How to Install Rigid Foam On Top of Roof Sheathing

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How to Install Rigid Foam On Top of Roof Sheathing

This type of insulated roof assembly limits thermal bridging through rafters

Posted on Apr 3 2015 by Martin Holladay
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A roof over a vented, unconditioned attic does not need to include any insulation. However, most cathedral ceilings and low-slope (flat) roofs are insulated roof assemblies: with this kind of roof, the insulation follows the slope of the roof.

Insulated roof assemblies can be vented or unvented. There are lots of different ways to insulate this type of roof, but one of the best methods calls for the installation of rigid foam insulation above the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .

There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing:

  • Rigid foam above the roof sheathing interrupts thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the rafters.
  • Rigid foam above the roof sheathing keeps the sheathing warmer and drier than it would be if all the insulation were on the interior side of the roof sheathing.

How much foam do I need?

If you plan to install rigid foam above your roof sheathing, you have two choices:

  • Option 1: You can install all of the insulation above the roof sheathing (in which case the rigid foam has to meet minimum code requirements for ceiling R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ); or
  • Option 2: You can install some of the insulation above the roof sheathing, and the rest of the insulation underneath the roof sheathing (and in direct contact with the roof sheathing).

If you choose Option 1, your rigid foam will be fairly thick:

  • In Climate Zone 1, you’ll need R-30 of rigid foam (about 8 or 8.5 inches of EPS, 6 inches of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., or 5 inches of polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. );
  • In Climate Zones 2 and 3, you’ll need R-38 of rigid foam (about 10 or 11 inches of EPS, 8 inches of XPS, or 7 inches of polyiso); and
  • In Climate Zones 4 through 8, you’ll need R-49 of rigid foam (about 12.5 or 14 inches of EPS, 10 inches of XPS, or about 9 inches of polyiso).

If you choose Option 2, the code dictates the minimum thickness of your rigid foam layer. According to section R806.4 of the 2009 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code., this approach requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” (In the 2012 IRC, the comparable code requirements can be found in Section R806.5.)

According to the relevant code table:

  • In Climate Zones 1 through 3, you’ll need at least R-5 of rigid foam (about 1.5 inch of EPS, 1 inch of XPS, or 1 inch of polyiso);
  • In Climate Zone 4C, you’ll need at least R-10 of rigid foam (about 2.5 or 3 inches of EPS, 2 inches of XPS, or 2 inches of polyiso);
  • In Climate Zones 4A and 4B, you’ll need at least R-15 of rigid foam (about 4 or 4.5 inches of EPS, 3 inches of XPS, or 3 inches of polyiso);
  • In Climate Zone 5, you’ll need at least R-20 of rigid foam (about 5 or 6 inches of EPS, 4 inches of XPS, or 4 inches of polyiso);
  • In Climate Zone 6, you’ll need at least R-25 of rigid foam (about 6.5 or 7.5 inches of EPS, 5 inches of XPS, or 5 inches of polyiso);
  • In Climate Zone 7, you’ll need at least R-30 of rigid foam (about 7.5 to 9 inches of EPS, 6 inches of XPS, or 6 inches of polyiso);
  • In Climate Zone 8, you’ll need at least R-35 of rigid foam (about 9 or 10 inches of EPS, 7 inches of XPS, or 7 inches of polyiso).

The purpose of the requirement that rigid foam installed above roof sheathing meet certain minimum R-values is to ensure that the roof sheathing stays warm enough during the winter to avoid moisture accumulation and possible sheathing rot. (If the rigid foam layer is too thin and the sheathing is too cold, the sheathing can absorb moisture from the home's interior. Although this process is often referred to as condensation, it is more accurately referred to as sorption.)

Note that if you follow this path (Option 2), the minimum R-values for the rigid foam layer don’t satisfy the full insulation requirement of the building code. You’ll still have to install some type of insulation under (and in direct contact with) the roof sheathing to make sure that the R-value of the assembly meets the R-30 requirement (in zone 1), the R-38 requirement (in zones 2 and 3), or the R-49 requirement (in zones 4 through 8).

If you follow Option 2, a wide variety of insulation materials can be installed under the roof sheathing. Among the possibilities: fiberglass batts, mineral wool, cellulose, or open-cell spray foam. (Closed-cell spray foam is not recommended for this purpose, since closed-cell spray foam prevents the roof sheathing from drying toward the interior if it ever gets damp. For the same reason, this type of roof assembly should never include an interior polyethylene vapor barrier.)

What if the R-value of your roof exceeds minimum code requirements?

One more caveat: If you are planning to thicken the insulation installed under the sheathing in order to achieve a total R-value that exceeds code-minimum requirements, you'll need to also thicken your above-sheathing foam layer to keep the ratio of above-sheathing insulation to below-sheathing insulation in the proper proportion.

For roofs with above-code levels of insulation, use the table reproduced as Image #4, below, to determine the minimum thickness of the rigid foam layer. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

Cold-weather performance of polyisocyanurate

While polyiso insulation has an R-value of about R-6 or R-6.5 per inch, this value only holds for temperatures above about 40°F. In lower temperatures, especially temperatures below 25°F, the effective R-value of polyiso drops noticeably, to a value that is closer to R-4.5 or R-4.0 per inch. (For more information on this topic, see In Cold Climates, R-5 Foam Beats R-6.)

Because of this fact, the use of polyiso to insulate roofs is best restricted to hot climates (for example, Florida or Texas). Cold-climate builders would be better off installing EPS or XPS rather than polyiso. (Note that most green builders try to avoid using XPS because the blowing agents used to manufacture XPS have a high global warming potential.)

