Is 40 PSI XPS board a overkill for basement under slab insulation?
mqcola
2 years ago
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mqcola
2 years agoRelated Discussions
Basement Insulation With XPS
Comments (19)Does this seem a little extreme here in my zone 5/6 Ohio? My first floor has R19. It's over Code. But Building Science Corp.'s Dr. Lstiburek's rule of thumb is that for any home north of the Mason Dixon Line, follow the 10, 20, 40, 60 rule. That's R Value for, respectively, basement slab, basement walls, above grade walls and attic. If you're planning to stay for awhile and the labour is yours, there may be an economic payback. The space will definitely be more comfortable. My biggest fear is trapping water with too much insulation. At Roxul sales displays, they like to run a water fountain through a chunk of their product. The water runs right through and is not absorbed. will my basement still be the coolest part of my house in the summer? Well, mine have been. But that's probably because of the leakage of HVAC-cooled air into the basement, where the cooling equipment is also located. Is it ok to just add fiberglass insulation without a vapor barrier, and close up with sheetrock? Unless you're in an extreme cold climate such as northern Alaska, leave the poly vapour barrier out. All you need is painted drywall as a vapour retarder. in the rim joist, I have 1" of closed cell foam covering the entire rim joist. Should I add XPS boards and seal, or just add unfaced fiberglass there as well? Either one works. See illustration below from Building Science Corp. Info. Sheet 511, p. 2....See MoreXPS, Please answer, once and for all!!!!
Comments (40)Thanks again Worthy for your contributions all across various topics. I think there is no perfect answer to choos between above two options, but your experience/Knowledge will help me to make best possible decision to minize the mold on concreat walls; rest all i think i can manage. Current State of basement wall: Smalls cracks fixed professionaly. Light Mild Green Mold/Marks bottom of the basement wall were cleanded with concrobium liquid before drylock DryLock anti-mold applied on wall after cleaning two coated applied; French Drail and Sump working; expected to be working; mostly dead dry; Never had water in the basement; in last 4years under my ownership (built in 1994); don't see any water damage/repairs from before also. I Need to decide this week and move forward between below two options; any final recommendation pls. I know no perfect answer with garantee. Option #1 XPS nailed/glued to concreat walll and sealed; As building science Lstiburek said, all concreat walls will crack/leak sometime for sure. Optin #2 imperable from basement wall from top seal to bottom into Cove Joint. ( To do this, I need to remove the fome inserted in the cove joint Tuck the imperable insulation. Means earth gases / vapour has way to get upto the top of the concreat wall but offcource it is tightly sealed all around. Thanks...See MoreFraming and insulation -limited by Canadian codes?
Comments (10)I just heard back from a local foam installer (seem reputable). He proposed 3" closed cell foam in the wall (R20.4 at low 90's% efficiency), and 4" directly in the cathedral (vaulted) ceiling (R29.2 at mid 90's% efficiency). I asked about these, "in my opinion", very minimal thicknesses in order to meet code, and the potential for ice damming. He argued that a completely sealed envelope will not cause any moist air to escape, hence no ice damming. He also believed that 4" would not cause any snow to melt either (said that cabins up north only have 3" without issues) What I like about this idea: the best air tightness possible (?). I don't have my drawings ready yet, so he could only give me a rough cost, but although this is more expensive than standard 2*6 with Roxul batts, is is significantly better and likely "only" a couple grand more, if I include in our own saved labour. In the grand scheme, I'm more than ok with that. in a single 2*4 wall, electrical would have to be installed first (ok, whatever), but the insulation behind the boxes would be very thin, and the wires would likely be IN the foam. How would that work in 20 years when for some reason we'd need to re-wire the place? Another big question: is the proposed thickness enough??? It also does not solve the thermal barrier issue. Would it make sense to increase the thickness a bit in order to cover the 2*4, then do another "interior" wall to install the electrical? (likely another stupid question, but could you also add the ventilation (HRV ducts) in this space?) Another option might be to do 2*6, filled 3.5-4"(?) with foam, then leave the rest open for electrical to be installed after. This leaves a full bridge though, but perhaps that's not that important?? For the ceilings we aren't limited by thickness at all, and can go with whatever. I'm just not sure what would make the most sense. Any additional thickness would drive the costs up to a point where it might not be financially viable anymore. Any input at all would be highly appreciated! Thanks...See MoreNeed advice quick - did you use foam insulation under basement slab?
Comments (12)Obviously a walk-out slab is at grade and not considered a basement at that building face so along the exposed slab edge it must have the under slab vertical insulation required in the Prescriptive Path chart of the IECC (if that is the compliance Path selected and assuming the slab is not heated. The IECC does not require insulation under a full basement slab because the heat lost to cold outside air is usually much greater than the heat lost to the earth below a basement slab. The rate of temperature loss through a material is proportional to the temperature difference between the surfaces. The temperature of the earth below a basement slab is permanently a bit warner than the temperature of well water which varies in the US from about 43 to 73 degrees. If the design temperature of the basement air is 70 degrees the temperature difference would vary from about 27 to 3 degrees. Therefore under slab insulation might be cost effective in North Dakota but not so much south of Kansas and Kentucky. In Michigan the difference would be about 20 degrees so insulation under a slab would be modestly cost effective and would also decrease condensation so R-5 (1" extruded polystyrene) should be adequate but if the price is right, 2" XPS can't hurt....See Moremqcola
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