Roof insulation during re-sheathing
Hello All, I’m new here, and thought I should start with a good, controversial topic: roof ventilation.
I’m planning a restoration of the exterior of an 1896, ballon construction Victorian in New Jersey. The building is about 1100 sq ft per floor, the roof consists of 4 gables. The attic was completely finished during the original construction, and no roof insulation exists currently.
We are about to tear off the roof (including original shake), and will re-sheath. My preferred approach for insulation is to lay-in foil-back fiberglass bats from above, and then place air baffles over those to assure an air gap for ventilation up to the ridges.
The roofers I talk to (1) do no want to take off the old furring strips before re-sheathing, and (2) complain about the logistics of these extra insulation steps when they have their “routine” that their crew knows how to follow.
Blowing in cellulose from below would eliminate all chance of ventilation, so I, and the insulation guy, don’t feel that’s the best approach. “Blowing” on foam from above, while the furring is still in place, could work, but isn’t my preference. (and is also an extra logistical step)
What do you think? Is my approach appropriate? Should I keep looking for someone that will work with me on the implementation? Are there other alternatives?
thanks,
dave
Replies
Dave,
I'd look for a smallish remodeling or renovation company to do a job like that. You need someone who has problem solving skills and is adaptable. Remodeling professionals are used to working in less than perfect conditions and dealing with things of this nature. A remodeler also has to be a "jack of all trades" sort of tradesman as opposed to a roofer. Most of my roofers don't want to deal with anything that isn't roofing. Maybe replace a piece of sheathing or two, but mainly they like to keep things simple and fast. A remodeler has experience with ventilation, insulation, sheathing and roofing materials.
(before anyone jumps one me, I'll admit I'm generalizing both remodelers and roofers for the sake of trying to keep it simple and get this fella a little help)
Dave, how deep are you're rafters? How much insulation can you fit in?
Your FG guy isn't fond of cellulose cuz he's a FG guy.
The roofer doesn't like this cuz he wants to tear of the old and put on the new. Stopping in the middle means he has no work for a few days, or has to tear down his set up and go somewhere else while you fiddle with the insulation.
What you probably need to do is seal the airflow up through the framing from the basement and any openings in the ceiling plane. This sort of thing takes time and some knowledge of what it is you're looking for.
The foam is the best choice, it will seal the air leaks at the same time providing more R per inch than anything else.
Costs more, does more.
Joe H
The local cellulose guy here claims that you don't need to ventilate the roof in this situation if you use cellulose - no airflow, no vapor transfer - but I don't buy it....
Toledo, Diesel, Joe,
Thanks for the ideas. The rafter depth appears to be 6"--i thought it would be more, but I checked in two places.
I haven't talked to a foam guy yet, it was the cellulose guy that was up-front and said that he couldn't provide any ventilation when blowing it in from below. He didn't take a position either way about whether that was good or bad--he said everyone has their own opinion. I'm in favor of using cellulose on the walls, and this guy probably will get the job--but I'm still unsure about the roof.
Yes, I need someone that likes the challenge as much as the paycheck! (I could hire myself, I guess) Trouble is, this isn't the job I want to get to know someone over--I'd go with a remodeler if I found a rock-solid referral. The roofing itself is a little complex, and steep--I feel much better with a seasoned crew doing that portion.
So, where does that leave me? What do you guys think?
1) technically, will i get good performance out of my approach of laying in foiled-bats from above, and leaving an air channel?
2) maybe I can pay the roofer extra for his time, and convince him to bring a smaller crew and work at a slightly slower pace, and I could provide a couple of guys to handle installing the bats from above while the roofers are there. maybe he could learn something along the way!
3) am i making a mistake not trying to apply foam from above? seems like coordinating the foam work and the roofing would be worst than the approach using the bats.
thanks, you all got me thinking again!
-dave
Anything you decide would be a compromise of conditions and results and cost. Foam is the best choice, but it will still slow the roofer some and increase his liabilities for keeping you dry inside.
I would be likely to opt for the denspack cellulose in this case ( Hear me Mike?) blown in frm below so that the roof and the insulation are two separate jobs, neither conflicting with the other.