If you are a cold-climate builder who wants to use polyiso, one possible approach is to create a foam sandwich, with polyiso on the bottom and EPS on the top. The EPS will keep the polyiso warm, so that the performance of the polyiso won’t be as affected by cold temperatures as it would if the polyiso were on the top of the sandwich.

For a more detailed discussion of the ramifications of the drop in performance of polyiso at cold temperatures, see Dana Dorsett's advice in Comment #3, below.

You need an air barrier at the bottom of the assembly

Before installing the rigid foam, make sure that the roof sheathing is airtight (or that you install an air barrier immediately above the roof sheathing).

There are at least two ways to do this:

  • If your roof has board sheathing, install an airtight membrane (for example, Solitex Mento, a product available from 475 High Performance Building Supply), a layer of synthetic roofing underlayment, or a peel-and-stick membrane.
  • If your roof has OSB or plywood sheathing, the panel seams can be taped (for example, with Zip System tape or Siga Wigluv tape); after taping the panel seams, install the roofing underlayment of your choice (for example, asphalt felt).

Multiple layers with staggered seams

The best way to install rigid foam above roof sheathing is to include at least two layers of foam with staggered seams. Staggering the seams improves airtightness and reduces the chance that heat will leak through the foam seams, causing embarrassing stripe marks (melt patterns) through thin layers of frost.

The first layer of rigid foam can be installed with cap nails. The second layer of rigid foam will probably need to be secured with a few long screws equipped with washers or roofing buttons. When the foam is first installed, it only needs to be held in place with a few fasteners. The layer above the rigid foam (either a series of 2x4s or an upper layer of roof sheathing) will hold the foam in place permanently.

To improve the airtightness of the insulation layer, it’s a good idea to tape the seams of the upper layer of rigid foam. Foil-faced polyiso seams can be taped with housewrap tape, while EPS or XPS seams are best taped with Siga Wigluv.

Some builders wonder whether there will be any problems if two or more layers of rigid foam have multiple vapor barriers (as happens when several layers of foil-faced polyiso are stacked on top of each other). The answer is no — this won’t cause any problems.

Once your final layer of rigid foam has been installed, it’s time to install either a second layer of roof sheathing (plywood or OSB) or 2x4s that establish a ventilation gap.

Do I need ventilation channels above the foam?

If you live in a snowy climate where ice dams are fairly common, you probably want to install ventilation channels above your rigid foam. During the winter, when the wind is blowing, air will exit the ventilation channels at the ridge vent, causing cold outdoor air to enter the ventilation channels through the soffit vents. This cold outdoor air lowers the temperature of the upper layer of roof sheathing, reducing the chance that there will be melting at the bottom of the layer of snow sitting on the roof. That reduces the chance of ice dams.

The usual way to ventilate this type of roof is to install 2x4s on the flat (creating a 1.5-inch-deep ventilation gap), with a 2x4 located above each rafter, extending from the eaves to the ridge. Depending on your rafter spacing, you’ll end up with 2x4s that are 16 inches or 24 inches on center. (Lstiburek calls this a “vented over-roof.” However, this approach doesn’t really create an over-roof; all it creates is ventilation channels.)

The 2x4s are fastened to the rafters below with long screws extending through the rigid foam.

How closely should I space the screws?

According to building scientist Joe Lstiburek, you don’t need as many screws to hold down these 2x4s as you would need to fasten furring strips to a wall. (For information on wall screws, see Fastening Furring Strips to a Foam-Sheathed Wall.)

Here’s what Lstiburek has to say: “How many [screws] and how far apart? The good news is that this is less complicated than installing continuous insulation on walls. … Less gravity, better friction. Uplift is the problem, not slippage and bending moment of the fasteners. The folks who do commercial flat roofs have this uplift thing dialed in. The same fastening requirements for uplift for flat roofs will work for these roofs while simultaneously handling the slippage issue. Except where it snows a lot. Snow likes to stick to sloping roofs. With lots of snow things get very complicated. The good news is that structural engineers have a good handle on this — especially ones who work in ski resorts — particularly Swiss and Austrian engineers.”

To summarize: if you don’t expect a lot of snow, you can install the same number of screws (for the upper layer of plywood or OSB, or for the 2x4s, if any, installed above the rigid foam) that are recommended for furring strips installed on walls — basically, one screw every 24 inches along each rafter, with a minimum penetration into solid wood of 1½ inch — and you’ll have more than enough screws. If you expect a lot of snow, however, you should consult an engineer.

What if I don’t need ventilation channels?

If you don’t need ventilation channels, you’ll probably need to install a layer of OSB or plywood roof sheathing on top of your rigid foam. The upper layer of rigid foam is secured to the rafters below with long screws through the foam.

If you plan to install through-fastened steel roofing, you may be able to skip the upper layer of roof sheathing. Instead, install 1x4 or 2x4 purlins, 24 inches on center, parallel to the ridge. Then fasten the roofing to the purlins.

Where do I buy long screws?

Some suppliers of long screws include:

SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. screws (often available at roofing supply outlets) come in long lengths and are often a good choice.

How hard is it to screw through thick foam?

Some builders have used the techniques described in this article to install as much as 10 inches of rigid foam above roof sheathing. That’s a lot. If you try to do that, you’ll probably need screws that are at least 12 inches long.

Trying to locate rafters underneath a thick layer of rigid foam can be tricky. While experience helps, it’s never a particularly easy task, and some find the work extremely frustrating.

