Posted By Dana1 on 05 Aug 2015 05:12 PM

Posted By ronmar on 05 Aug 2015 11:45 AM
Since you dredged this thread up TexasICF, can you elaborate on the makeup of your "more modern" sealed attic spaces? IE: A conventional attic space would be, from bottom to top: Drywall, vapor barrier, bottom chord, batt or blown-in, ventilated space, top chord and roof system. We are about to start construction and are planning a conventional attic. Just trying to get my brain around the makeup of a possibly better way

Thanks

Inserting a vapor barrier behind the ceiling gypsum would be a mistake in most of the eastern US, where outdoor humidity levels can sometimes exceed air conditioned room temperatures, resulting in condensation on the vapor barrier. 

Only in the coldest parts of the US would inserting an interior side vapor barrier in a ceiling stackup have any benefit.  In most places the potential risks exceed the potential benefits, and in the high-humidity high cooling load gulf coast states it's practically asking for trouble.

Even in much of eastern Canada (where building codes strongly encourage or outright demand) vapor barriers in ceilings it can be a mistake in an air-conditioned house.

A possibly better way would be to install a half-inch OSB air-barrier on the underside of the truss chords to support the high-R attic insulation, and use 2x4s perpendicular to the truss chords to support the ceiling gypsum, and as a wiring chase, etc. With slab-on-grade type construction an even deeper ceiling chase can be designed-in to accommodate cooling/heating ducts etc. That way the penetrations of the air barrier are much fewer and more secure, and the ducts stay inside the pressure & thermal boundary of the house.

Given the inherent air tightness of ICF walls it's downright shameful that many existing ICF houses don't even meet IRC 2012 air tightness levels  (3 ACH/50 pa ) due to the "sea of recessed lights" and/or other air leaks in the ceilings.

Posted By TexasICF on 05 Aug 2015 05:35 PM
Excluding ICF ceilings or ICF roofs --- It's clear from Dana's post that the ICF has nothing to do with contributing to Air infiltration. This is why I submitted all of my jobs to the MIT study only with sealed attics.

Posted By ronmar on 05 Aug 2015 07:18 PM
Risk from condensation of high humidity on the upper side over an air conditioned space? I could see that back east or south especially if there wasn't much insulation I am in the Pacific Northwest, no air conditioning here, only ventilation and heat in the winter months. My consideration of an air barrier was mainly to aid in air leakage control to try and get a low leakage air infiltration number, and to keep inside moisture away from the winter cooler attic space. I don't think there is too much of a chance to get condensation on the top side here.

The OSB layer sounds interesting though, as there will be a buttload of insulation up there. how do you seal the seams? Being in a seismic zone, there is a large re-enforcement bracing requirement for the walls thru the roof structure(ratruns every 48") which will bear some of the insulation weight.

Perhaps I didn't express myself clearly initially. Can someone describe the makeup of a non habitable, non storage sealed attic constructed with conventional wood trusses? Or is there no such thing using conventional trusses?

Thanks

Posted By billnaegeli on 05 Aug 2015 07:39 PM
ok i am about to start a whole new debate but i am down with TexasICF, we have been spray foaming the underside of the roof deck with open cell, including the rafters, we have not had any problems with this method and it is actually endorsed by Pathnet Technologies which is a quasi-public company. the thermal envelope is critical and it really does need to be airtight, i have some contractors who will spray foam the ceiling(on the attic side) then put in some batt insulation where they are trying to get a sealed attic, but as stated earlier - "can lighting" tends to be a problem for that application, but they are usually just trying to keep the cost down. it is a bit more expensive but the prices are coming down at least around the east coast we are in the 3-4 p/sqft range now, so it is a great option, not the least costly but in my opinion(for what its worth). the most cost efficient and effective if you are using ICF it just makes sense, now with that said if you have a roof that is a 14/12 or some crazy extreme pitch then yeah its gonna cost alot more? so then you may have to look at the other options, but the ideal attic is unvented for ICF applications, did i just ht the hornets nest? well see?

Posted By FBBP on 05 Aug 2015 10:59 PM
The only reason for a non vented attic is to keep the HORNETS OUT ;-))

You are absolutely correct that the thermal envelope is critical but why would you enlarge it? Even a low 4/12 pitch adds a few points to the envelope. Also you have that many more cubic feet of airspace to condition.
If foam is the answer, why not just spray the top of the drywall? Wiring, can lighting and all?

