Insulating an attic
We’re going to be refinishing an attic. It’s got gables as each end, and just a straight run in between. So, we’ve got to insulate the roof rafters.
For the two ends of the gable, we had put in our bid standard R-19 fiberglass insulation. But, for the roof rafters, we had put in our contract that it would be cellulose, as a lot of what I had read seemed to indicate that cellulose was definitely superior to fiberglass – and we wanted to do the best we could afford on the roof rafters – since so much energy can pass through that area.
The rafters of the attic are 2X10″ thickness.
Now, the subcontractor that the GC called in to bid on the job is recommending against cellulose. He says because it is a wet application, and with all the heat and humidity these days (it’s been raining like heck lately) that it would take forever to dry out. He also said that if we ever got a leak in the roof, the insulation would turn into mush (I thought I had read that moisture resistance was supposed to be one of the positive things about cellulose).
His plan instead is to start with a propavent for ventilation, then 8″ of R30 fiberglass, and then finish with a 1/4″ of solarguard which has an R-value of 9.
So, I’m in a bit of a quandry. I know that the sub the GC called in is a very reputable company. The fixed price contract I signed with the GC clearly calls for cellulose, not fiberglass. Should I demand we get cellulose, against the advice of the subcontractor?
Foam is just not in our budget.
cellulose is what you want, doesn't have to be sprayed wet, use the net system. Mike Smith can answer better.
well... the 1/4" of solarguard giving you R-9 is hype (in my not so humble opinion )
and if cellulse gets wet from a leaky roof, it is no worse thatn fiberglass getting wet from a leaky roof
the insulation contractor's problem is he wants to blow wet-pack and 8 " is too thick for good drying conditions... ask him what the incustry standard is for optimum drying
i'd use insulmesh and furring ( or leave out the furring ) and blow dry dens-pakMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
When did you get that middle name??
I knew you would have tyhe answer!!!!
Edited 7/24/2008 2:54 pm ET by frammer52
I asked him if they could use mesh and blow in cellulose, he said they don't blow in cellulose - they only blow in fiberglass which is obviously no better than using the batts.
"I asked him if they could use mesh and blow in cellulose, he said they don't blow in cellulose."
Sounds like you need to find yourself a new contractor. This one limits what he provides to his customers by what he wants to do -- not by what is best for the conditions. IMHO, mesh (scrim) and blow-in is great for a roof. And, if I wuz you, I'd have the gables blown in with celulose as well -- not much savings using FG (if any) if they're doing the roof at the same time.
Mike HennessyPittsburgh, PA
PS: Celulose settles less than FG, IMHO, since when it's properly installed, it's packed way tighter.
Edited 7/24/2008 3:17 pm ET by MikeHennessy
Wet application is not possible, though, correct? How long would it take to dry? Seems awfully humid these days.
I talked to another insulation contractor. Seemed like a pretty knowledgeable guy.
One issue that he brought up is that dense pack cellulose would crush the propavent. He said the vent would have no chance against the pressure. One possibility is to put in a more rigid vent system, use 1 by's and OSB or something, but that's quite a bit of carpentry work. I don't think that's an option for us.
So, the only cellulose option we have is a wet install with the right amount of binder so that it sticks appropriately. Unfortunately, this guy doesn't do that (that seems to be a primarily new construction technique and around here most insulation work is old work)
He said he's done a good bit of work with rigid foam insulation. He takes 2" boards, and triples them, to give an R42 - held in place with a little bit of expanding foam. Unfortunately, it's pretty expensive - $8 / square foot. That's in line with icynene.
edited: math error
Edited 7/24/2008 9:40 pm ET by CeltsFan
R52 in 6"? That's an R value of 8.67 per inch, which is much higher than anything I've seen. Polyisocyanurate will yield about R6.5 per inch after the gas escapes. I don't think you will find anything higher than that.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
R-52 for 6" of foam is fantasy. You sure you heard him correctly?