When Alex Cheimets performed at deep-energy retrofit at his house in Arlington, Massachusetts, he specified 6 inches of rigid foam should be installed above the roof sheathing. As GBA reported, “A box of broken 10-inch screws, a new set of impact drivers, and a week’s worth of frustration later, [the contractors] wondered if it was worth it. In the end, the roof worked out well, but [some of the team members felt that] the extra effort and cost were hard to justify.”

Nailbase or SIPs

If you don’t want to create a site-built sandwich of rigid foam and roof sheathing, you might want to consider installing structural insulated panels (SIPs) or nailbase instead.

A structural insulated panel is a sandwich panel made out of rigid foam (usually EPS) faced with OSB on both sides.

Nailbase is similar, except that nailbase has OSB on only one side of the foam.

If you decide to install SIPs, you should follow the recommendations of the SIP manufacturer. However, SIPs come with a few quirks. SIP seams are more vulnerable to air leaks than the seams between rigid foam panels in site-built assemblies, because it’s impossible to stagger the seams with SIPs. Some SIP roofs have experienced OSB rot near the seams; this occurs when indoor air has access to air channels that connect with the upper layer of OSB at the seam. To address these potential problems, installers should (a) always tape SIP seams on the interior and seal SIP seams with canned spray foam; and (b) strongly consider installing ventilation channels above the SIPs, especially in cold climates.

Nailbase is available in a variety of thicknesses from several manufacturers. For example, ABT Foam sells 9-inch-thick nailbase panels rated at R-33.

If you can’t find nailbase with a high enough R-value to meet your needs, you can install nailbase above a layer of rigid foam. This method has an important advantage: it allows you to stagger the seams between the two layers.

Martin Holladay’s previous blog: “Walls With Interior Rigid Foam.”

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Image Credits:

  1. Image #1: Green Building Advisor - Alex Cheimets job
  2. Image #2: Fine Homebuilding
  3. Image #3: Sipschool.files.wordpress.com
  4. Image #4: Martin Holladay

51.
Jan 11, 2016 4:27 PM ET

Edited Jan 11, 2016 4:29 PM ET.

Response to David Hicks
by Martin Holladay

David,
There are two schools of thought on this question.

1. Joe Lstiburek's take is that you need an air barrier at the sheathing level, or else you risk the possibility that warm interior air will get into the cracks between the rigid foam panels, creating convective loops that melt the frost on your roof in stripes. It happened to Joe, so he now insists on a tight air barrier at the sheathing level.

2. Other builders say that taped rigid foam can be the air barrier, especially if you have at least two layers of rigid foam with staggered seams (to prevent those embarrassing frost-melt stripes). If you go this route, it's best to tape each layer of rigid foam.


52.
Jan 11, 2016 6:17 PM ET

Thanks for the response
by David Hicks

I had in mind that plain vanilla osb with polyiso taped with inexpensive foil tape was more cost-effective than either trying to tape untreated OSB with Siga or using Zip panels and tape. But if two layers of polyiso and taping are needed, that defeats some of the cost benefit. Zip system and EPS is probably more effective since there is no ambiguity in the air barrier (it's all at the sheathing layer) as long as you don't mind the frustration of fastening many inches of EPS to the roof.


53.
Feb 2, 2016 11:09 AM ET

Flat reroof in Albuquerque
by Eric Harding

Dear Martin and GBA,
Great article. Thanks for in detailed reviews on this topic.

I am currently planning to reroof my flat roof home here in Albuquerque, NM. My roof construction is as follows (interior to exterior): gypsum board, polyethylene, R-29 fiberglass batts in 2x12 joists, half-inch plywood decking, and finally a tar and gravel roof. There is no dead airspace between the fiberglass and the plywood decking. This setup may be similar to that of the Arizona houses that Bill Rose investigated. The roof is currently un-vented.

At this point I plan to do a complete tear-off of the existing roof in order to inspect the plywood. Then the new roof will consist of tapered (1/4” over 1 ft pitch) polyiso foam, 75 lb. paper followed by 3 layers of 15 lb fiberglass paper with hot mopped tar in between each layer, and then gravel. Strips of modified bitamen will be applied to the parapet walls.

The question is whether or not we should install roof vents. The roofer is talking about installing one-way vents will “let the hot air out.” Should this roof be vented in this way?

A separate issue is the thickness of the polyiso foam. The minimum thickness of the foam will be 0.5”. If I understand your articles correctly, for my climate zone (4B), I am required by code to insulate the roof deck with R-15 so that condensate does not form on the plywood. However, to increase the minimum thickness of the polyiso foam to 2” would put parts of the roof above the parapet wall, and not to mention greatly increase the cost of this project.

Thanks for your help.


54.
Feb 22, 2016 2:58 PM ET

Edited Feb 22, 2016 3:00 PM ET.

Response to Eric Harding
by Martin Holladay

Eric,
Q. "The question is whether or not we should install roof vents. The roofer is talking about installing one-way vents will 'let the hot air out.' Should this roof be vented in this way?"

No. If you are installing rigid foam above the roof sheathing, you certainly don't want to ventilate under the roof sheathing. Your roofer is wrong.

Q. "The minimum thickness of the foam will be 0.5 inch. If I understand your articles correctly, for my climate zone (4B), I am required by code to insulate the roof deck with R-15 so that condensate does not form on the plywood. However, to increase the minimum thickness of the polyiso foam to 2 inches would put parts of the roof above the parapet wall, and not to mention greatly increase the cost of this project."

