960 report, “Condensation Between Panes of Double Windows.” It is available online at this link.
In that paper, Wilson wrote, "The inner pane assembly must be much tighter than that of the outer. Unless the inner pane assembly is extremely tight, openings past the outer pane assembly must be located both above and below the mid-height of the air space in order to induce air flow into the air space by chimney action. It will often be necessary to provide intentional openings between the air space and outside."
I was responding more to the 'toxic' windows post, not your comments which are basically legal requirements in my state. While I may feel restoring old windows is not particularly worthwhile, I don't see the presence of lead paint as way up on the decision tree. Doing anything with lead painted trim will release lead, whether fixing or replacing, and adequate precautions need to be taken. One could make the argument that leaving the old windows in might release less lead than tearing them out. I wouldn't make the argument but one could.........
Future historic pres
I don't think we have to worry too much about preserving the McMansions of the past 30 years. I can't imagine too many of them surviving past 50.
What should I do with old windows
Great article Martin, and I appreciate your addressing the topic, because there is a tremendous number of older windows, particularly in the North East, that are stimulating this concern/mental argument.
As you know, my belief is that insulating shades offer a quick, relatively cost effective solution to comfort and energy waste related to older windows. What else delivers as much as an R-4 (per your quote from Peter Yost’s research) value for the cost that a homeowner can easily do themselves – reducing the hassle surrounding a solution. Anyone in the US can go to CellularWindowShades.com and buy, for example, a custom shade that is made in America, 30” x 42” shade for $69, delivered. This solutionhandles a great deal of this problem in terms of protection from cold and hot spots, – or for an additional $78 they can get energy saving side tracks for the full R Value benefit Peter is speaking about. In our experience, the many available “how to” manuals, chat and videos makes this a successful DIY solution virtually 100% of the time. It also eliminates the “fuss” surrounding other contemplated solutions. Furthermore, cellular shades also help with privacy, glare control and decorating, which other solutions may leave undone. More benefit, they play well with conservationists – recommended by Preservation Trust Vermont who have used them repeatedly, successfully.
As for your reservation that they require management. Isn’t that their strength? On a freezing cold day they can be raised to allow ALL the light and sun’s warmth and lowered at night to trap it inside. The weather and the solar situation changes constantly – and so a homeowner needs the ability to manage their solution to be fully effective. Otherwise something working effectively on a dark cold night is applying the same impact on a sunny warm day. As you say, Peter may forget to lower his shades, but more likely on a mellow night in May. I suspect he is more likely to remember when it is 10 below and blowing in January.
You finish with:
‘What’s the bottom line? If you live in a cold climate, and your house has older single-pane windows and no storms, you should invest in low-e storm windows. If you live in a hot climate, you may want to invest in window film to reduce solar gain.”
Do windows in the north not get hot in summer and those in the south get cold in winter – particularly with our weather extremes?
“It’s hard to justify any other improvements to existing windows on the basis of energy savings. That said, many people want to repair or replace older windows for other reasons. If your older windows are drafty, or if you skin feels cold when you sit beside a window on a winter night, you may be happy to spend $400 or $500 per window for improved comfort.”
My sense, if its drafty and your skin feels cold, you may also be happy, and in fact will, lower the $69 shade - that you installed yourself.
Response to Gordon Clements
You sell window shades, so it's no surprise that you tout their benefits.
I stand by my statement that the installation of cellular window shades isn't cost-effective. Moreover, it's hard to claim credit for energy savings that require homeowners to operate the devices twice a day, all winter long. It would take extensive field testing with average families to determine the level of savings that these devices achieve.
@Norm - Benjamin Franklin was pretty late to the party. The Romans knew of lead's toxicity at least as early as 25 BC or so (although they were pretty bad at avoiding it).
RE: Cellular Shades
My old apartment was ground level, so we used a cellular shade for privacy and kept it closed 24/7. It fit pretty tightly, so it did work fairly well at insulation... but it wasn't at all moisture proof and as a result we had substantial condensation & mold in the winter.
As far as remembering to operate shades twice a day... anyone know of any motorized shades? I have a couple larger windows in a room that I'd like to insulate at night for comfort, and I want to automate the opening and closing...
Response to Tim C
I have never owned, installed, or operated motorized window shades -- but I know how to Google.
Google has more links on this topic than you can shake a stick at.
In regard to lead abatement - would it not be effective, if lead were the main concern, to undergo a more passive restoration of windows by leaving them in the casing and working from the outside?
Our 1910 bungalow has original sashes that remain slopped in exposed white lead on the exteriors, while the interior has been remediated. We have a 4 year old son, and do not want to raise high levels of dust for any reason. We have heard that any kind of disturbance, whether by renovation or replacement, even with extra precaution, will raise lead levels in a house. Why not pull the storm from the outside and use lead-rated stripper and soap on the exposed areas?
I suppose for the purpose of the forum, one should stick to recommending a lead abatement contractor in this case.
Thanks for a great article. We've been contemplating replacement of the aluminum frame windows in our mid-century modern home in the NW but this has swayed me toward keeping them.
One comment: it seems to me that pay-back periods should reference both current energy costs and future energy costs. Having said that, even cutting a 100-200 year pay-back in half by doubling energy costs does not necessarily affect the decision-making process.
