Under insulated roof
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Hi all,
I conducted some testing on a home in Northern Minnesota recently. Homeowners complained of higher than expected heating costs and uncomfortable areas within the home. The home is a 1999 four piece manufactured story and a half. The sloped ceiling is insulated to R-19, (not sure how this home made it to Minnesota), has a polyethylene vapor/air barrier and is ventilated. The home tested very poorly at 13.3 ACH50 with the majority of the air leakage in the knee wall storage areas of an upper level bedroom. Air sealing the knee walls is straight forward, but I would also like to suggest additional insulation for the roof. I am thinking of suggesting something similar to Pauline Guntlow’s recent blog Solving an Ice Dam Problem with Exterior Rigid Foam. According to the article, a minimum of 61% of the R-value would need to be rigid insulation above the roof deck for my climate. With the existing R-19 fiberglass under the decking, the roof would require around R-31 above. This suggestion would change the roof from vented to unvented. My concern is with the existing polyethylene. Would this have to be removed, which is difficult without removal of the finished ceiling, or would the temperatures of the roof sheeting rise enough to eliminate the risk of condensation?
Thanks,
Randy
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Replies
Randy,
No, the polyethylene would not need to be removed. While it's true that the polyethylene isn't ideal -- it will inhibit inward drying if there is ever a roof leak -- its presence won't cause any real problems. The solution you suggest will go a long ways toward solving this comfort and energy problems. I wouldn't worry about the polyethylene if I were you.
The trickiest part is the air sealing at the soffits and ridge. Do a good job there, and you'll have great results.
That is some major air leakage. More details would be helpful in deciding on a workable solution for this house. Is there no opportunity to insulate from the inside, what is the age of the roof?
The roof is original to the home, 1999, and showing some wear. Will need to be replaced in the near future. The homeowner does not want to remove the interior ceiling, which is requiring the exterior repair. The roof is constructed with 2 x 6's. Removing the roof decking and spray foaming from the top would only achieve R-30 or so, although it would substantially improve the air leakage rate. I zone tested the upper level bedroom with the blower door running, it hit 45 Pa, meaning its 90% connected to the outside. By far the worst room within a house I have ever tested. Another item I didn't mention in the original post is the forced air heating system is located in a 3 foot crawlspace. The concrete block walls are insulated with 2 inches of XPS, the rim has 6 inches of fiberglass. The heating system has two vents cut into the ductwork to keep the space above freezing. No returns. The air leakage through the rim joist is melting the snow the owner banks against the foundation. Heating the crawl space with the forced air unit is depressurizing the home. I'm thinking that while the air handler is on, the stack effect is overcome and air is moving from the leaky roof into the home, adding to the homeowners discomfort. Very interesting home to test.
Randy,
I am working on a similar project (MSP), R-22 Kraft faced batts in a 9 1/4" rafter space. Air leakage for this house was 3744 CFM50 or about 7.5 ACH50. Snow melts on the roof with any accumulation and forms ice on the leaf guards with icicles over the edge. I have addressed 2 areas so far with a dedicated air space, reused fiberglass batts and Polyiso on the underside of the rafters to bring the R-value to code.
One of the very interesting first discoveries was pressure testing after the first roof area was complete, 240 square feet of flat kitchen ceiling with 80 square feet of knee walls bordering heated areas (vaults).
We had only worked 320 SF of ceiling and knee wall out of a total 1,465 SF area and we had a 12% reduction in ACH50. Will be very interested to see the ACH50 results when the work is complete. There was no poly vapor barrier on this house and it most certainly had communication between the walls and the vaulted ceiling which we are addressing. My first impression is the bulk of the air leakage in this home at least is through the ceilings. My experience in doing energy retrofits is take care of the ceilings first. This minimizes the stack effect and suddenly brings some level of comfort to a home that had none before. Get rid of the high leaks and the cold air coming in down low will be much less.
If you are doing a roof over as you describe, attention to detail for the thermal boundary is critical. When you start to tighten up buildings the leakage areas go from diffused to more concentrated and this is where moisture issues can crop up. Rim joists, wall and roof connections need TLC.
Doug,
Thanks for the advice. Given the high leakage in the roof, I doubt the walls are much better. My experience has been once one area is tightened, such as the roof, other air leaks appear. They were always there, just hard to detect because of the overwhelming large leak in the roof. I doubt the roof insulation will happen, the building owner is 85 years old. Just wanted to give him several options.