Batts would be my absolute last choice. Can't even imagine why I would want to go that way
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I don't remember all of the details of you problem, but in another forum some one mentioned that he was having Hunter Cool-vent installed.
Basically it appears to add the insulation and venting on top.
http://www.roofingcontractor.com/CDA/ArticleInformation/features/BNP__Features__Item/0,3241,76729,00.html
If it's FG or nothing, I'd take the money and spend it on beer.
The FG is virtually worthless, might as well enjoy spending your money.
The cellulose is probably the most bang for the buck, the foam is the best for the most bucks.
Joe H
So that's two solid votes against FG, and two suggestions that lack of ventilation may not be a problem if the material is densely packed. (and the Hunter Panels look interesting, I'll read more about those).
I'm curious, why so solid a no to FG--is it difficulty of install, poor thermal performance, or other issues (performance over time?). My leaning that way was a combination of (1) looking for something simple that could be worked into roofer's schedule, and (2) a compromise on performance, since in the end, this building is never going to meet "2004" standards for thermal performance (no window upgrades, balloon construction, etc). I was making the assumption that even if FG was half as good as an alternative, it's not like I'm expecting to squeeze the last 5% of efficiency out of the structure over all. No solid math behind this thinking, just a bunch of compromises.
Maybe the compromise for efficiency, cost, and the ventilation debate is to create an air channel before placing the new sheathing, and then blowing in the cellulose from below. Votes?
thanks,
dave
Dave, Skip the vent channels.. Whole bunch of effort to gain zip.
FG works in theory, but the installation never matches the theoretical perfect installation.
The cellulose or foam will fill all of the cavity, it doesn't care what the shape is. The FG is not going to do the same, it is full of gaps and air moves through it like it wasn't there.
As the temperature drops, the R value of FG vanishes. Your FG guy probably isn't going to mention that one .
What kind of deal is that? The colder it is, the less insulating value?
Joe H
I am ignorant about the new foam. We went back after 10 yr to a urea formaldehyde foam job and all the foam in the stud spaces had shrunk back about 1 inch (all sides) and would disintegrate if touched. Will the new stuff do that?
Tradesman
Two answers to that Q -
One - they quit using that kind of foam years ago.
Two - The surfaces that the foam is sprayed onto must be at least 45°F or the chemical mix will shrink back from it aas it sets - but it happens right away, so is observable. Your experience indicates the former, rather than the latter.
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I'm curious, why so solid a no to FG--is it difficulty of install, poor thermal performance, or other issues
That pretty much hits the nail on the head. The "ideal" install for FG batts is in uniform bays that are square and true with no interruptions for wire, pipes, ducts or the like. This is rare enough in new construction as is. In remodeling work, it's non-existent. FG batts cannot be too loose, neither can they be too tight--both create air gaps that obviate the batt's existence. The R value in FG comes from the air trapped in the fibers. However, the glass fibers will eventually change temperature, which then affects the air trapped within. This is particularly true where one surface of the FG batt contacts a surface that is either cold or hot.
Since this is a finished attic, you have some conductivity issues (the interior finish is directly attached to the rafters which are attached the roof). With a 5 1/2-6" rafter depth, you are not looking at a lot of insulation. That suggests maximizing what you can get. That, in turn, suggests dense pack cellulose blown in.
Now, I'd be looking at getting at least an inch of insulation board up on the roof deck--especially since you are already doing a tear off. That will "break" the thermal conductivity of the deck-rafter-attic finish loop.
The only problem that creates is a bit more work at the edges of the roof, as you are making it thicker.Occupational hazard of my occupation not being around (sorry Bubba)
so this has all been very helpful...i hadn't measured the rafter depth, and was surprised to see only 6"--immediately got me wondering what i'd be able to achieve with so little space. And all the comments about FG are well taken--I was aware of them all, but conveniently overlooking them because I'd been thinking so hard about ventilation, and didn't see how to do it well with foam or cellulose. but you all seem less concerned about that then I have been--provided that foam or dense-pack cellulose is done right.
if i also look into a insulation board for under the sheathing, any particular type recommended? Is this a good time to think about a radiant barrier as well?
maybe we're almost done!
thanks,
dave
Finally, the light is penetrating the FG cloud.......
