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Louis S.
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Posts:3
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| 06/29/2000 9:56 PM |
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For a house to be built on the South Carolina coast, SIPS will go into the walls. One builder is recommending steel trusses instead of panels for the roof system, in order to save on the budget. Does this tradeoff make sense?
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Steve Andrews
Registered Users
Posts:334
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| 07/04/2000 12:45 PM |
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Why would the builder suggest steel trusses instead of wood trusses? There could be some structural reasons I'm not familiar with that apply to your hurricane loads. But for thermal reasons, steel trusses are not a good idea in climates with either significant heating or cooling loads. The webs between the top and bottom cords of the tursses help heat bypass the insulation, and the rafter tails which make up most overhangs act like heat exchange radiators. The net loss can be 50% of the R-value of your ceiling insulation. Additionally, they could cause "ghost lines"--a buildup of dirt over the drywall where it covers the steel bottom cords. So combining steel trusses for your ceiling with SIPs in your walls would seriously undercut the efficiency performance advantage of selecting SIPs in the first place.
The primary way to reduce the energy and "ghost line" problems would be to attach a layer of foam sheathing to the underside of the steel trusses prior to drywall.
If your budget can't sustain SIPs in the roof, wood trusses with a dense, blown-insulation product like cellulose insulation should provide a cost-effective performance alternative. Since the roof/attic system is the largest source of air leakage from a home, pay extra to have the attic sealed AFTER drywall but BEFORE insulation.
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Jim Lewis
Registered Users
Posts:10
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| 07/26/2000 11:52 AM |
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Steve,
Curious what you mean by "seal attic AFTER drywall...." How/what would you do?
One method I've heard about for adding cels in attic: after trusses, TACK on roof sheathing, put poly, if used, and DW on ceiling, then go back and blow in cels, shifting sheathing aside as necessary.
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Martin Coult
Registered Users
Posts:10
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| 07/27/2000 5:42 AM |
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I'm curious why anyone would want to build a technologically redundant forest of struts and reduce the potential floor area of their home by a third?
What a waste!
Martin Coult
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Steve Andrews
Registered Users
Posts:334
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| 07/27/2000 11:09 AM |
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Jim, to air-seal a non-panel attic AFTER drywall, you want to foam all the joints between the drywall and wood, plus other areas (described below). You could use a more flexible caulk such as urethane (from a tube, not a can), but it's messier to work with. Have some bubble-pack foil-faced insulation on hand for covering up larger holes.
From the attic, after drywall, I've taken photos showing sheshkabob skewers and writing pens sticking down into and up out of the joint between wood and drywall at interior top plates. The drywall doesn't always get fully butted up to or pulled into the top plate. If you consider the gap, there are probably several hundred linear feet of potential gap between drywall and top plates in the attic. (Many of those linear feet involve a nailer, which slows down air exfiltration up into the attic.)
The worst such gaps occur where return air ducts are placed high on an interior wall, and the cavity is used instead of a hard duct. either hot or cold attic air can be drawn down through attic insulation, through the drywall/top-plate gap, into the home's ductwork and circulated throughout the home; and when the HVAC blower shuts off, during wintertime operation air rises up from the home and into the attic. Double loser...
Besides the top-plate connection, other large structural leaks into the attic are through dropped soffits, recessed lights, pocket door cavities, and unsealed attic hatches.
How would you know if these are truly leaks? Well, a blower-door test and infrared camera inspection shows this type of leakage is commonplace in all homes (more so in attic with low-density fiberglass insulation, less so in homes with more dense cellulose insulation). I've performed several hundred such tests. But a simple test for energy geeks (such as yours truly) is to go up into your attic and carefully shift the insulation around to see if black streaking has occurred where the insulation covers up the above described air gaps; that sooting of insulation means it is acting as a filter for house air leaking up into the attic.
SIP roofs eliminate such leaks from partition walls into attics. All you have to worry about is sealing at the eave, rake and ridge lines--a fairly straight-forward proposition compared to a conventional attic.