A. If you are using polyiso, 2.5 inches of polyiso would be the minimum thickness for your climate zone. Where the roof extends above the parapet, simply trim the exposed edges with metal flashing. Parapets are useless (and in fact often inhibit the proper installation of insulation and also often inhibit proper drainage).

Here is a link to an article with more information on the type of roof you are describing: Insulating Low-Slope Residential Roofs.


55.
Mar 16, 2016 2:27 PM ET

Metal roofing attached to battens on EPS
by Justin Husted

Martin,

I'd like to attach 5V metal roofing to battens laid atop 10" of EPS, which would be on top of the OSB sheathing. My concern is, for V5, structurally, I don't think that I can just lay the battens atop the EPS. 10" tall battens also seems like a bad idea. To add more complication, I was thinking about encasing the EPS in 2" of closed cell foam to serve as the vapor barrier and the last bit of required insulation.

What's the best way to approach this? Will battens laid atop the Closed cell foam and bolted to the sheathing with 10" lag screws with rubber washers be sufficient? Would that compromise the vapr barrier?


56.
Mar 16, 2016 2:32 PM ET

Response to Justin Husted
by Martin Holladay

Justin,
Ten inches is a lot of foam, so it's time for you to consult an engineer.

One simple approach is simply to buy 10-inch-thick SIPs. (Here is a link to one of many SIP manufacturers that sells 10-inch-thick SIPs: http://sipsteamusa.com/packages/.)

The SIP manufacturer can provide advice on fastening -- they have in-house engineers.


57.
Apr 21, 2016 11:41 AM ET

Hi Martin, I've looked at
by Michaela Riley

Hi Martin,

I've looked at all the drawings on GBA, but I can't find one that details this approach when using purlins over the foam, rather than another layer of roof sheathing. Can you please confirm that I've got the stackup right? From the bottom up:

* Rafters filled with Roxul
* ZIP system with taped seams as the air barrier
* Asphalt felt
* Two layers of polyiso with staggered, taped seams
* 1x4 purlins fastened to the rafters with long screws
* Aluminum roofing fastened to the purlins

My main questions here are:

* I want to confirm that the roof underlayment goes *beneath* the foam in this scenario.

* Do I need to worry about the purlins having any moisture problems, or (as I assume from the article) is that a non-issue?

Also, I have opted for applied overhangs, per the details in this article: http://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulatio.... Their drawing does not show the use of exterior foam on the roof, though, so I have a couple more questions related to how this all fits together:

* Do I need to run sheathing over the top of the applied rafter tails to give the polyiso a continuous surface to rest on? I would love to avoid the extra weight (since I'm building a tiny house on a trailer), but I also don't want the polyiso to end up getting crushed into the rafter tails if we step on that area of the roof, so I'm assuming it's best to have sheathing there to distribute the weight.

* I will be attaching our fascia boards to the applied rafter tails, and I've ordered our roof drip edge long enough to extend from the top of the roof, down over the polyiso, and over the top of the fascia boards. To protect the edges of the polyiso from moisture, I am planning to run Grace Ice & Water around the edges of the polyiso and down to overlap the fascia boards, similar to the detail for the "Thick Roof" drawing shown on this article. Does that sound reasonable?

Thanks again for all your excellent advice! I don't know what I'd do without this website.

Michaela


58.
Apr 21, 2016 11:55 AM ET

Edited Apr 21, 2016 11:58 AM ET.

Response to Michaela Riley
by Martin Holladay

Michaela,
Q. "Can you please confirm that I've got the stackup right?"

A. You haven't given us enough information for an answer. In order to provide an answer, we need to know (a) your climate zone; (b) the thickness of the mineral wool insulation; and (c) the thickness of the polyiso.

Q. "I want to confirm that the roof underlayment goes beneath the foam in this scenario."

A. Roofing underlayment is required by most roofing manufacturers and building codes, and these manufacturers and most code enforcement officers would probably recommend that the roofing underlayment be installed above the polyiso.

You don't need roofing underlayment above the taped Zip sheathing, because taped Zip sheathing is waterproof. However, if you want to install asphalt felt between the Zip sheathing and the polyiso, the asphalt felt will do no harm.

Q. "Do I need to worry about the purlins having any moisture problems, or (as I assume from the article) is that a non-issue?"

A. Your purlins will be fine, until the day your roofing starts leaking.

Q. "I have opted for applied overhangs, per the details in this article: http://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulatio.... Their drawing does not show the use of exterior foam on the roof, though."

A. Look again. The PERSIST approach shown in that article includes rigid foam above the roof sheathing. Then 2x4s (usually installed on the flat) are installed above the rigid foam; these 2x4s are extended at the eaves and rakes to create a roof overhang.

Q. "Do I need to run sheathing over the top of the applied rafter tails to give the polyiso a continuous surface to rest on?"

A. You don't need polyiso on the roof overhangs.

Q. "I will be attaching our fascia boards to the applied rafter tails, and I've ordered our roof drip edge long enough to extend from the top of the roof, down over the polyiso, and over the top of the fascia boards. To protect the edges of the polyiso from moisture, I am planning to run Grace Ice & Water around the edges of the polyiso and down to overlap the fascia boards, similar to the detail for the "Thick Roof" drawing shown on this article. Does that sound reasonable?"

A. I don't think so. In most cases, a PERSIST roof has no rigid foam on the applied overhangs. See the detail drawing below.

.

PERSIST detail 3_0.jpg


59.
Jun 9, 2016 2:39 PM ET

Edited Jun 9, 2016 4:47 PM ET.

