I have a fair amount of drywall seam cracks in my ceiling but do need additional insulation(have cellulose now) in attic. What are options in this case? Concerned about additional weight.

Posted By georgec on 20 Feb 2014 03:07 PM
good point and to season the boards for a decade may not be an option also I kindof like the idea of a under deck look alike ceiling or if installed under the rafters as drywall would that would be fine too, but got to figure out a better way to seal it I guess.

is the tyvek paper a permeable membrane? I saw in various PH builds a similar membrane used to get the building airtight usually from the interior, wonder if something similar above the board would do

Tyvek is highly permeable to water vapor (>40 perms) but can be used as an air barrier. Some other housewraps such as Typar are slightly vapor retardent at about 10 perms, but nowhere near low enough permeance to be useful as an interior side vapor retarder. (Standard latex paint is  3-5 perms, less than half as vapor permeable as Typar.)

The membranes used on the interior side of PH and other high-performance houses are vapor-retarders, usually of variable permeance, that become far more vapor open when the proximate air is at a high relative humidity than when it's under 35% RH.  Below 30% RH they are all below 1-perm.





The common ones in use in the US are Certainteed MemBrain, and Pro Clima  Intello Plus, but there are a few others. At about 55% RH latex paint becomes the limiting factor on drying rates, looking at the MemBrain chart.  In winter when the ventilation air is much dryer the membranes become fairly vapor-tight, but in spring when the outdoor temps are high enough to cook out some of the accumulated moisture in the sheathing and insulation (if cellulose), the entrained air inside the wall goes to a much higher RH than the conditioned space air, but that high humidity causes the membrane to become more vapor open, passing moisture back into the house at a much higher rate than it ever got into the wall, making it something of a "humidity diode".

Asphalted craft facers on batts, and lighter-duty asphalted paper such as #15 felt exhibit similar behaviors, but in a less tightly specified manner than the polymer film type "smart" vapor retarders.

Posted By slenzen on 21 Feb 2014 12:45 PM
I have a fair amount of drywall seam cracks in my ceiling but do need additional insulation(have cellulose now) in attic. What are options in this case? Concerned about additional weight.

It really depends.  If the cracks are from the insulation weight as opposed to seasonal or wind driven dimensional shifts pouring on a ton more cellulose could be a problem, so you have to figure out why it's cracking.

Joists, or trusses?  You're more likely to run into the static load limits on trusses than joists, but with thin joists and long spans it's not out of the question when going for high-R.

so I know you argue at times that celulose insulation has enough air barrier properties in itsself, its also known that an air tight insulation cavity performs better, some will say air movement above the insulation will decrease its R value by as much as 30%, so considering one is willing to go the extra mile what is a good way to go about this?

on the larsen truss diagram showed a 6 mil poly sandwiched between the drywall and truss/inner chord, that is about as impermeable as it gets, I thought insulation is supposed to breathe moisture out not in, would poly be acceptable towards the inside of the house since latex paint limits the membrane anyhow, and maybe use a smart mambrane or even tyvec after all were just discussing air tightness as to improve the insulation performance, thou the moisture diode is interesting.

in most PH builds I followed the smart membrain was aplied on the inside, some I saw with a poly vapor barrier on the outside, I assume there is reasoning behind that.

OSB has been mention in order to prevent sagging. Does that work for a hot-humid climate like Florida where drying occurs on the interior. I'm looking putting in 12" is that even a concern in terms of weight per square foot with trusses spaced 24"?

Yes, OSB is mechanically stronger by far and more rigid than 5/8" gypsum. But do you need it?

The dead-weight loading of 12" of cellulose at real-world open blown densities is between 1.25-1.5 lbs per square foot. Half-inch gypsum is only specified for ~1.3 lbs per square foot at 24" o.c. trust/joist spacing but 5/8" goods will handle it.