Randy,
You said the 2X6 space is ventilated. Do you mean there is a ventilation baffle above the insulation or just a gap between the insulation and the sheathing? If there is a gap, I believe you will need to fill that space to bring the air permeable insulation in contact with the sheathing (in addition to sealing off the ventilation channels). Sorry if I am misunderstanding the situation.
Randy,
I agree somewhat with what you said about air leakage but sealing the high leaks is priority one. Thermal buoyancy, stack effect are in play and the high leaks, well they really leak. Walls are mostly located in the neutral pressure plane and will show up as leaks with the blower door but will not so much in normal day to day conditions. I have witnessed in our own house the value of a hot roof (very airtight and highly insulated, R-100). It is very dramatic from a comfort and energy use standpoint.
Steve,
There is a ventilation baffle present. I thought about removing it, but I'm not sure it would matter. If the roof is no longer ventilated and the original roof sheathing is above the condensation temperature, I don't think the 1 inch space will effect anything. Correct me if I'm wrong.
Doug,
I agree with you 100%. But, here is a situation I have wondered about. On occasion, I have had jobs where air sealing the attic isn't a possibility, so we concentrate our efforts on the lower air leaks such as the mud sill, rim joist, bottom plate... the leaks up high still remain. Did we shrink the neutral pressure plane by raising where the lower leaks are occurring? Are we leaking more air through the outlets and windows? Probably. I have a feeling in the next few years, this will no longer be a concern. AeroSeal will be the norm with most houses achieving 1 ACH50. There are a couple contractors that I'm aware of using the process in Minnesota now, and a friend of mine who is an insulating contractor is looking into purchasing the equipment. Somehow I missed their booth at IBS this year, but did catch a training session about the process in Duluth. Looks very promising.
Randy,
Here is the language I was remembering (from https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling#ixzz5A0id96Mb):
If you want to combine air-permeable and air-impermeable insulation, there are two possible ways to proceed. One option (according to the code) requires: "In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control."
Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly) . The table calls for a minimum of:
R-5 foam for Climate Zones 1-3,
R-10 for Climate Zone 4C,
R-15 for Climate Zones 4A and 4B,
R-20 for Climate Zone 5,
R-25 for Climate Zone 6,
R-30 for Climate Zone 7, and
R-35 for Climate Zone 8.
After you have installed at least the code-mandated thickness of rigid foam above your roof sheathing, you should install the balance of your required insulation (in most cases, by installing an air-permeable insulation material like fiberglass, cellulose, or mineral wool) below the roof sheathing. Note that both types of insulation — the rigid insulation above the roof sheathing, and the fluffy insulation below the roof sheathing — need to be in direct contact with the roof sheathing.
Read more: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling#ixzz5A0id96Mb
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To fit R19s (6.25" manufactured loft) in to vented channel cathedralized ceiling with a 1" vented channel can I presume they are 2x8 rafters (7.25 " deep), and not 2x6 as stated. Depth matters- be sure.
It's possible to blow cellulose into the vent channel prior to the re-roofing, drilling through the roof deck. leaving the vent channels empty would create strong potential for a thermal bypass. If it's 2x6 that would result in ~R20, assuming it fully filled the cavity, compressing the R19s a bit. With R20 in the cavities it'll take R30+ (6" or more of 2lb roofing polyiso) above the roof deck for dew point control. If' they're 2x8s it would be about R27, and would need R40+ (8" of roofing polyiso minimum) above the roof deck for dew point control.
To prevent direct wetting of the roof deck from above, install a self-sealing waterproof membrane such as Grace Ice & Water shield on the roof deck before adding exterior foam (yes, this creates a moisture trap.) Keep spot checking the moisture content of the roof deck with a pronged wood moisture content meter. Ideally it'll be under 15% m.c. everywhere, but if it's 20% or higher it's safer to dry the roof deck before sealing it from above.
Re: high/low air sealing
Might be better to put your effort into corner/windward/leeward air sealing. Wind can create ~10-14 pascals of pressure with peaks much higher. At a no-so-common -20C, stack effect at 3m causes only +/-3 pascals.
Also be sure that forced air heating isn't causing pressure imbalance in any rooms. That can easily exceed 5 pascals.
http://www.wbdg.org/resources/air-barrier-systems-buildings
Attached is a photo of the knee wall/roof framing attachment. The space is only 5 1/2 inches. I was able to stick my hand into the opening to confirm the presence of the air chute. Poor air sealing strategy. Removal of the insulation in the knee wall should allow for removal of the air chutes through most of the roof. About 1/4 of the roof would not be accessible. I think the cost of the insulation over roof deck is going to be a deterrent to the homeowner. I will be suggesting other less costly fixes, none achieve the roof R-values I would like to see.