Insulation stack Zone 5 question
by Timothy Robinson

The attached file shows a roof detail for an ICF house planned for Omaha, Nebraska. You'll see that the top chord of the 30" roof trusses rests on the ICF wall. Between the trusses and sill pieces, I intended to use sill sealer and spray foam to keep the joint between the top of the wall and roof deck airtight.

I've tried to follow the guidance across multiple articles and, not counting the nominal R-value of the non-foam layers, estimate the insulation above the roof deck to approach R-55, but, in any event, exceed the R-49 required.

In case the attachment doesn't open/display correctly, the assembly stack from top to bottom is:
a. steel roofing screwed to osb sheathing
b. 5/8" T&G OSB sheathing
c. 4" XPS
d. 5.5" Polyiso
e. synthetic roof underlayment
f. 3/4" T&G OSB with seams taped.

Other than the general question about whether I've misunderstood something in the articles designing the insulated roof this way and you see serious red flags:

1) What type of roof underlayment between the roof deck and first layer of polyiso is necessary with the taped OSB seams and foil facing of the staggered foam panels?

2) I can't figure out what type of tape is used for the OSB joints. Is it one of the aluminum tapes?

Many thanks for this valuable resource.


60.
Jun 9, 2016 3:08 PM ET

Response to Timothy Robinson
by Martin Holladay

Timothy,
Q. "What type of roof underlayment between the roof deck and first layer of polyiso is necessary with the taped OSB seams and foil facing of the staggered foam panels?"

A. If the OSB seams are taped with a high-quality tape, and you are also taping the rigid foam seams, you have good air barriers. Strictly speaking, you don't need roofing underlayment on the lowest level of OSB -- but because the work on the roof might take a while (during the construction phase), some ordinary asphalt felt might be a good idea to help keep the OSB dry.

Q. "I can't figure out what type of tape is used for the OSB joints. Is it one of the aluminum tapes?"

A. The three best tapes for taping OSB are Siga Wigluv, Huber Zip System tape, and 3M All Weather Flashing Tape. Here are links to two articles with more information on tape performance:

Return to the Backyard Tape Test

Two Wingnuts Describe Their Backyard Tape Tests


61.
Jun 19, 2016 3:56 PM ET

Insulation stack
by Timothy Robinson

Perfect! Really appreciate it, Martin. This greatly helped me avoid some bad/more expensive ideas from contractor and architect.


62.
Aug 13, 2016 3:35 PM ET

hip roof in zone 6
by kim dolce

We are renovating a mid-1800's carriage house in mid-coast Maine (zone 6). The building was completely gutted prior to purchase and the exposed beams and roof structure sold us on the place. What we didn't factor in was the difficulties and expense of insulating the unvented hipped roof with two skylights on opposing sides of the "peak". We decided it would be worth covering up the exposed roof decking and rafters to insulate at a reasonable cost. Not as reasonable as we had hoped. Three quotes all came in over $10k, so we're now considering insulating exclusively with rigid foam above even though the roof is still in fairly good condition. If this option proves expensive at least we'll save by not having to finish the interior ceiling and we get to keep the feature that sold us on the building in the first place.

I'm not getting comparable information from roofers so turned to this site for some knowledge. All fully understood that I planned to keep the ceiling exposed and only insulate from above. The first roofer wouldn't touch the job, it just isn't in his wheelhouse. The second (a fairly sizable company) thinks 6"+ inches of foam is too much and came back with a recommendation of using a 3.5" of polyiso nailbase panel faced on one side with MDF, followed by asphalt shingles which they said this would give us R20 on the roof, no mention of ventilation. When I balked at R20 and repeated there would be no insulation on the interior, they responded with R20 is pretty good considering it has nothing now. (FYI, the town I live in only follows state code for plumbing and electric so no one is going to insist I get to R49.) Roofer #3 has no problem stacking up 6"+ of foam on the roof, but insisted he would only do the job if he installs prefabricated metal roof vents in every other rafter bay. I had a hard time grasping that one and his explanation didn't do much to help my understanding.

My questions – First, can you tell me if the 3rd roofers insistence on vents in every other rafter bay is appropriate for this situation? And if so, can you explain how all my heated interior air in the winter won't just escape through those vents?

In regards to this article and my situation, assuming Option 1, all insulation above the roof sheathing, and being able to skimp a little on R value and code, would installing less than R49 above (but hopefully more than R20) cause any issues and if so what sort?

I'm a little confused regarding the section on ventilation channels. Does an air gap, in and of itself, between the foam and the upper layer of sheathing make a difference or must there be a path for air to travel from soffit to ridge and if so, is there some way to create this above what is currently an unvented hipped roof? Basically, does this paragraph apply with a hip roof?

Thank you so much for this column and the opportunity to learn.


63.
Aug 13, 2016 7:13 PM ET

Reply to Kim
by Charlie Sullivan

Kim, I'm sorry you are having so much trouble finding someone do do this. You could certainly survive with R20, but there's no good reason to stop with that little. But then the suggestion of venting every other rafter by is very hard to make sense of. The only things I can think of that he might mean are either putting some venting above the foam between it and the new roofing, which is a good idea. Or maybe he think you are going to put drywall over the rafters, and have uninsulated closed cavities there, and he wants them vented to the indoors.

There might be builders who are generalists and do some roofing who understand insulation better than the roofers you are talking to.

You could also try talking to EcoCor, which is a very innovative green building company at least somewhat near you. They have a system of cellulose insulated wall and roof panels that are primarily for new construction, but they also do retrofits, and might be able to offer something good for your situation. And if they can't, they might know a builder to refer you to.