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Steve Andrews
Registered Users
Posts:334
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| 07/27/2000 11:15 AM |
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Martin, conventional wisdom has it that the best economic justification for a SIP roof is steep-pitched situations. When SIPs pitched up at 10/12 or higher enclose spaces, those spaces provide very livable additional living space, not just a well-insulated cap to a house. I gather you are in the business of enclosing such spaces through retrofits in urban UK, where land is at a premium. Could you describe what the costs are? Perhaps walk us through a case study under a new post dedicated to the subject?
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Jim Lewis
Registered Users
Posts:10
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| 07/27/2000 12:30 PM |
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Holy Moley, Steve! CAULK every wood-to-DW joint in a trussed attic?? AFTER installing DW?? You got some monkeys you've trained to do this?
If you wanted the joints caulked, why not do it AS you DW? (Incidently saving by 50% on caulk, since you'd use one bead in center of each wood member.)
But, if you are going to blow in cels, it's my understanding that the cels itself does the sealing, and is a major reason to use it over FG (fiberglas).
Let me take time out to applaud your on-going fairness, Steve. You are always careful to be honest about your knowledge of the pros and cons of SIPs.
Martin, I don't understand how use of trusses is going to "reduce the potential floor area of ... home by a third"? I hear you, though, about "redundant forest of studs"; I HOPE to be able to use SIP for roof, and possibly walls, if I can work through some practical issues. (For roof, size of beam to carry panels; for walls, mainly, wiring; for both, finding a trustworthy crew to erect. Of course, finding a trustworthy crew is the key no matter what method of construction you choose.)
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Jim Kistner
Registered Users
Posts:20
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| 07/27/2000 2:00 PM |
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Martin,
I think its due to esthetics. Many subdivisions here not only have size requirements (SF of floor area), but also dictate such things as roof pitch. The floor area requirement usually also requires, even on a multilevel house, a minimum SF size on the main level. So, if you don't need more SF than the main level requirement you end up building a house with a steep roof that could have livable area under it that remains unfinished.
Maybe some day enough individuals will realize that this is a very wasteful practice and start demanding more reasonable restrictions on the land they purchase for their homes.
[This message has been edited by Jim Kistner (edited 07-28-2000).]
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Steve Andrews
Registered Users
Posts:334
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| 07/27/2000 5:59 PM |
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I have a colleague in the insulation business who estimates it costs about $60 in labor to seal the average 1,000 ft2 trussed attic (including top plates) prior to insulation. I've seen the difference it makes; it's worth it....if you have someone who knows how to do it, understands air-flow paths from house up into attic. I've seen money spent much less effectively trying to air-seal homes. Remember: the attic is the average home's leakiest single building component.
If you want to cut your labor costs in such a trussed ceiling, you can do so by buying more expensive materials, such as a froth pack of two-part spray foam; buy enough to seal your rims as well, if you install panels on a standard platform frame.
You CAN use an adhesive or gasket at all top plates when you install drywall; but I question the effectiveness a bit more, if the drywallers aren't careful. (You have to caulk or gasket both sides of the top plate.)
Blown cellulose insulation is NOT an air barrier. It does reduce air flow through it significantly, as least compared to fiberglass, thanks to its higher density. In truss-roof applications, I am a fan of cellulose for the above reason...given the typically unsealed attic.
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Jim Kistner
Registered Users
Posts:20
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| 07/28/2000 11:16 AM |
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Steve,
How about wet blown cellulose in the attic? Since, I believe it has an adhesive mixed with it, it could conceivably provide an air seal. Around here wet blown cellulose is used in the walls quite frequently, but, in the ceiling the installers usually revert to the dry, dense pack method for some reason?
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Steve Andrews
Registered Users
Posts:334
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| 07/28/2000 12:55 PM |
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Jim: In my opinion, what you suggest here wouldn't be a good economic tradeoff. It's my understanding that when you spray cellulose wet (and/or with an adhesive), you have to pay for a product that is treated with a more expensive fire retardant than is normally used in attic-blow cellulose. Even when you pay extra to "wet-spray it," and you have a 6-mil poly vapor retarder above the drywall, you may have application problems...right in the very spots I've referred to--drywall-to-top-plate joints-- where the ceiling poly might potentially increase the size of naturally occuring gaps there. Why not just seal the attic after drywall?
I doubt that the adhesive binder added to cellulose would actually increase tightness--compared to just dry-blown cellulose--in a flat attic.
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