How do you outsulate a SIP roof ?
eyremountllc
| Posted in Green Building Techniques on
We have a SIP roof that will be covered with standing seam metal. Because the lumber between SIPs roof panels present thermal bridges, we wanted to add two inches of EPS to break it.
Now the question is how to put the assembly together. We already have High Temp underlayment on the SIPs panels. Can we put the EPS on top and install the metal roofing on top of that?
Or do we need to put EPS, add another layer of plywood (or furring), underlayment and then the metal roof?
I figure the important thing is to protect the SIPs, so it makes sense to put that layer of underlayment on the SIPs.
Roger
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Replies
I would consult the SIP manufacturer before adding insulation to the exterior of a SIP panel.
Roger,
1. Installing a layer of rigid foam on top of your SIP roof to address thermal bridging is an excellent idea.
2. Ideally, your layer of rigid foam will be airtight, so you should tape the seams with a high-quality tape. EPS can be hard to tape unless you are using foil-faced EPS. Another option is to use foil-faced polyisocyanurate, which is easy to tape.
3. You don't want to install metal roofing on top of the rigid foam, in my opinion, without intervening furring strips. Check with your roofer to find out whether furring strips installed parallel to the ridge will work, or whether the standing-seam panels need a continuous plywood layer. If your roofer prefers plywood to furring strips, that's the way to go.
4. I don't think you need roofing underlayment on top of your rigid foam.
5. If you end up using a layer of plywood or OSB on top of the foam, you can improve the roof system by installing 2x4s from soffit to ridge, 24 inches on center, between the rigid foam and the plywood. These ventilation channels will give you a cold roof -- cheap insurance if you are going to the trouble of getting your details right.
In my opinion it is never a good idea to outsulate SIP Panels. This is true for SIP Walls and Roofs. Depending on your climate you might be OK but the Risk that the outer OSB will rot away over time exists. Wrapping material that can rot (e.g. OSB) between foam, which slows down drying potential dramatically does not make sense in my opinion. A thermal bridge created by wood members are not all that bad after all. There won`t be condensation on the surface and therefore no mold growth (happens with real bad thermal bridges created by concrete or steel). I know it is probably to late now because it seems like the panels are already up but a better way to improve the thermal performance of your roof, is to just use thicker SIP`s. If you do an energy model you just calculate the true R-Value instead of the unrealistic ones advertised by the manufactures. As long as the R/in difference of the materials (in this case wood R/in about 1 and EPS R/in about 4) is less then factor 5 a simple linear calculation will do the job just fine. I don`t work with SIPs so much but I have also seen them made with I-joist, which is a good approach to minimize (not eliminate) thermal bridging.
Philipp,
In my opinion, the risk of rotting OSB is greater without the "outsulation" than with the outsulation. Plenty of SIP roofs have developed OSB rot at the seams. Granted, this is usually due to exfiltration, not thermal bridging, but an added layer of rigid foam would reduce the risk of both problems.
The extra layer of exterior rigid foam will keep the OSB warmer and dryer -- and therefore safer -- than would otherwise be the case.
Makes me wonder if there are any advantages/problems with insulating from the inside.
Martin,
I agree with the problematic of the exfiltration occur with SIPs. This is the biggest reason why I usually stay away from them. But also I would be careful (as I mentioned above) to modify the SIP system just because there is to my knowledge no long time experience. We plan buildings to last 50-100 years and even though diffusion might be less critical than air leakage I think it is wrong to completely disregard it. I mentioned before that more complex assemblies might need to be analyzed differently than simple ones. Perm rating plays a role and not only temperature. I would like to see SIP`s with an alternative outer sheeting (Plywood or even better Dense glass or Magnesium Oxide Board).
Keith,
Inside insulation is often blocked by the trusses supporting the SIP roof. Otherwise, I think it's the easiest method.
Thanks guys and especially Martin, for the great detail! I am glad you confirmed that EPS outsulation will not only provide thermal break but protect the SIPs.
I need to think about how much more to add to the SIPs because it's already kind of ridiculously thick (12inches).