64.
Aug 14, 2016 5:50 AM ET

Kim, I agree with Charlie. If
by Martin Holladay

Kim,
I agree with Charlie.

If you want to negotiate with the roofers:

1. You can tell the roofer who likes nailbase that "Nailbase is available in a variety of thicknesses from several manufacturers. For example, ABT Foam sells 9-inch-thick nailbase panels rated at R-33, while Atlas EPS sells 11¾-inch-thick nailbase panels rated at R-48."

2. You can point out to the roofer who wants to install ventilation channels underneath the roof insulation that ventilation channels always belong on the exterior side of the insulation, not on the interior side of the insulation. After all, why would you want to introduce cold exterior air into a channel on the interior side of the insulation?


65.
Aug 14, 2016 6:43 AM ET

to Martin and Charlie
by kim dolce

Martin, I certainly agree that beefing up on the R20 is good, should I assume from your response that being shy of R49 with no added interior insulation will not cause problems?

Nice to know my reasoning is sound on the through the ceiling vents. If I can convince him to vent above the insulation, can that be a simple air gap or must it be vented for air flow from soffit to ridge? And is that feasible with my hip roof?

Charlie, thanks for the lead. I'll seek them out for another opinion and quote. The roofer who suggested the vents is a general builder who does a lot of roofs and he stood inside with me and agreed how cool it would be to keep the ceiling exposed. Maybe he had an off day.


66.
Aug 15, 2016 2:32 AM ET

sunroom roof
by Sherry Litasi

I'm planning a south side sunroom addition with a simple metal shed roof. I'm in rural southwest NM at 6300 ft - it will get up to 90 in the summer and down to the 20's in the winter (and some snow). I have limited height against the existing brick wall, so the slope will be about 1/12. I am buying 1x6 t&g blue pine (beetle kill) and want to expose between the rafters - as follows: rafters, t&g, underlayment, 2" foam (polyiso?), 2x4 purlins, then metal roof.

My concerns: 1) having enough ventilation to avoid overheating in the summer, 2) will 2" polyiso be enough and is polyiso the right choice, 3) does this make sense?

Thanks for your great articles! So much to think about!


67.
Aug 15, 2016 5:56 AM ET

Response to Kim Dolce (Comment #65)
by Martin Holladay

Kim,
Assuming that you don't have to meet any minimum code requirements for R-value -- and it sounds like you don't -- then you can install below-code insulation levels without worrying about moisture problems. If you end up with R-20 foam or R-24 foam instead of R-49 foam, your roof won't rot. You'll just have higher energy bills than a house with code-minimum insulation.

On a hip roof, there isn't much point in spend extra for ventilation channels above the rigid foam, since soffit-to-ridge venting is impossible on a hip roof.


68.
Aug 15, 2016 6:01 AM ET

Response to Sherry Litasi (Comment #66)
by Martin Holladay

Sherry,
I urge you to read this article: How to Build an Insulated Cathedral Ceiling.

It sounds like you are located in Climate Zone 4 or 5. In your climate zone, building codes require a minimum of R-49 roof insulation.

You are planning to install only R-12 insulation; that's woefully inadequate. It's fine to use polyiso insulation if you want; but you'll need to install at least 9 inches of polyiso, not 2 inches of polyiso.


69.
Oct 21, 2016 1:43 PM ET

Does this method Totally preclude no insulation under sheathing
by Roger Smith

I am assuming that this article almost exclusively applies to a conditioned and unvented attic, right?

My attic, in a Zone 5 dry arid climate at 5000 feet, is currently passively vented and unconditioned. I favor installing air wash channels under the sheathing but I also want to reduce daytime temps. When the outside air is in the 90's or 100's the temps in the attic can reach 150. For this reason I am leaning towards some form of polyiso or a combination of polyiso and EPS on top of the sheathing. Am I going the wrong direction with this plan?


70.
Oct 21, 2016 1:52 PM ET

Response to Roger Smith
by Martin Holladay

Roger,
There are two types of attics: vented unconditioned attics and unvented conditioned attics. You have to decide what type of attic you want.

If you have a vented attic, it makes no sense to install rigid foam above the roof sheathing. A vented attic is basically outdoors -- so installing rigid foam above your roof sheathing is a waste of money and a waste of rigid foam. You might as well put the rigid foam in the branches of a tree in your back yard.

If you want to install rigid foam above your roof sheathing, it means that you are committed to creating an unvented conditioned attic. For more information on this type of attic, see Creating a Conditioned Attic.

If you don't have any HVAC equipment or ducts in your attic, it makes sense to have a vented unconditioned attic. This type of attic is less expensive to insulate, and it usually performs better in all respects than an unvented conditioned attic.

If your attic has HVAC equipment or ducts, you may want to create an unvented conditioned attic.


71.
Oct 23, 2016 10:30 PM ET

So how do you reduce daytime attic temps...
by Roger Smith

or do you just live with it and [over]compensate with a ton of insulation on the attic floor?


72.
Oct 24, 2016 5:10 AM ET

Edited Oct 24, 2016 5:12 AM ET.

Response to Roger Smith
by Martin Holladay

Roger,
As long as your attic doesn't contain any HVAC equipment or ducts, the temperature of your attic is irrelevant. The reason that building codes require R-38 or R-49 insulation on the attic floor is that analysis shows that this level of insulation is cost-effective. It's not only cost-effective -- it's enough insulation that you don't have to worry whether your attic temperature is 110°F or 150°F. In either case, the insulation is enough.

