Insulation retrofitting of a 3rd floor finished attic space
Hello GBA,
I am in Northern NJ, and I have a question on the proper way to retro-fit a finished 3rd floor with insulation, for both comfort and energy efficiency.
In many old colonials and victorians in my neck of the woods, what was once un-finished attic space has been converted to living space. The rafters typically have had gypsum board installed, with a flat ceiling framed above and kneewalls dropped, leaving kneewall “crawlspaces” to either side. The floor of this newly finished 3rd floor is typically un-insulated, as is the case with most homes of this era.
The decision on where to define the envelope can be tricky with regard to potential moisture issues. I often air seal and insulate the high flat attic, dense pack the finished sloped ceiling, then air seal and insulate the knee wall. I am concerned primarily about 2 issues:
1. Am I creating a potential moisture issue at this sloped ceiling where I have just dense packed?
2. Am I creating a major problem by making a portion of the roof “hot” while the rest of the roof is cold? Note that these homes typically have NO attic ventilation.
When the customer’s budget allows, I prefer to gut the whole room and properly spray foam the roof and gable walls, but that is not always an option financially. Nor is installing outboard rigid foam.
Lstiburek just wrote an article slamming this type of dense packing practice…
Thoughts? Should I just run away if the customer won’t gut it?
thanks!
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Replies
Grant,
Dr. Joe was only half right. The example he highlighted in his Don't Be Dense article was an incompletely insulated flat roof with no exterior drying potential and uncontrolled indoor humidity. No wonder it failed.
Bill Hullstrunk, the technical manager for National Fiber Cellulose (and an instructor of superinsulation) rebutted the BSC jeremiad. Bill said "Tens if not hundreds of thousands of roof assemblies have been successfully dense packed since the 1980’s and today it is common practice to dense pack roof assemblies in both retrofit and in new construction. Unvented roof assemblies dense packed (3.5 lbs/cuft) with cellulose are more robust than those filled with foam due to the hygroscopic properties of the cellulose insulation which provide active moisture transport and dispersal."
The key to your project is to define the air barrier well (make sure there is sealed blocking between the joists under the knee walls, for instance), make sure indoor humidity levels are within the safe zone, and do the best you can with what you're given to work with.
For added security, you can always install rigid vent baffles under the roof deck through the slant ceiling sections - even 1" of space will help. But most sloped residential roofs can breathe to the outside (unless they are reroofed with peel-n-stick membrane, which is foolishness).
Grant,
For more opinions on dense-packing cellulose in unvented rafter bays, and a discussion of Lstiburek's article, see Joe Lstiburek says, Don't dense-pack unvented cathedral ceilings.
Here's my view:
1. If you want to fill unvented rafter bays with insulation, use closed-cell spray polyurethane foam.
2. If you want to install cellulose in rafter bays, first install site-built ventilation channels using 1x1 sticks in the corners and plywood.
3. If you want to convert an existing attic into conditioned space with kneewalls, then the insulation should follow the roof line all the way down to the perimeter of the attic rather than following the kneewalls.
Grant,
One more point: you should check with your local building inspector to see if it is legal in your jurisdiction to install dense-packed cellulose in unvented rafter bays. In many jurisdictions, such installations are now illegal.
The 2006 IRC includes a section (R806.4) that requires the use of "air-impermeable insulation" in unvented rafter bays unless you also install a significant quantity of rigid foam insulation on top of the roof sheathing.
Thanks, Robert, for the reply.
I think I would prefer to dense pack the slopes in questions....it can be impossible to install vent baffles the entire span of the roof slope from either above or below due to roofing nails, access, length of the span, etc. I lock the bottom of the roof slope in with spray foam as a backer, then dense pack the slopes from above. Comfort and energy issues seem to be resolved for the customer with this application. Needless to say, we are air sealing below the kneewalls and any flat attics within the kneewall "crawlspaces"....
thanks again.
Thanks too, Martin.
I'd like your opinion on one other issue here, if possible:
Closed cell spray foam to me is the obvious choice, but what if there is already drywall installed, and we need to access ONLY from above/below? I have never worked with a closed cell foam product that I felt could be installed in this method....hence the dense packing. Are there any foam products you are aware of that can be installed into an already finished, un-vented, sloped ceiling? Lets say the span of the slope is 12 feet.....
thanks!