Rodger,
Don't do it. Phillip is right.
Use mineral wool instead. It has the insulating value that you need, and the vapor permeance that the SIP needs. There is zero tolerance in a SIP system. Placing the OSB layer between two impermeable layers is not a good idea. It will probably not stand up to any building science scrutiny if a certification from an entity that pays attention to these issues is later sought. Perhaps this project will seek a post construction certification?
Martin, with all due respect, Phillip is right on this one. Moisture could even be present when the assembly is overlaid with a non permeable material at this time of year. There are enough ways that moisture could find it's way into the OSB layer over the assemblies life. Once there...
It's like riding a motorcycle without a helmut. Sure most times your fine. It's only takes one occurrence for catastrophic results!
I know we like data in this arena. There is plenty of it around: take a SIP and just add water... the results are not pleasing!
Albert,
SIPs make be nervous, and there are many questions concerning the longevity of OSB. There are plenty of examples of OSB problems with SIPs.
That said, I stand by my statement: an outer layer of rigid foam will lower the risk, not raise the risk, of OSB rot.
Roger, Is the SIP roof already in place?
Or are you still in the design phase?
It sounds like you have already started building and now you are asking for advice?
After reading the different opinions...it sounds kinda confusing and frightening to me.
I don't see any evidence that SIPs are worse than standard construction. A badly built house will rot, period. A run of badly sealed houses in Alaska of all places does not define a building system.
Many other houses built at the same time period as when SIPs became popular are falling apart for a number of reasons.
Some of us don't get to plan for an infinite amount of time and get ideas on the fly. Sounds like his questions are well timed to me.
Sounds like Mr Lin is trying to build a quality structure
Well, the unfortunate thing is that we won`t find out who is right about this for a long time. If I am right and there might be a diffusion problem causing condensation and slowly rod the OSB it could take decades until the problem is detected. We also still don`t know what climate Roger is in. This would maybe help the guessing work. My advise would be to do a hygrothermal analysis (with WUFI for example) for a rather unusual assembly. This of cause costs money and is not for free like the GBA advice.
Good luck
I am in the D.C. area. We had planned on putting EPS on the SIPs from the beginning but since the SIPs were put together I started having doubts about how to assemble the system.
We ran WUFI and will do it again.
Roger
I think the suggestion from this paper is to add a vented over-roof ...rather than above insulation
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/files/bsi-036_complex_3d_airflow_networks.pdf
Roger,
Obviously, the most important factor is airtightness. Vapor diffusion is not likely to be an issue. So use a belt and suspenders -- both spray foam and tape at the SIP seams.
I don't see why you are worried about condensation. If you have enough rigid foam on top of your SIPs, the OSB will stay warm. If it isn't cold, there won't be any condensation or moisture accululation. That's why the rigid foam helps.
Needless to say, you want to be sure the top level of OSB is dry when it is covered with underlayment and foam. Do the work on a dry, sunny day.
I have been wondering if the same vented over roof concept ..might be the "fix" for the problem that Garth pointed out in the Airtight Sheathing Blog....the problem of the damaged air barrier when replacing the roof on a Marc Rosenbaum type roof assembly.
https://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing
see comment 54,55 & 56
The other issue that I brought up was certification. I did know that Rodger was in the DC area as he was at last weekend's Passive House conference in DC. (good conference by the way).
A 12" roof SIP that is adding exterior foam in a mixed climate probably means it's a Passivehaus project. Sandwiching OSB in mid foam layers might be ok, but it certainly not ideal. From the OSB up, a diffusion open assembly is certainly better than a closed one.
Sure Martin in a perfect world, you can do take the right steps and prevent air or water infiltration to the OSB. However over the lifespan of the building if there is a failure, it's not just removing sheeting and re-applying, it's a significant repair of replacing the entire panel.
That's the point of using a permeable material like mineral wool. If there is a problem with water over the next 100 years, it can dry out and this simple choice can be the difference in whether or not there is a roof failure.
Back to it being a Passivhaus project... I've heard of a project that PHIUS declined to certify because they disagreed with the assembly and it's building science. They thought the wall would rot.
