Cellulose settling in wide wall cavities
Situation: dense-packed cellulose in a 12″ thick double stud wall cavity, 9-10 feet tall. New construction, likely mesh stapled to inner wall studs.
I’d like to know of any good info on whether or not there is likely to be any long-term settling of the cellulose in such a wide cavity. I understand all the basics, that packing to around 3.5 #/cuft density makes it over twice the natural settled density of loose cellulose, so that it always wants to expand, not shrink more.
On Rick Karg’s (cellulose insulation expert from Maine) site there is a paper that suggests the possibility of settling and the resultant gap at the top of a wide, tall cavity with smooth retaining surfaces. The example was 12″ cavity perhaps 14 feet tall, with OSB on one side and poly VB on the other, unless I remember the numbers incorrectly.
I seem to recall having read somewhere of someone stuffing a piece of FG batt into the upper end of each part of the cavity, and blowing cellulose up against that, compressing it. The idea was that if any settling of the cavity occurred over time, then the FG bat piece would expand downward to fill what otherwise would be a slight void.
Does anyone know where I may have read that? Does anyone have experience with long-term settling of dense-packed cellulose in cavities?
Replies
Check this video out.
It doesn't answer any of your questions but is sort of relevant and pretty cool too.
http://www.youtube.com/watch?v=MgfgazrrSlw
The loose fill insulation blower in the video puts anything I have seen in N.A. to shame. Though, that said, there may be some progressive insulation contractors with something similar.
I blew cellulose into 2x6 stud cavities, and did notice some slight subsequent settling. Next time I'd be a little more aggressive in ensuring the density of the fill.
Try Taunton's Green Building Advisor. You might find someone with Passive House experience that has dealt with that issue too.
Also worth a read...
http://www.greenbuildingadvisor.com/blogs/dept/musings/installing-fiberglass-right
Maybe I missed something in the video ... like the point. Interesting stuff. Overkill? Maybe? But the read out on the blower door ... am I getting this right? 0.44 at 50 pascals? That ain't worth spit. Maybe I'm rusty in what to look for or what I'm looking at? All that work, I'd expect a value of 0.10 or less easy with that type of construction.
I sent the link to that video to a friend of mine who has spent a couple of winters at the Passive House Institue in Germany, and he seemed impressed with the .44 ACH (air changes per hour). I may have mistaken his repsonse, but I believe that .44 ACH is under the the Passive House requirements for the blower door test.
Cheers,
Andrew
My understanding is that the ACH at 50 pascals is divided by something like 20 to give expected ACH under normal atmospheric conditions. Of course "natural" leakage is highly dependent on temperature difference and wind speed/direction. So maybe that 0.44@50 is like 0.02 to 0.03 natural, quite enviable.
That's probably what I'm forgetting ... that the 50 pascals is at a higher [i.e. forced] leakage rate. I've done blower door tests myself but I guess CRS has got me a bit. It's been 15 yrs since I did that and I guess it didn't really stick for me as I was actually helping someone else.
Out of curiosity, I just found this by quick search:http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/940111.htmlIt discusses the origin of dividing by 20 to come up with average air infiltration and also presents some refinements to adjust for type of construction, location, etc.In any case, "your actual mileage will vary," and ACH @ 50 pascals is only an indication of tightness and not a way of calculating what the actual infiltration will be. When it's windless and mild outside, the infiltration rate could be zero. That's why I am irked by folks who argue against tight construction, saying "the house has to breathe."
I knew I was missing something (more than what is sitting on my shoulders). Thanks for the link/reminder. Yeah, the average actual is based on the 'average wind speed in winter', I think. At zero wind speed, though you still have leakage due to the thermal stack affect (warm air out high cracks, cold air in low). R-values are set and tested the same way ... actual can be somewhat different depending on the construction.
".. actual can be somewhat different depending on the construction."And that's what bothers me when a builder figures he can rely on leakage to provide fresh air. There is no way to design in the correct amount of "untightness." Whatever leakage results will vary from nil at some times to either still insufficient or far too drafty at the worst times, with no way whatsoever of controlling it short of opening windows.
In the old days ... by nature of the construction, the houses tended to be REAL LEAKY ... that way it would leak right (i.e. a minimum or more) a lot of the time. Modern construction ... leak using a ventilation system and control it for when you need it (e.g. occupancy or humidity). Controlled air leakage is always more energy efficient than uncontrolled leakage.
There have been opinionated discussions about this in Riversong's larsen truss wall thread. Either search for it or try hitting up Riversong himself.
He was for it, and said he had a system to prevent settling.
Iam
Here is the Larson wall truss thread. I'm pretty sure Dick was in that discussion.
99930.5
Rich
thanks for chasing down the linkI remember learning a lot from that thread
Iam
You are welcome.
I keep it in my favorites.
Rich
I didn't pour through all the posts on that thread, but I may have chimed in at some point. I did look yesterday at the writeup and pics of his Modified Larsen Truss project on the (I think) Build it Solar site. In that link, he describes having the 18-20" of loose attic fill contiguous with the top of the wall cavity, so that any settling that did occur would result in a small bit of the attic insulation dropping down to fill the void.If his outer wall cavity is continuous insulation, 20 ft tall, then at 3.5 lb/cuft density the weight of the fill produces about 70 lb/sqft or a half of one psi pressure at the bottom of the cavity. I wonder what the internal pressure and density of a packaged 30# bale of cellulose are.As to fire stopping, I imagine some BI may insist of something solid every 10 feet. Riversong says he got the BI on his projects to accept the dense packed cellulose as firestopping, and there is sound reasoning to support that it would function that way. Remember the pics on TV of a guy holding a clump of loose cellulose in his hand and melting a penny on top of it with a torch?I'm inclined to think that a 10 foot wall at 3.5 lb/cuft density, kept dry, would not settle. But it sure would be nice to have evidence from demos done after a couple of years to confirm that it wouldn't settle. That's why I asked.
Dick
Sorry, but i have no experience blowing super deep walls.
Just links.
Almost all of my experience is blowing existing walls 2x4 or 2x6.
I would feel better with a netting or OSB barrier at the 10' level of a tall wall.
I think it would make it easier to blow too.
Did you catch that thread on the Remote wall system? It might be of interest to you.
http://forums.taunton.com/tp-breaktime/messages?msg=120232.1
Rich
Edited 7/22/2009 1:40 pm ET by cargin