That said, some GBA readers install R-60 attic insulation, and you're free to do that if you want. But putting insulation above the roof sheathing is misguided if you have a vented attic. Instead of adding insulation above your roof sheathing, put it on the attic floor where it belongs.

If you intend to use your attic for storage, and if you are worried that the attic will get so hot that your wax candles (or whatever vulnerable items you are storing) will melt, that's another story. In that case, you probably want to design an unvented conditioned attic.


73.
Oct 24, 2016 10:46 AM ET

Much much thanks Martin...
by Roger Smith

Thanks for clearing that up and steering me in the right direction. This site is awesome made all the better by contributors like you.

Cheers


74.
Jan 12, 2017 8:54 PM ET

to vent or not to vent my cathedral ceiling
by Ehren Maedge

Hey all,

I’ve read many articles on GBA on this topic and really appreciate the dialog. I’ve learned a lot.

I’m in climate zone 3 and am building a new home and am to the point where I need to make a decision on how to finish the roof structure. The roof is a 12/12 pitch. Simple gable style.

I’ve got a cathedral ceiling throughout consisting of 2x12 TJIs running along a ridge beam. I’ve thrown out the idea of using spray foam on the underside of my sheating due to the fact it's too expensive. As I see it I have these two options and I’m interested in opinions.

Option 1. Create a vent channel underneath the sheathing using cut and cobble method. foam battens created from 2” polyiso glued in place on the top cord of the TJI. That gives me 1.5” of ventilation space. 8 1/2” is left for fiberglass batts. high density R-30 batts are 8 1/4” high so that just fits. The rigid battens are R-10 so that gets me to where I need to be for code. I will have wiring and sprinklers in that ceiling.

Option 2. Create an unvented roof putting foam on top of the sheathing. My roofing is steel and is designed to be mounted on battens/purlins. From what I’ve learned here, seal the seams of the OSB roof sheathing with zip system tape to create an air tight barrier. Put down roof felt. Put full sheets of 2” foam on top of the sheathing and felt tacked in place. I know two layers of 1” foam would be ideal but that’s more expensive per R. Finally, lay down 1x4s parallel to the ridge at 24” spacing and screw those through the foam, felt and sheathing into the rafters. Roofer mounts his roofing clips to the 1x4s and installed the roofing panels. The roof is vented at the ridge and soffit. Inside we have an unvented rafter bay we can fill with standard R30 batts. It would give me extra room for wiring and sprinkler plumbing.

In looking at both options, it appears that they cost about the same in materials. There’s the same square footage of rigid foam required. Option 1 requires glue/caulk/tape to seal the rigid foam and Option 2 requires zip system and housewrap tape and 1x4s.

For labor, it seems that option 2 would be faster given there’s no cut and cobble of rigid foam. Foam is going down in full sheets on the roof and 1x4s are getting tacked in after.

My inspector seems to be ok with either option.

I know the bonus is I get a thermal break with the insulation on the outside. It gets hot here during the Summers so I’m looking to keep a 130 degree metal roof from transferring heat into my loft.

Any thoughts or advice appreciated.


75.
Jan 13, 2017 5:23 AM ET

Edited Jan 13, 2017 5:25 AM ET.

Response to Ehren Maedge
by Martin Holladay

Ehren,
Either approach will work, so the decision is up to you.

If your roof has a skylight or chimney, these features would argue against the vented option (since the features would interrupt the continuity of the ventilation channels).

If you choose Option 2, it would be best if you could fill the entire rafter bays with fluffy insulation (fiberglass, cellulose, or mineral wool) rather than only filling a portion of the cavity depth. If your batts don't fill the full depth, you have two problems: (1) how to make sure that the batts stay in contact with the roof sheathing, and don't slump down, and (2) what material to use as an interior air barrier (which, ideally, needs to be installed in contact with the interior side of the fluffy insulation).

I suppose that you could try to staple up strips of a European air barrier material on the interior side of the batts, but that's fussy work. It's much easier to let your drywall be the interior air barrier -- and if you decide to do that, you need to fill the full depth of the rafter bays with insulation.


76.
Jan 13, 2017 1:04 PM ET

Martin, Thanks for your words
by Ehren Maedge

Martin,

Thanks for your words of encouragement and advice. I really appreciate it. Understood on the need to keep fiberglass batts in contact with sheathing.

One other thing came to mind. You mentioned in another article on GBA that venting the top side of sheathing with a cold roof (steel roof fastened on battens allowing air flow directly underneath the roofing but on top of the sheathing) while at the same time venting underneath the sheathing is incompatible. Why is that? I ask because of the intense sun we get here, even if I chose option 1, I like the idea of raising the roof off the sheathing 3/4" and letting some air flow underneath creating a "sombrero" of sorts. It seems like this would keep my roof cooler rather than having heat from the steel roofing get transferred directIy to the sheathing. Why is doing a cold roof a bad idea if I choose option 1?


77.
Jan 13, 2017 1:15 PM ET

Response to Ehren Maedge
by Martin Holladay

Ehren,
You must have misunderstood what I wrote. I probably wrote that if a roof has rigid foam above the roof sheathing, you don't want to have a ventilation channel underneath the roof sheathing (because it makes no sense to install insulation on the exterior side of a ventilation channel).

If you want two ventilation channels -- one above the roof sheathing, and one below the roof sheathing -- that's perfectly OK, as long as all of the insulation is on the interior side of the ventilation channels.