Rodger, if this roof assembly passed pre-cert, then fine. If it's a change, then I'm just trying to point out that a questionable assembly may affect certification.
Regardless of what WUFI says, I'd change the insulation material to something that will let water pass. We do WUFI Pro models as a service for those that don't run it often enough to be sure that they are not misunderstanding the assembly or the report. The one thing that we stress is that is still just a model. It assumes things like zero air infiltration. Actual results can vary. You should not use WUFI to talk yourself into building an assembly that is contrary to the basic principle of what happens when you mix OSB, water and time.
Martin, I agree with almost everything you say. But ones more: The Diffusion/Condensation issue is not just about temperature but rather a combination of permeability and temperature/dew point. The image shows a simplified, stationary (anchored in the German DIN-Norm) hygrothermal analysis for a 12" SIP assembly without EPS, with 2" EPS and with 10" EPS for my climate (Climate zone 6). Wherever the two curves touch there will be condensation and the more the bottom curve is interrupted the more condensation there will be. Clearly 2" EPS is riskier in that prospective than no foam even though the sheathing is "warmer" and you are right about more foam is better than less. That is how the rule of thumb 1/3 insulation inside and 2/3 insulation outside comes from. Whenever I see a potential problem with this analysis I use WUFI which takes drying potential into account. I agree that convection is worse all I am saying is that we hopefully build to last and if diffusion is a problem with these "high performance" envelopes we have yet to find out (maybe decades from now).
Albert & Philipp,
What do you think of the Joe Lstiburek suggested detail in the Insight 0-36
A "vented over roof" as the suspenders instead of outside insulation?
John,
Although you didn't ask me, I'll chime in: I think it is a good idea.
Sorry Martin...I thought you were promoting the outside insulation approach...or I would have included you
I think every European "compact roof" that I have seen while web-browsing employs a similar concept.....
I also agree. Good idea and works with SIP and other compact roofs also.
John,
A SIP roof has several weaknesses: OSB is prone to rot; the outer layer of OSB is cold; there are often internal framing members that act as thermal bridges; and the seams often leak air.
There are several ways to improve the performance of such a roof:
1. Tape the seams.
2. Create ventilation channels above the SIPs. This helps protect the OSB by venting away moisture, but it may increase air leakage through the seams, and it doesn't address the thermal bridging problem.
3. Install a layer of rigid foam over the SIPs. This reduces air leaks and addresses thermal bridging.
4. Install a layer of rigid foam over the SIPs, followed by 2x4s laid soffit-to-ridge to create ventilation channels, and then another layer of plywood. This helps address air leaks and thermal bridging, while creating a cold roof.
Martin,
I really like this detail you outlined here " Install a layer of rigid foam over the SIPs, followed by 2x4s laid soffit-to-ridge to create ventilation channels, and then another layer of plywood. This helps address air leaks and thermal bridging, while creating a cold roof."
Can I use 1x4 pressure treated plywood instead of 2x4s. Is 1in enough to create the ventilation channels?
Roger
Roger,
I don't know what "1x4 pressure-treated plywood" means. Do you mean you want to buy 1-inch-thick pressure-treated plywood and rip it into 4-inch wide strips? (Does anyone even make pressure-treated plywood that thick?) That plan sounds laborious and expensive.
2x4s are cheaper. There is no need for pressure treatment -- these ventilation channels will stay very dry. And you really want your ventilation channel to be at least 1.5 inch high, if possible.
One more point about all of these complicated methods of improving the performance of a SIP roof: they raise the question of whether it makes sense to choose a SIP roof, in light of the inherent thermal weaknesses I mentioned.
Better roofs include:
1. A PERSIST roof (sheathing, peel-and-stick membrane, multiple layers of rigid foam, ventilation channels, plywood, roofing).
2. A hybrid roof with rigid foam on top of the sheathing and dense-packed cellulose in the rafter bays.
3. An uninsulated roof over a ventilated unconditioned attic, with a deep layer of cellulose on the attic floor.
4. A roof with rigid foam topped by ventilated nailbase.
Thanks Martin! That's exactly right, this little discussion has led me to rethink about SIPs as a roof... Thanks everyone for the great input.
