Cellulose with borate
rfhjr
| Posted in General Questions on
I live in the Winston-Salem, NC, area and was planning on insulating the walls with cellulose. Should I use the cellulose with borate or the regular kind?
Also since the building only has vinyl siding with no wrap, I was thinking of putting an XPS foam board on the outside wall then putting in the dense-pack cellulose. Would this be better? Or should I just use cellulose?
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Replies
Rob,
Most or all brands of cellulose include borates (added to reduce flammability and to make the cellulose unappetizing to insects). Some brands also include ammonium sulfate, which (like borate) is a fire retardant.
For more information, see "Cellulose Insulation."
Some contractors tout the advantages of cellulose that has no ammonium sulfate, since ammonium sulfate-treated cellulose can become corrosive if the cellulose ever gets wet. I'm an agnostic on this issue -- I think that the worries over corrosiveness of cellulose with ammonium sulfate are overblown, because properly installed cellulose shouldn't get wet. But if the corrosiveness stories concern you, you should buy cellulose treated with borates (and no ammonium sulfate).
Installing a continuous layer of rigid foam on the exterior side of your wall sheathing is an excellent way to reduce air infiltration, improve the R-value of your wall assembly, and reduce thermal bridging through the studs. That said, most green builders choose EPS or polyiso over XPS, for environmental reasons. For more information on this topic, see "Choosing Rigid Foam."
For more information on installing a layer of rigid foam on the exterior side of your wall sheathing, see this article: "How to Install Rigid Foam Sheathing."
"I think that the worries over corrosiveness of cellulose with ammonium sulfate are overblown..."
I see what you did there.
Thank you for the information I was thinking of using XPS blue foam board as I have heard it repels water better than polysio. Since the building has no exterior wrap only vinyl siding and OSB I did not know if this would help. Also does the type of netting used make any difference there is an Insulweb netting which I did not know if it is better than the others.
XPS does not repel water better than foil faced polyiso. Foil is completely impermeable to liquid water and water vapor.
Polyiso is hygroscopic, but the only exposed polyiso is at the cut ends, and unless it's chronically being wetted (say, under a damp basement slab) it will not take on water. Foil facers are slightly easier to tape (with temperature rated foil tape) than XPS, and polyiso delivers a higher R/inch. When installed under vinyl siding the foil facer adds another ~R0.5-R1 to the performance as well (due to the low-E foil facing the air space behind the siding.
Polyiso also has a higher kindling temperature than XPS, and (like polyurethane) does not melt the way polystyrene (EPS & XPS) does, but chars in place.
Polyiso has a higher R/inch than XPS, and (usually) cost less per R per square foot.
For design purposes XPS should be derated to no more than it's warranteed R4.5/inch, not it's labeled R5/inch. As it's HFC blowing agents are released over several decades doing their environmental damage it's performance drops toward it's fully depleted R4.2/inch.
On the exterior of the assembly in a zone 3 climate polyiso can be assumed to perform close to it's labeled performance, falling off only slightly (if at all) at the 1% outside design temperatures. Whenever the mean temperature through the foam layer is above ~35-40F it beats fresh, new XPS on performance, inch for inch. Whenever the mean temp through the foam layer is above ~15F (which is pretty much always, in Winston-Salem) it beats the warranteed performance of XPS.
Dow Thermax polyiso is more expensive than most, but does not need to be derated at lower temperatures. Like polystyrene it's performance monotonically gains performance sl0wly as the mean temp through foam drops.
Rob,
The purpose of exterior rigid foam is to act as insulation, not to repel water. Building codes require, and building scientists recommend, that every wall have a water-resistive barrier (WRB) such as housewrap. For more information on WRBs, see "All About Water-Resistive Barriers."
For more information on installing cellulose -- with or without InsulWeb -- see this article: "How to Install Cellulose Insulation."
I appreciate your answers to my questions. Unfortunately the building was built in the 1970's and they probably did not have insulation options like builders have now. If I remove the vinyl siding in the future I will follow your suggestion.
I've seen houses that were built in the 1970s with an inch of foil faced foam on the exterior- it was around, but not very common.
When adding rigid foam to an existing building it's important to get the window flashing details right, which in most cases would mean adding a weather resistant barrier layer between the foam and the sheathing, lapping the window flashing properly to the new weather resistant barrier. If the house has NO window flashing it's prudent to add some. The inherently back-ventilated characteristic of the vinyl siding meant that bulk water incursions at the windows getting behind the siding and onto the sheathing or even inside the framing bays still had a reasonable shot of drying to the exterior quickly enough to not do any damage.
When a layer of foam is added the drying capacity drops precipitously, whether XPS or foil faced. Using a crinkle type sheet weather resistant barrier such as Tyvek Drainwrap (tm) between the foam & sheathing does a pretty good job of letting bulk water drain via gravity, and provides a hint of capillary break.