If insulation external to the sheathing on walls will protect the sheathing from condensate, why would the R40 of insulation not protect the vapour barrier from condensation?
If the condensation point is the vapour barrier, would it not be the drywall if there was no vapour barrier? Resulting in saturated and falling ceilings?

Having been born and raised in southern Ontario and having most of my siblings still living there and a number in the construction trades, I have never heard of a condensate issue connected with a vapour barrier in the ceiling. These have been required for over three decades so I would think that it would have been apparent by now if there was issues. Southern Ontario being farther south then much of the east coast and approaching the same humidity levels.

Posted By arkie6 on 06 Aug 2015 12:25 AM

Posted By FBBP on 05 Aug 2015 10:59 PM

You are absolutely correct that the thermal envelope is critical but why would you enlarge it? Even a low 4/12 pitch adds a few points to the envelope. Also you have that many more cubic feet of airspace to condition.
If foam is the answer, why not just spray the top of the drywall? Wiring, can lighting and all?

In my area and most of the south, many if not most homes are built slab-on-grade with all of the duct work located in the attic.  And many homes have the air handler located in the attic as well (I pity the HVAC tech that has to work on those in the summer).  That is the only time I see an advantage to spray foaming at the roof line.  But the cost, assuming it is insulated to code minimum (often not), is many more times the cost to air seal at the ceiling and meet or exceed code minimum blown in insulation at the ceiling.  I can have R49 stabilized cellulose blown in above my ceiling for less than the cost of 1" of closed cell spray foam at the roof line.

Posted By jonr on 06 Aug 2015 10:53 AM
If overhead ducts are needed, I'd consider some type of plenum truss design that allows them to be part of conditioned space.

Posted By smartwall on 06 Aug 2015 01:31 PM
Raised heel truss between the icf walls and R=60 celly blown in contact with the icf walls for about $1.20 per sq ft blows spray foam out of the water for performance at a great price. As far as recessed lights, you would have to be a moron not to use the high efficiency sealed cans on the market now. One thing here that makes foam less efficient is the fact that you can have 12' high trusses, so that means your conditioning a lot more cu ft of space than if you insulate the ceiling.

Posted By Dana1 on 06 Aug 2015 03:29 PM

Posted By ronmar on 05 Aug 2015 07:18 PM
Risk from condensation of high humidity on the upper side over an air conditioned space? I could see that back east or south especially if there wasn't much insulation I am in the Pacific Northwest, no air conditioning here, only ventilation and heat in the winter months. My consideration of an air barrier was purely to aid in air leakage control to try and get a low leakage air infiltration number, and to keep inside moisture away from the winter cooler attic space. I don't think there is too much of a chance to get condensation on the top side here.

The OSB layer sounds interesting though, as there will be a buttload of insulation up there. how do you seal the seams? Being in a seismic zone, there is a large re-enforcement bracing requirement for the walls thru the roof structure(ratruns every 48") which will bear some of the insulation weight.

Perhaps I didn't express myself clearly initially. Can someone describe the makeup of a non habitable, non storage sealed attic constructed with conventional wood trusses? Or is there no such thing using conventional trusses?

Thanks

No air conditioning in the PNW?  Even THIS year? (My Puget Sound region relatives heating with mini-splits were happy to run them in air-conditioning mode extensively in the past 6-8 weeks!).  But the outdoor dew points there stayed well below room temps, unlike more humid regions, so you're right, the condensation risk is very low.  But you still don't need or want polyethylene in the stackups.


In the Pacific Northwest there is zero rationale for a vapor barrier in the ceiling, since the outdoor & attic air temps are (nearly) always near or above the dew point of the conditioned space air.   In climates where the attic might average 20F or colder over the winter there is a vapor pressure delta between conditioned space air (with a dew point of 35-40F), and the much colder attic, which moves moisture via vapor diffusion to collect in the wood or insulation layers colder than the indoor dew point air.  Indoor dew points are a bit higher in the PNW at about 45F, but so is the wintertime outdoor temperature averages.  In the summer the average outdoor dew points average below 50F, and never above 70F, which is about the coolest most people would air condition a house.  Still, if you're using broadsheet goods simply as an air-barrier, use housewrap, which would limit the size of the puddle when the roof leaks, and make discovery of the leak sooner. 

