the more I look at the potential of remodeling our 100-year-old house, the more I think about super-insulating the walls. I'm looking for experts and advice. (I know I can find lots of advice here, and lots of experts, too. Anyone fit into both cells of this particular Venn diagram??)


Here's the scoop; the windows need to go. That implies I might as well rip out the sash weight space and install wider windows. THAT means ripping up the metal exterior trim and metal siding. The difficulty of repairing the metal siding suggests that I just take all of it off, retrofit in rainscreen firring strips, and put up new Hardi-Plank or some such. (I hate the siding anyway.)

By the time I've gone that far, I start thinking that 4 inches of polyiso and a a full-blown "deep retrofit" wall assembly start making sense. Fortunately, my wife knows little about construction and remodeling -- I don't have to justify the cost of all this based on a five-year payback.

So, first thing... I'm open to ideas on detailing openings. I'd really like to put the drainage plane at the exterior of the foam, set the windows clear out in bucks, and have foot-deep windowsills. Good idea, or bad? And if I do set the windows out at the outer surface of the foam, what's the best way to flash them?

Second -- under the metal siding is a second layer of old cedar clapboard. I really don't want to demo it. (With what is certainly lead paint, I'd have to landfill it, instead of using it as kindling.) If I just screw foam over the top of the clapboard and chase the edges with foam (so I actually have the best air barrier at the drainage plane) do the little triangular prisms of dead air under the foam matter from an insulation standpoint?

So you know, the rest of the way in, the walls are 4" studs, dense-pack cellulose cavities, and plaster and lath. No vapor retarder anywhere in the assembly, but I presume the decades of lead paint serve pretty well as long as I seal the junction boxes and penetrations in the plaster.  

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First, no one can give you advice without knowing your climate zone (your profile page says Madison WI - is that where the house is?). Secondly, are there any historic zone limitations on exterior renovations? Third, what does your local building energy code require - here in Vermont any renovations that involve opening up walls or roofs must be brought up to new house energy standards.

There are many ways to superinsulate an existing home and all present challenges and caveats. Some people have had allergenic reactions to open-cell spray foam and have had to vacate their homes. Closed-cell is expensive and eliminates the ability of the walls to breathe as does rigid foam board on the exterior. The more insulating and air-tight a building envelope becomes - the less heat and air flux - the more important diffusive drying becomes.

A rainscreen might become essential if you use non-breatheable materials, but is unnecessary with a breatheable envelope which also makes flashing details much simpler.

Do you not want to renovate the interior of the walls? Often with retrofits in cold climates, building inward makes more sense from a hygrothermal persepective than building out.

I've been designing and building super-insulated homes for 30 years and have done a number of deep energy retrofits as well. Much of my work these days is consulting on projects like yours.

Duh! Yes, the house is in Wisconsin.

And, yes, I'd been thinking of staying away from interior demo. The work that NEEDS to be done (replacing the failing windows) is going to involve significant siding work anyway. (And, it's not a big house to start with. So that's why I'm exploring the idea of going whole hog and superinsulating the walls, and doing it from the outside.

Thanks for the heads-up about historical review. I forgot about that. I'll do that research. As for Code -- If I am going to go for this, that won't be an obstacle. After all, if I do all this work and end up only good enough to bring the energy load in line with new construction Code, I'd consider that a failure.

As for drying -- first of all, I think that R-28 outside the studs is enough to keep the structural part of the wall above the dew point pretty much all year. That should give me some margin. The house (after a good deal of shell work) is now at 2,000 CFM50. I'd like to hope that chasing the window sash weights and some other detailing as part of the window replace would get it to 1,500. I don't know how realistic it is to try to get tighter. If I could get it to 1,000 CFM50 and need an HRV, I'd be willing to do that.

The CFM50 numbers don't tell me much without the context of square footage, volume, geometry and shell surface area of the house, but if it's "not a big house" those numbers seem quite high after "a good deal of shell work". I would urge you to set a more ambitious air-sealing goal for the project, since that will not only have a significant impact on energy-efficiency, but also on durability. I would aim at no more than 3 ACH50. Above that and well-insulated homes tend to have moisture accumulation problems. Above 5 ACH50 and they also tend to be too dry inside (while still creating interstitial moisture problems).

A leaky house is not necessarily a house that can dry itself out if it gets moist, but more likely a house that will be prone to moisture problems once the insulation level is increased. While we all strive to keep moisture out of the thermal and structural envelope, all homes get wet at times and the ability to dry is just as important as preventing wetness.

This requires a balance between rate of wetting, rate of drying and moisture buffering capability, which means hygroscopic materials such as cellulose and wood rather than foam and plastic. In a cold climate, the primary drying direction is from inside to out, so a good rule of thumb is for the outer skin to be five times as permeable as the inner skin (which just happens to exactly meet the current IRC specifications for vapor retarder and breatheable weather barrier).