Find out what foam your roofer is familiar with or recommends & then check back.
If he sez "We don't do that".........keep looking.
Joe H
Edited 7/13/2004 12:46 am ET by JoeH
I'm back! I see there's been more than one additional dialog on roof insulation and venting. Hopefully you recall some of the background in my initial message.
I've been exploring a combination of cellulose blown in from below into the existing 6" rafter space (after the re-roofing!), combined with insulated foam nailboard product (local distributor recommends Atlas vented nailboard).
The local code official likes the job overall, but does want to see adequate ventilation in the rafter space. He is happy, under the conditions, to achieve this with a limited application of cellulose, rather than dense packing it. This suggests maybe 3-4 inches, for an R 9 to 12. In addition, he, Atlas, and the distributor all insist that the vented nailboard product be used with asphalt shingles. His concern is the outer sheathing more than the shingles themselves.
After talking to three roofers about the insulation in general terms, none were very interested. I have not approached them about this approach yet. Labor is more (edge work and use screw fasteners are the big items), but the overall impact on job-flow is probably the least (unless I skip it altogether, of course).
Cost:
for 2400 sq feet of roof, I estimate :
$2200 materials differential (vented nailboard is $1.55 per sq ft)
$3500 labor (this is 60% of what I think current labor cost is on lowest estimate gotten so far. It includes the additional edge work. "lowest" is not my goal, but this guy does get good refs, including from the code official.)
$500 fasteners and misc. materials
So that's about $6,000 to get the R6.93 that the vented nailboard promises. It does provide the thermal break to the 6" rafters, so that's a plus. But it does appear to be a lot of work, when I look at it objectively.
If I go with only the cellulose, and limit it to 3-4 inches, then I get R9 or 12, only, and no thermal break.
So, for those of you kind enough to follow all this, that's where I'm at.
1) can cellulose really be blown in to partially fill the cavity? I haven't asked yet.
2) technically, what do you all think?
3) personally, what do you all think? In the end, would you be happy you did it when you were done? :)
-dave
dave i'm thinking this roof is mostly gables , ( 4 gables ).. so there is mostly ridge and no eaves.. certainly not ebnough eaves to give balanced flow from eaves to ridge.. so the idea of venting is becoming an exercise in futility
no, you cannot "partially" fill a bay with blown cellulose.. you can either fill it or dens-pack it..
i've done the nailboard foam with the built -in vents.. in retrospect.. what a waste of money for your application... we used it to cover an old steel roof and add outside insulation where inadequate FG insulation was already in the existing chalet ceiling /roof..
i would sit down with the BI and try to get him/her to let you do a hot roof ( fully packed rafters...) then i'd find a shingle mfr. who was going to warrant their shingle on a non-vented roof..
non-vented roofs are allowed if you are using SIPS, so there should be a provision for a non-vented roof without SIPS... but your BI and the shingle mfr. will have to get on boardMike Smith Rhode Island : Design / Build / Repair / Restore
Mike, what is SIPS?
well the "damn the ventilation" chorus is back!! I've tried to attach two photos--I haven't done this before, but hopefully it will work. Yes, the roof is substantially gables. The vented-nailboard shows diagrams with airflowing in all sorts of directions...i wasn't real clear on how much of it manages to enter an eave on one pitch and finds it's way up one of the gables...i don't think hot air does that sort of thing naturally...
if everyone here in NJ would sign on to the dense-pack approach, I could live with that--in fact, for a modest premium, I could add some UN-vented nailboard (only $1.11 sq ft, not quite double the 1/2" OSB price), and with less work than the vented product I'd end up with a damn good roof, I think.
please let me know if you are able to see the photos.
-dave
great looking house.. but it also looks like a bout 50% is impossible to EFFECTIVELY vent..
BTW.. you should download the freeware "Irfanview" so you can resize your photos below 1K... yours are about 4 times bigger than most dialups can view easilyMike Smith Rhode Island : Design / Build / Repair / Restore
My suggestion would be to insulate outside the roof deck.