But it's no more difficult and often easier to get a reliable air seal using a rigid air barrier than it is with flexible goods, sealing the seams of the gypsum (or OSB) and can-foaming/caulking any wiring/plumbing penetrations.  Air-sealing flue penetrations to rigid goods with sheet metal is also easier than with more flammable/melt-able flexible sheet materials. There are fancy tapes suitable for sealing the seams of OSB (perhaps more appropriate for exterior sheathing than this application) but it can also be air-sealed with fiber reinforced duct mastic, and any wiring/plumbing penetrations can be can-foamed or caulked with acoustic sealant, etc.


OSB is a "smart" vapor retarder.  When installed on the interior it'll usually stay dry enough (6-12% moisture content) to run about 0.5 perms most of the time, which is well into class-II vapor retardency ( and an order of magnitude more vapor tight than standard latex paint.)  But if the RH on the attic side increases dramatically (as it would in the southeast in the summer, or if there were a roof leak), as the moisture content of the OSB rises it becomes more vapor open, allowing the moisture to diffuse into the conditioned space to be managed by the air conditioning rather than collect all summer and grow mold in the insulation (or worse), the way it can with a poly vapor barrier.  OSB also supports a lot more insulation weight over bigger spans without sagging or breaking than ceiling gypsum.  It's a cost-adder, to be sure, but it adds resilience.

Posted By ronmar on 06 Aug 2015 08:56 PM
Highest indoor temp I have seen so far this year is around 74F. This in a very poorly insulated house that started it's life as three shacks drug together in the 1940's. We are out on the Olympic Peninsula and we always run a little cooler than most of the rest of the state.

My reasoning for sheeting in the ceiling stack was primarilly to aid in air control. We have a 40' roof span so there will be a bit of movement between the tops of the internal walls and the bottom chords of the trusses. A air barrier that remains more flexible over time, particularly one covering the top corners of the drywall appeals to me. I hadn't thought of using housewrap though, I will have to look into that, thanks...

Posted By dmaceld on 07 Aug 2015 12:36 AM

Posted By ronmar on 06 Aug 2015 08:56 PM
We have a 40' roof span so there will be a bit of movement between the tops of the internal walls and the bottom chords of the trusses. A air barrier that remains more flexible over time, particularly one covering the top corners of the drywall appeals to me. I hadn't thought of using housewrap though, I will have to look into that, thanks...

I don't recall where I saw it, in some of my research or a workshop or where, but there is a way to deal with this. Basically you don't fasten the ceiling sheetrock to the joists any closer than about 12" to 18" of the walls. The wall to ceiling sheetrock joint will then cause the ceiling rock to flex up and down as the joist moves up and down.

I trust the more experienced here will chime in with some clarifying comments.

Do some research on buildingscience.com about vapor and air movement. You can get plenty good air barrier with sheetrock and paint and still have a vapor permeable layer. You want to allow vapor to move in and out, which poly sheeting won't permit. Vapor movement does not equate to air movement.

Posted By ronmar on 07 Aug 2015 01:03 AM
Yep, that is how you attach sheetrock to allow the bottom chord of the trusses to float. You also use slotted clips to attach the walls to the trusses. Secures the wall laterally, but allows the truss chord to move vertically in the slot.

But since it still moves/flexes the drywall, my concern is long term ability to maintain a seal.

Posted By FBBP on 07 Aug 2015 01:11 AM
Yes, you can deal with the movement issue as dmaceld describes. We have used this method for years in our colder climate. The more severe the climate and the steeper the pitch of the trusses, the more differential in the expansion of the bottom and top truss chords. But it means that you are relying on the paper tape joint in the corner as you air barrier. I believe this is ronmars point. He recognizes the need to have a long lasting flexible joint at interior wall and ceiling joints. If you place framers poly (18" wide) between the double top plates prior to standing up the wall, you can then fold this poly up the edge of the top plate and horizontally over the ceiling. When the ceiling poly is applied it over laps the wall poly and is sealed with acoustic caulk for a long lasting flexible joint.
I see no reason why housewrap would not work but have not tried it.

If we have conditions such that water vapours are going to condense on the upper side of the ceiling (regardless of the material) will any vapour permeable air barrier allow sufficient vapour though to make a difference to the near ceiling insulation's vapour content if the whole attic space continues to produce condensate at that point? If it did, would you have to run the a.c. even longer to remove this vapour that is being transmitted through the semi permeable barrier thereby causing more condensate to form above the ceiling?

If the conditions prevail, it is unlikely that enough moisture moves into the house to make a difference. As soon as the condition reverse to a more drying condition, the vented attic space should provide more drying then a permeable barrier can provide so I'm not quite sure why the concern with the vapour barrier.