Grant,
There are some foam products that can be installed in closed cavities, but I don't recommend them.
Remember: drywall is cheap and easy to patch.
Unfortunately, the current IRC restrictions on insulating unvented cathedral ceilings are like the old code requirement for interior vapor barriers - they are "insurance" against poor practice and inadequate materials.
Done well, there is no reason that dense-pack cellulose at a minimum of 3.5 pcf cannot be used safely. In fact, I believe that closed-cell foam can be much more dangerous to a cathedralized roof assembly if the roof cannot breathe to the outside since it can trap bulk moisture in the roof deck in the event of a leak (and studies have confirmed this).
The primary advantage of closed-cell foam is its higher R per inch in limited spaces. But IECC R-value standards can be averaged over the entire roof assembly, so a reduction in the slant ceiling can be offset by increasing the R-value in the flats.
Also, the code language requiring "air-impermeable insulation" can be interpreted variously. While dense-pack doesn't meet the extremely stringent air barrier material standard, since it qualifies as a fire-stop and will dramatically improve the air tightness of a house envelope it can be considered an air-semi-impermeable insulation.
Robert,
I disagree with your statement, "the code language requiring "air-impermeable insulation" can be interpreted variously." According to the 2009 IRC, air-impermeable insulation is defined as "an insulation having an air permeance equal to or less than 0.02 L/s-m2 at 75 Pa pressure differential tested according to ASTM E 2178 or E 283."
According to Joe Lstiburek, who helped write and promulgate this section of the code, dense-packed cellulose "doesn't come close to meeting the definition of an 'air-impermeable' insulation -- not even within an order of magnitude difference." It leaks too much air when tested according to either test method to meet the code definition for an "air-impermeable" insulation.
I almost said that Dr. Joe probably helped write that section of code, for it certainly reflects his prejudice. I think he's wrong and it's as unnecessarily restrictive as the old vapor barrier requirement.
And, even though the building codes are getting ridiculously detailed in their prescriptive requirements, a building inspector always has the responsibility to interpret the code.
When codes specified "fire-stopping will be nominal 2x wood", I had three different inspectors in three jurisdictions allow me to use dense-pack cellulose in lieu of wood as fire-stopping.
Robert,
Okay, Robert.
X or 10X -- what's the difference? It's just an order of magnitude.
Here's the bottom line: the technique is risky and clearly illegal according to the most recent versions of the code. But, if you want, you may be able to talk your local building inspector into accepting it.
Maybe. But I think the definition of an "air-impermeable insulation" doesn't allow enough wiggle room for such an order-of-magnitude "interpretation."
And I will continue to argue that it should, and probably some day will.
Just as the codes used to differentiate between vapor impermeable and everything else, code now recognizes that there are different orders of magnitude that are appropriate in different applications - and we recognize four classes of vapor permeance.
If we applied the same intelligence and discernment to air barriers (which are the new "vapor barrier"), then we would allow the proper order of magnitude to the appropriate situation.
The fact is that Bill Hulstrunk is right - that there are tens if not hundreds of thousands of successful examples of dense-packed unvented cathedral ceilings - and Dr. Joe is using a much too narrow lens to denigrate a technique that has multiple benefits over spray foam (as are you).
Did anyone address Art's second question?
"2. Am I creating a major problem by making a portion of the roof "hot" while the rest of the roof is cold? Note that these homes typically have NO attic ventilation."
Will this type of insulating reduce ice dams or cause them to increase?
Is there a recognized method of opening the soffits, using spray foam over 1x1 sticks & plywood (as Martin suggested for cellulose), baffle vents over the slanted part of the roof, and then adding a ridge vent?
April,
If I visualize this correctly, the sloping roof has three sections:
1. The lowest section is above a triangular attic behind a kneewall.
2. The middle section is insulated.
3. The upper section is above a conventional attic above a flat ceiling.
This arrangement is typical of Cape Cod homes.
If you want roof ventilation, you can certainly provide it. To do it right, though, you need soffit vents and ridge vents, as well as ventilation channels above the insulation installed in the center section of the roof. You also need to be sure that the triangular attic behind the kneewalls has been carefully air-sealed.