John,
Are you ok?
I looked up the BSC 036. In the second paragraph, there it was: a reference to the dreaded unvented "cathedral ceiling"! I've gotten the impression from past posts that you may not be in favor of cathedral ceilings...
Yes. This looks great.
Martin,
Three of your 4 rely on foam. I've got another detail that is pretty elegant in it's simplicity, just built in Portland OR, that I can't post until I'm back in the office. It's all open web truss and cellulose, and diffusion open, and topped off with a vented vegetative roof. It's from the genius of Green Hammer in Portland.
Albert,
I will stand by my rant that the road to AFFORDABLE high performance is Martin's Option #3.
I also realize(thanks to you & Lstiburek) that folks are willing to pay more and/or suffer an energy/durability penalty for fashion, ego and "delight".
I think Martin's 70's house is delightful.
And Albert...you were right about "Mrs. Gambrel Roof Guy" ;--)
John Brooks,
I think you're right. When we start seeing MASS production of REALLY high performance, affordable houses in the U.S., I expect they'll almost all use Martin's #3: "An uninsulated roof over a ventilated unconditioned attic, with a deep layer of cellulose on the attic floor." = lowest cost, least fuss, best chance of achieving desired results.
I agree that Option 3 is the champ in affordability and simplicity. Live and learn...
As promised here is another build-up to consider. The design and project comes from Green Hammer in Portland OR. Regionally one of our best Passive House builders.
The project is a farm workers association headquarters (PCUN) near Portland OR. and is one of our regions first commercial projects achieving the Passive House Standard.
I'm posting the build-up because it's elegant (I think) in it's simplicity and uses no foam. It's pertinent for this discussion since it uses a vapor profile that is designed to place permeable layers and then venting on top. It passed the county where foam would usually be required; Installed quickly as quoted below and was possible by using a highly permeable, yet air and water proof roofing membrane (in this case SIGA Majcoat at 34 US perms) above a small air gap in the cellulose. The classic "drying to the exterior assembly" all made with simple materials.
In Europe the top layer would be a woof fibre board that is stiff enough to walk on, but is vapor open at a perm rating of 18US perms or higher. Since the boards aren't available, the idea was to switch to a very high quality membrane since they are available and then add battens to create venting and that carry the OSB or Plywood roof sheeting above. Finally the roofing material on top. In this case a vegetative system.
It went together really well. Looks to be a very economical way to dense pack cellulose in open web trusses and skip the foam.
I'll add that below the truss is taped OSB with a service cavity below.
Here is an except from an email that Gene was kind enough to send the next day: "Yesterday we blew the insulation and installed the SIGA Majcoat on the CAPACES Leadership Institute. There was more than 8,000 Cubic feet of insulation to install, so our insulators had 2 of their blower trucks and a good crew on site. They were blowing cellulose as fast as they could and the the rain was heading our direction.
Less than 45 minutes after the insulation was completed we had finished the Majcoat and were busy taping seams. Because there is an airspace above the insulation, I am confident that the small amount of moisture that got on the insulation will be able to dry to the exterior." -Gene Wixon, Construction Manager.
Btw...
John: I'm glad to see that good Texan's will "stand by their rant". Also... thanks for revisiting & "lightening up" on the "Mrs Gambrel roof". You're probably still right that super insulating a Gambrel is a fools errand (my term), but what's life without a little "delight"?
best,
a.
Way to go Albert and Green Hammer. I hope this kind of construction gains reputation quickly. It is very sustainable, cost competitive and sound from a building physics standpoint. At least in my opinion. I think for cold roofs there is not a lot of arguments about how to but for compact roofs this is my preferred way. There are two projects on the way in Minnesota with similar assemblies that I have been involved in.
Albert, you and Green Hammer deserve a "Not-So-Foamy" award
kinda reminds me of a vaulted Thorsten Chlupp arctic wall
that's right! I was reminded of his sunrise house walls too.