Looking for a modern approach to an uninsulated 1928 kit house
We’re in Westchester County, NY in a 1928, 1500 sq 2 story balloon-framed Bennett Homes kit house. Wall construction is original plaster/lath over nominal 2×4 covered by t&g cedar backer-boards, tar paper, and finished with cedar lap siding and paint. These original walls have been well preserved behind vinyl/eps that we plan to remove and then sand, patch, and paint.
The question is how to handle insulation. Do we insulate the walls with blown-in Rockwool, use vapor barrier paint inside, and tighten up the place….. or leave the walls empty. We’ve been hearing that the lack of a modern vapor barrier means that any insulation will tend to trap moisture that can be absorbed by the framing, creating conditions for mold, terminates, and possibly rot. Will a vapor barrier paint really prevent that? We’re installing an ERV but since we like to live with open windows 6 months a year I don’t think we can count on that.
Will hollow walls be a huge factor if we foam the sill and top plates, provide for respectable R values in the attic and replace the circa 80’s window inserts? We’ve been here one winter with radiators and while the thermal images show cold spots in the corners it didn’t feel drafty. Perhaps that will change dramatically when we replace the radiators and all the supply pipes that feed them with Mitsubishi Hyper Heat units.
There are many articles on these topics but haven’t found one that addresses all the issues in concert that we’ve been wrestling with and would appreciate your thoughts.
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Replies
Flashing details and roof overhangs matter. Can you post some pictures of the house, and perhaps detailing around the windows?
Vapor barrier paint on tight drywall is more than adequate for US climate zone 4/Westchester, and if there is good drainage behind the shingle siding even that might be overkill.
The last thing you'd want to do is install a true vapor barrier on the interior side, such as 4-6 mil polyethylene. If going for a broadsheet membrane type vapor retarder something like Intello Plus or (much cheaper) 2 mil nylon (Certainteed MemBrain) would be more forgiving than even vapor barrier latex.
Also, blown cellulose would be more forgiving than blown rock wool, redistributing and safely sharing the seasonal moisture burdens.
Thanks for the quick reply Dana. The walls are not open so the only option is vapor barrier paint applied to the original plaster walls, as you said, if it’s even needed.
also, I had assumed that cellulose would absorb moisture and settle further whereas rockwool would resist absorption? what’s the science behind choosing cellulose?
the windows are being replaced and the casing is in good shape other than the sills that need to be replaced. Thanks in advance!
If the original plaster was painted with an alkyd/oil paint at any point in it's history it's already highly vapor retardent. A layer of vapor barrier latex won't hurt, but it's probably not buying you anything.
It's still going to be important to air seal the walls, filling in any cracks or picture-nail holes, air-sealing any & all electrical & plumbing penetrations, air sealing the electrical boxes themselves, etc. The air sealing doesn't have to be absolutely perfect in your climate, but tighter is always better.
Cellulose (like wood) takes up and releases moisture in the form of adsorb fairly freely as temperature and humidity conditions vary, due to the hollow tube-like microstructure of cellulose fibers. Water in the form of adsorb does not damage or affect the insulative function of the cellulose. By wicking and redistributing the moisture in the assembly it keeps the structural wood drier. The amount of moisture safely buffered by cellulose is substantial- many times that of rock wool.
But the moisture cycling in & out of the cellulose does cause it to flex and change dimensions slightly with moisture content a phenomenon referred to as "creepage", that can cause the material to move and settle. But for any climate there is a definable installed density at which the natural springiness of the compacted material overcomes any creepage settling unless the moisture cycling is for some reason severe (are you insulating a steam bath?). In your area that would be about 3 - 3.2lbs per cubic foot or more. In the cooler climates of New England it might take 3.5-4lbs per cubic foot.
Researcher Torben Valdbjørn Rasmussen of Aalborg Universitet in Denmark has spent a chunk of his career studying insulation, insulation settling and in particular the relationship between density and settling of cellulose, as outlined in this paper (in English):
http://www.nordtest.info/images/documents/nt-technical-reports/NT%20TR%20565_Density%20of%20loose-fill%20insulation%20material%20exposed%20to%20cyclic%20humidity%20conditions_Nordtest%20Technical%20Report.pdf
A primer on the importance of hygric buffering in buildings built with moisture susceptible materials from Building Science Corp:
https://www.buildingscience.com/documents/insights/bsi-039-five-things
More, from Greenbuildingadvisor;
https://www.greenbuildingadvisor.com/article/hygric-buffering-and-hygric-redistribution
Cellulose is also very air retardent compared to blown rock wool, and even more air retardent when dense packed to 3lbs or more. The higher pressures of dense-packing also forces fiber into every crack seam and hole where air can move, tightening up the building (a lot!). It's good stuff. Dense packed fiberglass at 1.8lbs density or higher is about as air retardent as 3.5 lbs cellulose and has a higher R/inch than 3.5lb cellulose, but an order of magnitude less hygric buffering capacity, more comparable to rock wool.
The house in the picture appears to have 20-24" roof overhangs, which offers a lot of forgiveness on imperfections (or even absence) the window flashing details, but it's advisable to figure those out and correct any deficiencies before insulating below the windows (whether cellulose or rock wool). A st of tight exterior storm windows (ideally low-E) would also mitigate against wind-driven rain getting around the window too. (Low-E storm windows cost more, but "pay back" sooner due to the higher performance.)
Dana, thank you for taking the time! That is a tremendously helpful response. Shout out to @Akos here too. He made a great point, which I'll paraphrase to help me articulate the fork in the road we seem to be facing here... the house has a 100-year WRB made of tar paper that appears to be in good condition. Yet after we blow in the cellulose we will have reduced the capacity for the system to dry out. This certainly speaks to the point you made about double-checking that the proper flashing is in place, sills are sealed, etc. The notion of peeling paint would be very counterproductive. The labor cost to repair and paint the cedar siding is significant. For the same budget we could rip it all off down to the cedar backer board, replace the tar paper with a modern WRB and reside with above-average vinyl. Am I wrong in summarizing this as a choice between A) a design without insulating the cavities but living with a beautifully restored energy pig vs B) throwing out the old-growth cedar to live in a tighter more efficient home that is less charming but more comfortable? I'm finding that the middle ground is less certain, yet we need to decide on one approach so that we can size the HVAC system correctly.
There is a third option.
If you are doing interior work, or planning to do interior work down the road where you open up the walls from the inside, you can build a small ventilation channel just behind the existing sheathing.
This could be a piece of 1x1 stapled into the corner of the stud bays to space some house wrap/foam/OSB from the sheathing. You can then insulate the stud bays with either spray foam or dense pack while keeping the air gap behind the siding. This would add a lot of drying capacity.
thank you. that’s a great thought and makes sense. in our case we’re installing new HVAC based on whatever our design requires, so it’s hard to see how this will all come together down the road and until we’d be living with an undersized system. but more importantly we’re not planning to open the walls from the inside. Thankfully, our plaster is nearly flawless. but I get it, and this system sounds like the right way to go for those with the right access.
First and foremost with balloon framed house, you need to air seal. This means either spray foam or one of the dense pack options.
The one thing to keep in mind, that although it does sound like you have a WRB, your siding will still have less drying capability, which might mean peeling paint. Unfortunately there is no way of knowing if this is an issue, one idea could be to only insulate one section of the walls and wait a couple of winters.
One thing to watch in these old houses, sometime the interior walls are framed without top/bottom plates creating a nice chimney for the hot air inside the house to the attic.
Having insulated a 2.5 story balloon framed house with dense cellulose, the winter time comfort was night and day, definitely worth it.
thank you! great points!