He told me that each 2" solid foam board (I think they were also foil faced - if that matters) had an R-14. So, sorry for the poor math, but 3 X 14 would be R-42. So, R52 is incorrect.
Keep lookin'. I KNOW you can get vents that work with blow-in.
Kinda' makes me all warm & fuzzy to know I have apparently found a good insulation contractor. ;-)
Mike HennessyPittsburgh, PA
Talked to another insulation contractor. His opinion was that densepack cellulose wouldn't crush the propavent. He also said that we wouldn't need the propavent if we densepack, that you can go with a "hot roof".
I've read the hot roof vs. non-hot roof stuff. Personally, I feel a little more comfortable going with the traditional cold roof - especially if there's not much downside (what do you really lose other than an inch of space and some plastic propavents?)
Very few people I've talked to do wet spray around here. It's pretty much all dry.
Talked to another insulation contractor. He also felt that they could do densepack without crushing a propavent. He said they could also use plastic vents instead of styrofoam, which are a bit stronger.
So, the score so far is:
Densepack "Yes" - 2
Densepack "No" - 1
Lot higher score than that if you count our posts.
Gotta toot my own horn here - this IS the definitive attic reno thread, if I say so myself - http://forums.taunton.com/tp-breaktime/messages?msg=84041.1 .
I used Johns-Manville "Spider" dense-pack glass; no ventilation (Atlanta area)
The verts have the glue sprayer turned on; 'glas will stack up.
Had dinner with the client last week, a year later, he says their gas/elec consumption is no more than it was before - and with ~1200 new square feet up there!
Forrest - did some purty work, oncet
Any issues with installing cellulose on an angle; e.g. in roof rafters? You'd think it might have a tendency to settle out towards the eaves.
Would wet spray cellulose crush a propavent?
Is open cell foam even an option (cost aside)? I would think that would crush a propavent as well (if it bows vinyl windows you'd think it would crush those flimsy vents).
i don't think it would crush the PropaVents we useBUT.. if i dens-pak a rafter bay , my BI allows me to skip the ventingMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
What the others said. You should still use a vent channel just under the decking, tho'. It helps the roof stay a bit cooler and has the added benefit of protecting the insulation from getting wet in case of a minor leak. (The downside is that it's a beach to find the leak if you get one, since it always comes out in the same place -- the bottom of the vent channel.) Make sure you understand the need for, and how to achieve, proper venting and that your contractor properly installs it. Under certain conditions, improper venting, especially in an attic, can rot out a wall or roof from the inside in short order.
FWIW, I like celulose, and use it whenever I can over fiberglass. Insulation contractors tend to pick one side and badmouth the other -- often without basis. I get the feeling their advice depends on their profit margins on the various products. FG has its uses, but for a retrofit, if you can't afford foam, celulose does a much better job, IMHO, in stopping airflow through the insulated space -- one of the major sources of heat transfer in old work.
"His plan instead is to start with a propavent for ventilation, then 8" of R30 fiberglass, and then finish with a 1/4" of solarguard which has an R-value of 9."
So you are going to leave the ceiling unfinished? And just have the solargaurd exposed.
That is the only way that would work.
Now there are a bunch of solarguards, from window films to vinyl fencing. But I assume that this is the one.
Radiant barriers have an very low R-value, 1-2 or even less, based on the standard R-value tests. That puts a hot plate on one surface of the material on test and measure the heat flow to a plate on the other side.
The only way to use a radiant barrier and get any affective heat control is for it to have an air space on one side.
Look at the footnotes on that page you you will see that they are testing an ASEMBLY, not the material.
And there are questions about if that can be realated to the real world the same way that other insulation with a R value can be.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Signed contract with GC who is using a sub who only does wet spray cells.
Why would you put FG in the gable end walls and something else in the roof.
Search here for "Mooney wall" or ask Mike Smith for an explanation.
Skip the venting, dense pack the roof and the end wallk.
The wet spray isn't going to work for you, and the guy you're dealing with isn't offering anything that will.
R-9 in 1/4" is bullsheet -- forget that too.