Are you hesitant to add insulation to exterior walls of existing homes without also adding an exterior vapor barrier?
Have you been doing this for years and noticed no ill effects from moisture?
There are valid building science reasons to question whether or not adding insulation without adding a vapor barrier might be problematic. But then again, we think people have been doing this for a long time (20+ years?) and we’ve not heard of wall assemblies suffering moisture issues as a result.
So we’d like to hear: what have you seen? (Conceptual discussion is okay too, but we're primarily interested in hearing about your direct experience.)
We (at the Residential Building Systems group at LBNL) ask because we’ve heard this issue debated by home performance contractors whom we respect, and don’t really have a good answer. There is a huge cost difference between just doing drill-and-fill and doing both insulation and a vapor barrier. This cost difference can determine whether or not a retrofit happens. If drill and fill alone can safely be done, then we want to cite that approach as a cost-effective way to increase comfort and reduce conditioning demand. But we don’t want to introduce moisture-caused structural integrity or IAQ issues in the process of adding insulation.
Of course this is isn't a question with a yes/no it's-always-okay vs. it's-never-okay answer. We suspect that there are scenarios of particular concern for this issue, based on (among other factors) cladding material, region and era of construction, climate zone, and wall orientation. We're just trying to get a general sense of people's experiences.
The biggest issue is checking the cavities first for any signs of water intrusion & dealing with those issues first - many old homes were not flashed properly or had other issues which really didn't matter as they could dry out quickly. If you add insulation in you have now prevented the walls from drying out as easily leading to a ton of issues. The other check is what type of wiring is in the walls as most older ones that can benefit from it have K&T, etc...
Can it safely be done & cost effective - yes in many cases, but you had better be sure there aren't other issues there first which can be tough as no one has x-ray vision
One quick nit-pick, no to vapor barrier - it is a WRB that should be applied outside and flashed properly, the bulk of issues occur due to air leakage & bulk water intrusion
This is really helpful, Sean. Thanks for your input. And please, nit-pick away!
I agree completely with Sean
Further, it can depend on what type of sheathing the house has. In the mid-Atlantic we see mostly:
Balloon framing, no sheathing, siding on studs - insulation against the back of the siding
Balloon framing, 1X8 sheathing boards with gaps up to 3/8"
Platform framing with 2X4 foot gypsum with small gaps
Platform framing with plywood.
Any of these may have tar paper or not, or may have vinyl siding over everything. So how well the dense packing occurs is more or less critical in some construction. Of course the best would be to get the house with no siding on it so you CAN install a proper WRB
Where we operate, we have not seen any problems (except bulk water from outside) in over 30 years.
Please consider this publication:
|Title||Insulating the old house: a handbook for the owner|
|Author||Greater Portland Landmarks, Inc.|
which describes our experience with sidewall insulation in the 1970s here in Portland, Maine. During the 70s "energy crunches" we insulated many houses. By the end of the 70s We found that filling the stud spaces of older houses with insulation often traps moisture in the walls leading to exterior paint failure, and structural deterioration. These findings were confirmed as more deterioration was found in these insulated houses during the 80s and 90s.
I am currently working through this issue with my own home. The more complicated answer is that it depends. It depends upon your climate zone and more specifically your micro-climate. My conclusion is that insulating just the cavities is always problematic. The bottom line is that structural members are thermal bridges and a source of significant energy loss. This combination is always dangerous particularly where there is a significant Delta T. Regardless of the WRB and the sheathing the potential for condensation build up is always present in this type of an assembly.
I live in a very dry climate and I am unwilling to risk the potential moisture gain that comes by leaving the dew point within the insulation cavity. My solution is to add a good WRB/ Air barrier to the existing sheathing and then add board insulation to the outside of that, eliminating any thermal bridging and moving the dew point out of the insulating cavity. My preference is mineral wool because of its moisture management properties. Attaching furring strips through the mineral into the existing studs allows for a properly attached rain screen and an air gap for drying.
I understand that this may seem complicated, but I reached the conclusion that pushing my dew point as far outside the insulation cavity as possible was worth the brain damage. If you are interested I will update you on costs, which I understand is the biggest driver in this conversation.
I work in a building that is 120 years old in Charleston, SC. The "Charleston Single" has no sheathing, just wood siding on balloon framing. It was retrofitted in early 2000. Because of historic preservation laws here we were not able to install sheathing. Instead an enkamat (http://www.globalplasticsheeting.com/enkamat-enkadrain/) was installed against the siding before the installation of fiberglass or cellulose (in different wall sections) to allow for a space for the wall to breathe but allow a tight air barrier on on the interior wall of the home (not the ideal solution but the one that was decided at the time by a group of folks that were not building scientists). We have several Plexiglas demonstration walls throughout the building and there is more than one spot that is showing moisture intrusion on the fiberglass. Nothing is showing on the cellulose but the demonstration wall for the cellulose is only about 1/20th of the insulated wall area. It has been 13 years and there has been no noticeable fungal growth - but we cannot see through all the walls. My point in that on a home without sheathing it may be impossible to stop bulk moisture from entering with a strong wind driven rain. And we have a demonstration wall that proves that.