Installing strapping 16" o.c. perpendicular to the truss chords and ring-shank nailing gypsum to the strapping half-inch goods would take 2.2 lbs per square foot. The weight of a full OSB layer hanging on the truss chords, which adds another ~1.5lbs per square foot. On long spans assuming 1.5lbs of OSB + 1.5lbs of insulation + 1.6lbs of gypsum = 4.6 lbs per square foot of load can approach the chord loading limits with 24" o.c. trusses. With 1x strapping you have less dead weight loading, and (in most cases) won't even have to look up the chord loading limits.

Having the truss chords mechanically linked with strapping (or OSB) also adds a lot of rigidity to the assembly, limiting the dynamic flex differences from truss-to-truss from wind-loads or vibration.

On new construction, you can easily add strapping. On a retrofit, it seems like cellulose over fiberglass is a good choice in many cases. But is there any practical way to add weight bearing capacity in a retrofit? I could imagine first putting in insulation just up to the top of the joists, and then laying boards over the top of the insulation. If the insulation was too thick that would leave air gaps which would not be a good thing. If the insulation was too thick, compressing the insulation with the boards would put stress on the gypsum board. So it seems like you need to aim for it to be just right. I'm thinking that 1x4 boards, spaced a few inches apart would actually be a good way to do this, because the cellulose that would fall into the 2" cracks between them would help prevent any continuity between any air pockets that result, while most of the cellulose weight would still be supported by the boards.

Posted By Dana1 on 20 Nov 2015 02:39 PM
Yes, OSB is mechanically stronger by far and more rigid than 5/8" gypsum. But do you need it?

The dead-weight loading of 12" of cellulose at real-world open blown densities is between 1.25-1.5 lbs per square foot. Half-inch gypsum is only specified for ~1.3 lbs per square foot at 24" o.c. trust/joist spacing but 5/8" goods will handle it.

Installing strapping 16" o.c. perpendicular to the truss chords and ring-shank nailing gypsum to the strapping half-inch goods would take 2.2 lbs per square foot. The weight of a full OSB layer hanging on the truss chords, which adds another ~1.5lbs per square foot. On long spans assuming 1.5lbs of OSB + 1.5lbs of insulation + 1.6lbs of gypsum = 4.6 lbs per square foot of load can approach the chord loading limits with 24" o.c. trusses. With 1x strapping you have less dead weight loading, and (in most cases) won't even have to look up the chord loading limits.

Having the truss chords mechanically linked with strapping (or OSB) also adds a lot of rigidity to the assembly, limiting the dynamic flex differences from truss-to-truss from wind-loads or vibration.

Hey Dana - have you ever run into a chart that give installed cellulose weights in different humidities?

So with 12" of cellulose 1/2" drywall would likely cup later while 5/8" would not on trusses at 2' spacing. But the most favorable method would we strapping every 16" with 1//2" drywall?

agagent - we typically use 1/2" drywall specified for ceilings. It is a fibre reinforced produce that will usually out perform 5/8 at less cost and less weight.

http://www.buildgp.com/toughrock-cd-ceiling-board

humidity will alway play a part in the equation. Wet board is weaker then nice crisp board.

We install Majpell (A Siga product from http://performancebuildingsupply.com) on the bottom of the trusses as our dedicated air seal - stapled and taped, then strap under it (at 16"OC), run our wiring between the strapping. Then we build interior walls, and hang the 1/2" drywall ceiling. The Majpell and strapping support the insulation and give us an airtight, breathable membrane. Zero sagging anywhere.

Posted By Roberth on 12 Apr 2012 01:15 AM
How much cellulose insulation can a ceiling hold. I have heard people say that cellulose can weigh too much for a ceiling. what can a 1/2 inch or 5/8 inch dyywall handle at 16 or 24" on center. What about plaster.

According to me, no one should expect ceiling insulation to draw moisture away. The life of insulation depends upon the shape of ceiling. In case of slope style ceiling you must fill rafter with cellulose.