If you want to omit ventilation, you can. However, you have to be sure that the insulation used in the center section of the roof meets code requirements for unvented roofs.
And if you want to avoid ice dams, you have to be sure that your air-sealing work was impeccable and that all of the insulation installed -- insulation over flat ceilings, insulation behind kneewalls, and insulation in sloping roofs -- meets or exceeds the current minimum code requirements.
Thanks Martin,
Yes, I'm talking about the roof line of a typical cape cod as you described, though we will actually be retrofitting insulation in a third story finished attic- the primary purpose being to eliminate/reduce ice dams. The difference is that I wanted the lowest section behind the knee wall conditioned, which means insulating along the underside of the roof instead of the attic floor.
It has been suggested to me to spray foam the lowest section directly on the underside of the roof, and then blow in cellulose on the sloped ceilings and flat ceilling. In this case, attic ventilation would be omitted.
But is it possible to use this method and also add the insurance of ventilation? Or is that overkill? I'd hate to put a lot of money into insulating with no affect on the ice dams.
April,
You didn't mention the thickness of the cavities being filled with cellulose nor the anticipated R-value you will achieve.
Installing cellulose insulation in unvented rafter bays is a code violation, unless you also install rigid foam insulation above the roof sheathing. Although the code is unequivocal, some local code officials are routinely talked into approving this insulation technique.
In most older homes, it's hard to get enough R-value in a sloping rafter cavity using cellulose to stop ice dams.
Nothing like a blast from the past....
So we have theoretically separated the roof into 3 "sections"....kneewall roof, sloped roof, upper roof.
My two questions:
1. If one or two of these sections, say the kneewall roof and the sloped roof, have insulation applied to the underside of the roof deck (or the rafter bays dense packed...), will this have an impact on ice damming? I tend to think that, although we are absent any ventilation to wash the underside of the deck with Outside Air, the roof deck should stay sufficiently cold to prevent ice damming, provided adequate R-value is provided and air sealing is meticulous.
2. Is there an issue here with the un-even aging of the shingle and/or roof deck?
Also, my biggest concern with dense pack is, as always, in the installation. I feel that it can be easy to get proper density in the center of the cavity (here we are "controversially" discussing enclosed rafter bays), but that we lose that density at the extremities of the cavity. Where the density ceases to be sufficient, we can get air movement. And that is no bueno.
Also, curious your thoughts on the attached approach. I reason that if the top and bottom of the enclosed rafter bays are meticulously airsealed, with the CCSF acting as a "plug" that you can dense pack up against, that this method could work. You will have properly dense packed cellulose in the sloped section, and the spray foam at the tops and bottoms of the rafters will eliminate any path or pressure differential to guide airflow through this section should proper density not be achieved. I don't think diffuse moisture would cause a problem through the wall board, and I can't see how we'd get an ice dam here.
Thoughts?
Art,
I agree that you don't need ventilation to prevent ice dams. As long as air leakage has been addressed and the sloped roof has been insulated to a high R-value, an unvented sloped roof can work well.
The problems we see with this approach include the following:
1. It's important to follow code requirements. Air-permeable insulation can't be used for an unvented rafter bay unless there is rigid foam on top of the roof sheathing.
2. Many failures occur because of air leaks and convection currents.
3. Many attempts to implement this approach use insufficient R-value because the rafters aren't deep enough for the required R-value.
We live in NW Pennsylvania, zone 5. There is 12 inches of cavity space.
This was the exact recommendation:
8" of spray foam along roof deck = R 50
dense packing along slanted roof= R 44?
10" blown in cellulose on flat ceiling= R 31
4" spray foam to gable ends of the knee wall space= R 25
Rigid foam above the roof sheathing is not an option at this point so it looks like venting would be necessary. That would decrease R value on the slated ceiling to around 40. It also means finding a way to vent the lowest part of the roof between the roof sheathing and spray foam.
Thanks for brain storming. It doesn't appear to be an easy fix. It woulld be expensive to use spray foam to fill the rafters above the sloped ceiling, and it would be tedious to correctly vent the attic since it currently has no ventilation.
Hmm.