Even at my age, I can properly poly and seal a 3000 square foot ceiling space in under five hours. I suspect it would take two young lads three times that long to solid sheet a multi room ceiling and then you would still have to provide seam sealers that will remain adhered and flexible to OSB forever which I'm not persuaded can happen due to the expansion and contraction of the OSB to itself and to it's adjoining materials.

Posted By Dana1 on 07 Aug 2015 12:38 PM
Experience in the muggy southeastern US (with average mid-summer dew points in the 70s F ) shows that latex paint is sufficiently vapor permeable to keep the paper facer on the insulation side of the ceiling gypsum mold-free in air conditioned buildings. 

The same would be true of OSB, which becomes more vapor open than latex paint when it's moisture content rises sufficiently to grow mold.

The expansion & contraction rate of OSB with moisture & temperature is the same as the wooden truss chords they would be attached to.  If tapes and goops work on exterior sheathing attched to studs over the long term (and they do), they'll work even better in the more benign climate of the interior side of the assembly.

Posted By ronmar on 07 Aug 2015 02:00 PM

Posted By FBBP on 07 Aug 2015 01:11 AM
Yes, you can deal with the movement issue as dmaceld describes. We have used this method for years in our colder climate. The more severe the climate and the steeper the pitch of the trusses, the more differential in the expansion of the bottom and top truss chords. But it means that you are relying on the paper tape joint in the corner as you air barrier. I believe this is ronmars point. He recognizes the need to have a long lasting flexible joint at interior wall and ceiling joints. If you place framers poly (18" wide) between the double top plates prior to standing up the wall, you can then fold this poly up the edge of the top plate and horizontally over the ceiling. When the ceiling poly is applied it over laps the wall poly and is sealed with acoustic caulk for a long lasting flexible joint.

Exactly! This was also very much what I was envisioning for the wall to ceiling seal. Strip of poly folded over the wall top plate before the wall is stood up. The wall-ceiling sliding clips would be placed on top of the poly(and sealed) so that the connection to the moving truss is situated outside the air seal. Sandwiching the poly between a doubled top plate sounds interesting, less chance of tearthru where the clips attach... The approach of creating one predictable air barrier and good sealing at any wall top plate wiring/plumbing penetrations appeals to me. As opposed to a possible breakdown or leak at every drywall to top plate union, every wall penetration and every wall or ceiling taped joint. Thru good detailed drywall air sealing most all of these can be dealt with effectively except the drywall ceiling taped joints on a moving ceiling. I am also of the opinion that internal moisture is best controlled thru proper ventilation and not on the possibly unpredictable or perhaps insufficient permeability of the building materials. Wow I sure drug this thread away from it's intended topic, Sorry. Perhaps I should start a new one on air sealing.

Posted By arkie6 on 07 Aug 2015 04:20 PM
Another way to lock your ceiling drywall down at the interior partition wall top plates is to install 2x6 blocking between the truss chords over the 2x4 top plate.

Posted By FBBP on 07 Aug 2015 04:24 PM


"Sandwiching the poly between a doubled top plate sounds interesting, less chance of tearthru where the clips attach."
No to mention much safer! If you are a wall walker, you don't appreciate slippery poly under foot when you are moving trusses. Especially when wet or frosty! Just remember to stick the acoustic nozzle up any hole in the plates and give it a good squeeze to seal around any wires or plumbing.
We generally lap the ceiling poly down the ICF walls about one foot and seal it to the foam. The drywall then sandwiches it tight. Don't know if you have vapour hats available for electric out lets where you are but if you travel north every box store will have them. Vapour hats, h.d. 6 mil poly and acoustic caulk and there is no reason not to have a 100% sealed ceiling that will last for a long time.

Posted By Dana1 on 07 Aug 2015 06:31 PM


There a many ways to skin this cat, with lots of details to consider, some work better in some climates than others.  It's been discussed at length on this site before- count on it.

With a 40' span bottom chord loading specs become an issue at high-R with cellulose, and even if an OSB or 5/8" gypsum layer could take the load, you need to verify that the truss chords are designed & rated for it.  With 2x4s perpendicular to the truss chords under the OSB as a lighting & wiring chase the dynamic loads get shared among trusses, but the static load goes up due to the weight of the extra framing & OSB layer, even though you can back off on gypsum weight a tad. 

The OSB gives the chord load a bit more structural strength whereas gypum would add none, but unless you want to do the engineering on that you should treat it as dead load, and use only the truss designer's load spec for the chord.