Moisture balance

The current practice - both for renovations and new construction - is just the opposite, and this can create potential problems that need more complex "solutions" such as vented rainscreens.

I would also ditto Bud's caution about reducing roof overhangs, as the ratio between overhang and wall height is one of the most important criteria for a weather-durable home.

Don, it has been awhile since I read this article and it isn't exactly brand new, but may have some points in it.


To add to Roberts comments, adding to the outside requires considering the overhang at soffits and gables.  I elected to extend my soffits and diminish the overhang at my gables.  However, I am adding a new roof as well.  Moving the new windows out is easy, but the in-swing doors may need to remain on the inside which involves some detailing on the exterior, ie they only make certain size threshold extensions.  Or at least that's is all I could locate.

Adding to the exterior has some other advantages, at least for my home, the added insulation extends below the floor level to the foundation and above the top sill plate, an area that always seems to get minimal attention.

If you have knob and tube wiring buried in those walls, that may also be a consideration.

My one concern about your project would be the lead paint.  Where it started out as just "cover it and forget it" it has evolved to be a greater concern, maybe not for you, but for future buyers.  Whether a super major renovation would be possible at this time or not, it would really add peace of mind to get rid of the lead once and for all at some time.  I know removing it 20 years ago would have cost a lot less than removing it today and I can only guess but, removing it 20 years from now is bound to be even more expensive.



Since 2008, there are strict EPA regulations on any renovations involving pre-1978 homes with lead paint. Though there is a homeowner exemption, it's still worth doing this work safely.

National Grid's Deep Energy Retrofit has performed over 40 whole house superinsulation retrofit projects in Massachusetts and Rhode Island with exterior insulation on the walls and roof (typically interior insulation in the basement).  You can check out the case studies, including envelope details and sections, at Building Science Corporation (BSC) website:

However if you can wait until January 2013, National Grid has commissioned BSC to create a "Deep Energy Retrofit Builder's Guide" which will give step-by-step instructions as well as all required drawings and details. 

In January 2013, The "Deep Energy Retrofit Builder's Guide"Deep Energy Retrofit Builder's Guide" will be available as a free downloadable pdf at National Grid's energy efficiency website as well as the Mass Save website (

Please note, National Grid is the electric and gas utility in all of Rhode Island, some of Massachusetts, and some of New York. 

I love Building Science Inc., Dr. Joe Lstiburek's irreverent humor and wisdom, and John Straube's broad knowledge, but they have a heavy bias toward exterior foam as the solution to almost everything, and show prejudice against other more environmentally-friendly options.

In today's new construction and retrofit market, it's hard to sell anything else, because wrapping a structure with foam is almost always the quickest (and most profitable) option, which also appears to take care of several issues at once, including air-sealing and thermal bridges.

Ironically, Dr. Joe stresses the moisture balance (as I've illustrated above), with both moisture buffering and drying being as important as preventing wetting, but his strategies are almost completely dependent on moisture sealing, shedding and deflection with little allowance for drying and even less for buffering.

I teach a course in HygroThermal Engineering (, and focus on the many unintended consequences of our design decisions. I start with a law of Murphy: all house envelopes will get wet at some point, and then focus on how to buffer and diffuse that moisture safely while also doing the best to prevent moisture intrusion. This requires the use of moisture-open and hygroscopic (i.e. natural) materials as much as possible and, in a cold climate, an exterior skin that is five times as vapor permeable as the inside skin.

These are rules of thumb that have worked well for more than 30 years. You can violate those rules only if you compensate adequately, which means really understanding the principles of mass (moisture and air) and energy flow and how they interact.

As far as I'm aware, there have been only two studies performed that deliberately injected a controlled "leak" into a wall or roof structure to either measure or model drying capability. In both cases, with relatively impermeable exterior foam insulation, the roof took 6 months to dry and the wall (with inside vapor barrier) did not dry out in the two years of the test. Ironically, the exterior foam (the "warm sheathing" approach) also raised the temperature of the sheathing and framing sufficiently to initiate and maintain mold growth and rot. Unintended consequences.

When I use rigid foam board for cold-climate retrofits, I always place it on the interior in addition to cavity insulation, use foil-faced polyiso (which has far lower global warming contribution than XPS), tape and foam the rigid insulation for an air barrier, strap for a radiant air space, and then drywall. Otherwise, if space allows, I'll cross-hatch or build a secondary interior frame to deepen the cavity and use dense-pack cellulose (the most environmentally-friendly and effective insulation) for moisture buffering, fire resistance and insect/mold/rodent resistance, using the drywall as the air barrier.

If the only option is exterior foam, then avoiding moisture-vulnerable materials like OSB becomes important, flashing and weather barrier details become critical, and a rainscreen becomes necessary. Often, the roof overhangs need to be extended as well, with gutters and downspouts to manage rainwater.


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