Response to Asa Bradford (Comment #38)
1. Raising and lowering window sash generates friction that transforms old paint into paint dust. Even if you try to remove the paint from one side of your sash, you may not have eliminated all of the paint that generates dust.
2. Unfortunately, plenty of well-meaning homeowners have tried a DIY approach to lead abatement, and some of these efforts have resulted in tragedy (namely, increased blood lead levels in the family's children). I will not attempt to explain lead safe practices here. I will only comment that homeowners and their children have been poisoned by a variety of methods, including scraping, sanding, and the use of heat guns. It makes sense to consult an abatement contractor.
Response to Chris Koehn (Comment #39)
You are right that many people who perform payback calculations like to polish their crystal balls and try to estimate future energy price inflation. I discussed these estimates and their effect on payback calculations at length in one of my articles. Here's the link: Payback Calculations for Energy-Efficiency Improvements.
New field study available for window restoration
Field studies on window restoration methods were performed in 2011 as part of writing the Window Preservation Standards, http://windowstandards.org/ I recommend this publication be added to your list of window web sources for energy retrofits on old windows.
For the WPS, real world windows were restored in the field by a variety of methods, then field tested for air infiltration. Several combinations of common restoration and weatherstripping methods met the IECC 2012 standard. The best performer did not include a storm window, but simply included a new, weatherstripped stop across the sill on the inside where the bottom rail meets the sill. Such sill stops are uncommon on historic windows, but common on any modern window. Adding one for a couple bucks offers excellent payback on a window restoration job.
As for lead paint on old windows, remember that some parts aren't supposed to be painted and weren't painted originally. These unpainted parts include wherever components slid across each other in friction: the track of the lower sash, the side edges of the sash, and margin of the face of the sash stiles where it ran along the stops. The problem is that homeowner and painters who didn't know better painted these areas. This is bad because it introduced paint where it is most likely to fail and several coats of paint in these locations degrades the air infiltration quality of the window. And since these painted surfaces are in friction, they fail to dust and chips. Proper mitigation would remove paint from these surfaces and leave them unpainted.
Thank you for the pointers Meyer. I should have pointed out that we are fortunate (?) to have windows that never received such mistreatment. All sliding surfaces of the lower sash remain unpainted, and only the exterior rails have lead paint. In this case, it seems it would be better to remove the exterior paint without dismantling the windows.
I do believe that remediation is built on principles of conservative approach. I would add that preserving historic character is easier to justify in cases where there hasn't been excessive butchering in the past. If you have to replace half the useable material on the window, you are getting into murkier territory regarding the value of restoration. If, as Meyer said, the windows were painted over in places they shouldn't, you now have a bigger problem. With so many details to consider, it really seems like the ultimate decisions are personal and emotional. The real payoff is whatever gives you peace of mind, whatever that is!
Thanks for the article! One of many excellent write-ups from this resource.
seems like the math or ROI would apply to new construction also
Thank you Martin;
As I design my own house, and begin to contemplate how much to spend on new windows for it, I know there is a diminishing return on investment. After a point you start paying a significant amount for less and less gain. If you double the R-value, how long will that take you to break even on your heating bills? The take away message I get from the article is it probably isn't worth spending too much on "better" windows.
"In fact, your new replacement windows will save you so little money on your energy bills that the payback period for this investment may be more than 100 years — far longer than the new windows are likely to last."
It would seem the million dollar question is where is this breakover point? What is good enough, and when is it good enough? I live in Colorado at 5000' elevation, and I can't tell you how many houses I have been in with 10 to 20 year old, relatively quality windows and it's clear that the window seal has broken, fogging it under the right conditions and ruining the R-value. I have heard it has to do with our elevation. Windows are one of the most expensive parts of a house, so it seems criminal that they aren't made to last longer.
Interior storms can work. Insulated shades trouble
Martin wrote (#16) (a) “Interior storms are more likely to have problems with condensation and mold than exterior storms.” (b) “Whenever an opening has two windows, it's better if the exterior window is a little leaky and the interior window is as airtight as possible.…”
Your point b above jibes right with the Building Science/Baker study and the AGWilson paper. But as with Jodi (#19), my experience with interior and exterior storms is that their installation is often prompted by drafty windows and the added storm window, whether exterior or interior, tends to be tighter than the original window. In these cases the exterior storm window is more likely to cause problems with condensation than the interior storm window.
Either strategy can be a good one if we keep point b above in mind. There are interior storms that are being done well and they can be pretty darn airtight.
Regarding pleated or insulated shades, I’ll second Tim (#36). While they can make the window opening feel warmer to the occupant, they create a similar potential for condensation since it’s really, really hard to make the shade airtight. Airborne moisture is going to reach the cold window glass. It’s a super easy way to get condensation (ice!) and mold on the window and even the back of the shades.
Pleated shades won't give insulation, only cellular shades will give adequate insulation for temp and sound. Cellular shades comes with different R-value so highest is the R-value the better.
I'm in a cold climate with historic single pane windows with permanent exterior aluminum storms. They're all very leaky. Can I add interior storm windows too? Are there any risks to having both interior and exterior storm windows installed?
If you already have storm windows, the solution to air leaks is not to add another storm window. The solution is to install weatherstripping to reduce air leakage. Weatherstripping manufacturers make a wide variety of products to address all types of windows.
Interior storms can lead to moisture problems, depending on where the air leaks are.
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