Article from Building Science Corp., Dr. Joseph Lstiburek BSI-095, May 13, 2016
Buildings and cities age - as do humans. Sometimes we get better and sometime we do not. Some of us eat healthier and get more exercise. Some of us do not. Some of us look better as we get older. Some of us do not. So where am I going with this?
We all want to live longer and live better. I think we want our buildings to do the same. It is instructive to see how some cities and buildings handle this. Think New York, Paris, London, Prague...you get the idea.
Let me start out with a couple of observations based on being around a long time. What is aging our buildings are three things: water, heat and ultra violet radiation. Of the three, water is the worst. And the thing about water is that it is not how wet something gets, but does it dry after it gets wet. And the thing about drying is that it requires energy. And there is the rub. As we make our older buildings "more efficient" and "more comfortable" we reduce energy flow and they stay wet longer and that is a problem. So can we make older buildings more efficient and not have them age faster? Can we in fact have it all? Can we make them more efficient and have them last longer? Yes, but the approach will irritate some. Or do we end up screwing it up so badly that all we have done is euthanize them?
So we have been doing water control for a long time - both the rainwater kind and the ground water kind. What is the big deal? Well, the urgency of getting it right has changed. Huh? Uninsulated mass buildings could tolerate wetting because of their high drying potential. We had lots of time to get around to fix stuff. But not any more. We are insulating them - and insulating them with not just a little insulation but a lot. And when we do that and don't get the water control right we get ugliness Freeze-thaw big time. Yes, we are seeing more and more freeze-thaw. And now you know why. There is no such thing as a free thermodynamic lunch. There are consequences for reducing energy flow.
Are there consequences everywhere? Yes, but they are "worser" in some places than others. What do we know? Well for freeze-thaw problems you need cold and you need wet. Take Map of Hygrothermal Regions and look at the critical hygrothermal regions for freeze-thaw - cold and very cold climates since they get the most freezing cycles. Now take Map of Rainfall Exposure and look at the critical rain exposure regions for freeze-thaw - moderate rain exposure and higher. Now superimpose them (Freeze-Thaw Risk). That's where you absolutely have to intervene on the water control side before you insulate big time. If the stuff does not get wet it can go through infinite freeze-thaw cycles. It is hard to screw up a building in Edmonton - easy to screw it up in Chicago.
So let's say I have neat old building in New York (Neat Old Building) and I want to insulate it on the interior and not screw it up (Typical Interior Condition). What do I have to do? Easy, do the water control first. Then you have all kinds of options. If you don't do the water control don't insulate. If you don't do the water control and you insulate you are basically euthanizing your building.
So let's say I've convinced you to do the water control and you actually do it. What options do you have? One of the easiest and most robust is Robust Interior Insulation Approach - high-density spray polyurethane foam applied directly to the interior of the mass wall. This works in all climate zones. Now, lots of folks, some for good reasons, some bad reasons absolutely refuse to use high-density spray polyurethane foam. I get it. Whatever. So now what?
How about High-density Mineral Wool. Apply a cementitious rendering on the interior the mass wall and coat this rendering with a vapor semi-permeable fluid applied water control layer. The rendering fills in the big gaps allowing the fluid applied layer to be applied in a continuous fashion. Now, apply high-density mineral wool directly against the fluid applied water control layer. Done. Caveat - this is limited to climate zones 4 or lower. Bummer. The assembly is not airtight enough and vapor tight enough from the inside to work in colder climates - unless, unless the interior relative humidity is kept low during the coldest two months - less than 30 percent in climate zone 5 and less than 20 percent in climate zone 6 and in colder places forgeddaboudit.
We also have Cellulose or Fiberglass. Maybe you can get away without a cementitious rendering. Just stud out the wall - do not - absolutely do not stud out the wall with metal studs - now insulated the cavities with cellulose or fiberglass and call it a day. No caveat? Nope. Big caveat. Same caveat as for High-density Mineral Wool. Climate zones 4 or lower…or control the interior relative humidity.
Aw man, can't we do something else? Actually, yes. You can use a membrane "smart vapor barrier". You need to protect it - that's what the strapped wall horizontal framing does. And you have to be "fastidious" on the air sealing of the membrane. This gets tricky between floors and at punched openings - but it is doable. And here is the good part - no caveat climate zone wise. You can go to ugly cold places with this. Just remember the "fastidious" part. So why is the membrane called "smart"? Ah, it changes its vapor transmission based on relative humidity. Basically, it is vapor closed during the winter when the vapor drive is from the interior to the exterior. And it can become vapor open should the assembly need to dry to the interior if the cavity moisture levels rise.
Let me repeat once again-if you don't do the water control don't insulate. If you don't do the water control and you insulate you are basically euthanizing your building.