Strip off and re-sheathe if necessary (your post was unclear on that). Use 3/8 OSB or ½" Black Joe for this first layer of sheathing, and cover it with a double-coverage of 15# felt. Then lay on 3" Poly-isocyanurate foam board set into a 2x4 crib lag-bolted on edge through the sheathing and into the rafters. (You may have to special-order the foam board to get it in 3" thickness; Atlas Roofing makes this stuff, among other suppliers.)The 2x's will be proud of the insulation by a bit under a half-inch. Lay on horizontal 1x3 strapping over the crib, then sheathe over again with ½" ply. Put on a double coverage of 15# felt, and shingle over all. (This assumes you are putting on asphalt shingles; if you're putting on shakes, skip the second layer of sheathing and lay the shakes on that.)
Vent the soffits so airflow goes up above the outer sheathing; vent the rake boards also on all the gables. Be generous with the venting; too much is not a problem in this configuration.
For the lower-slope sections, you should put Ice-Gard on the eaves or even the entire section if it's small, like the turrets.
You should definitely find a good renovation outfit to handle this job; the actual shingling is the easy part and presents no problems that a reno guy with roofing experience won't be able to handle. But a straight-ahead roofing contractor will be out of his element with all the rest of the job, and neither of you would be happy with the result....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
What is "black joe"? I always thought people were referring to Celotex, but I can't imagine you would recommend that for sheathing.
Jon Blakemore
Black Joe is an old brand name for a tar-impregnated version of compressed paperboard, which is also known to some as 'Ten-Test' (another brand name that's passed into the vernacular). As structural sheathing it's a non-entity, but an enormous number of homes are built in this rigorous climate using it as exterior wall sheathing where racking shear is not an issue. It performs well in that application as it is weather resistant, inexpensive, and easy to cut and handle.
In the application I specified for this roof, there are no structural issues at the level where it would be used; it would simply serve as an intermediate weather membrane and provide a surface to which the polyiso panels could be 'glued' in place with bituminous pitch. The 2x4's that make up the vertical cribbing between panels of polyiso would actually be carrying the load of the roof deck and shingles, and transfer that load directly to the rafters to which they are to be lag-screwed through the Black Joe.
Black Joe also has a medium-small R- value, but I can't remember how much offhand. Think of it as extruded very-dense-pack cellulose....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
No advice but I must say-
Very nice house!
Jon Blakemore
2)
TECHNICALY? - YOUR BUILDING INSPECTOR IS IGNORANT!
Reducing from denspack cells or BIBBs to a loose fill will both reduce the R-value and will increase the convection heat loss while allowing for moisture to move through with the convection air and make the ventilation an absolute necessity. He is trying to enforce ventilation while ignoring all the basic principles building science has learned about insulation in the past generation.
But be forgiving of him, it is in the nature of government bureaucrats to be always fighting the last war out of ignorance.
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"The R value in FG comes from the air trapped in the fibers."
may I pick at that statement a little bit?
It implies that the fibres are hollow and therefore contain the insulating air within each fibre, in the way that Hollofil does in sleeping bags. This is not correct. The FGB is spun glass fibres and not hollow.
The more accurate statement, which I'm sure you meant to say, would be, "The R value in FG comes from the air trapped inBETWEEN the fibers."
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how are you going to ventilate the gables anyway ?Mike Smith Rhode Island : Design / Build / Repair / Restore
This probably won't apply exactly to your situation, but about 17 years ago two of us finished an attic on a 1920's house here in west central Illinois for a lawyer (we didn't want any problems).
We got the CIPS energy consultant involved and he said he had a theory about this that went back 25 years with success. He said "fill the 8 inch rafter spaces FULL of fibreglass unfaced insulation". We held it up with stapled string until the drywall was on. We drywalled the underside of the roof rafters showing all the roof geometry. It included a loft and looked pretty sharp.
He said the venting space would bring in moisture & problems, so fill it full. He said the unfaced fibreglass would let the moisture come and go through the drywall. The job has stood the test of time with no problems. There is no vent. There is no vapor barrier. The paint has not peeled or the drywall cracked.
This guy was also against plastic. He said the moisture will get in and the plastic will hold it. (I know this goes against the conventions, but the job has shown no problems.)
More facts about this house: man made slate roof/ no wall cavity insulation, however T&G 3/4 beaver board inside studs with plaster over/ wood sheathing & tar paper with brick veneer outside/ forced air heat.