78.
Jan 13, 2017 3:26 PM ET

got it
by Ehren Maedge

Thanks Martin. Makes sense now.


79.
Jan 14, 2017 11:30 PM ET

ventilation channels metal roof
by Whitney Scurlock

Hi Martin,

Thank you for another great article.

I wanted to make sure that there isn't any significant problem with 2x4s being perpendicular to the rafters over the foam rather that parallel, as you recommended in the article for reducing ice dams? This would be to avoid adding additional 1x4 purlins or plywood over the 2x4s for attaching a metal roof. Also, if a channel is only open on one side, will it significantly increase risk of ice dams?

Also, is there any structural concern about adding solar panels to a standing seam metal roof that is attached to 2x4s 24" o.c. rather than plywood base?

Thank you for your advise!


80.
Mar 5, 2017 12:57 AM ET

no venting for hip roofs?
by user-6779378

In comment 67, you said not to put in ventilation channels with a hip roof. If no ventilation channels, how does one vent a hip roof? Thank you.


81.
Mar 5, 2017 1:39 AM ET

Hipped roofs
by Malcolm Taylor

Typically you build hipped roofs as un-vented assemblies. If you are using metal on the roof you can vent the hips the same way you vent a ridge. There are also some manufactured vents for hipped roofs with asphalt shingles, but I have no idea whether they work.


82.
Mar 5, 2017 5:59 AM ET

More on hipped roofs
by Martin Holladay

It's possible (sort of) to vent a hipped roof over an unconditioned attic, with a combination of mushroom vents and (in some cases) a short ridge vent.

If the house has cathedral ceilings, though, I don't recommend a vented approach for a hipped roof.

The only way you might succeed would be with two layers of strapping (probably 2x4 strapping) above the rafters -- one layer perpendicular to the rafters, and another layer parallel to the rafters). Each layer of strapping would be installed at either 16 inches o.c. or 24 inches o.c. You'd still need to be creative when it comes to installing vent outlets (again, probably a combination of mushroom vents and a short ridge vent).

-- Martin Holladay


83.
Mar 5, 2017 10:20 AM ET

Also consider DCI SmartVents
by Jon R

Also consider DCI SmartVents or similar.


84.
Apr 27, 2017 2:04 AM ET

Best setup for my remodel with 2x6" rafters
by Mats Lundgren

Hi Martin,

I have started a remodel of my home in Woodside, CA (I believe climate zone 3). The house has cathedral ceilings and is framed with 2x6".
My plan is to use normal fiberglass between rafters (5.5") which will give me R-19, correct?
And then add polyiso on top following your instructions above. Looks like 2 layers of 1" will give we another R-13 taking me to R-32 which is above the min R-30.
But going for 2" (R-13) + 1" (R-6.5) poly-iso (Rmax Thermasheat 3) would get me to R-38.5 which I guess is a better idea?

Additional questions:
1. You suggest air barrier for the first layer of plywood/OSB (on top of my 2x6") rafter and use "Siga wigluv tape" which sounds good but for the final layer of Rmax Thermaheet (where we also want it airtight) can I use HVAC foil tape (I assume cheaper)?
2. For the new insulation I place between the 2x6 should I use kraft faced fiberglass with the kraft paper down towards the inner ceiling? House has on the interior side drywall and then cedar planks and I am doing all the insulation from the top as I am also re-roofing.
3. If I use OSB board is there any value to add OSB Techshield radiant barrier version? I guess not since the 2 layers of OSB (On top of rafters and on top of polyiso) will both face an insulated area?

Thanks,

/Mats


85.
Apr 27, 2017 5:23 AM ET

Edited Apr 29, 2017 6:18 AM ET.

Response to N/A N/A
by Martin Holladay

Q. "My plan is to use normal fiberglass between rafters (5.5 inches) which will give me R-19, correct?"

A. Correct.

Q. "But going for 2 inches (R-13) + 1 inch (R-6.5) polyiso (Rmax Thermasheet 3) would get me to R-38.5, which I guess is a better idea?"

A. Correct.

Q. "You suggest air barrier for the first layer of plywood/OSB (on top of my 2x6) rafter and use "Siga Wigluv tape," which sounds good, but for the final layer of Rmax Thermaheet (where we also want it airtight) can I use HVAC foil tape (I assume cheaper)?"

A. Yes.

Q. "For the new insulation I place between the 2x6 should I use kraft-faced fiberglass with the kraft paper down towards the inner ceiling?"

A. You could. Unfaced batts would also work.

Q. "If I use OSB board is there any value to add OSB Techshield radiant barrier version? I guess not since the 2 layers of OSB (On top of rafters and on top of polyiso) will both face an insulated area?"

A. You're right: the radiant barrier wouldn't help, because a radiant barrier is only effective if it faces an air space.

-- Martin Holladay


87.
Apr 28, 2017 8:55 PM ET

Follow-up question
by Mats Lundgren

Hi again Martin.

We intend to install recessed lights (Airtight Title 24 approved cans) in the cathedral ceiling. Since we are using LED lights we can use shorter cans (Halo H25ICAT) that will fit between 5.5" joists for most of the areas but were we have the great room we have 20 feet ceiling and need deeper cans that can adjust for the roof slope and provide 1200 lumens (like Halo HL612ICAT). These cans are typically inside an internal metal box to handle the tilt function. With a depth of 7" they will go through the first plywood layer but I should still be able to cover them with 1 layer of 1" polyiso (R6) plus 1" square cut piece for the rectangular piece. I believe this is enough?
For ceiling between the can and the plywood on top of the rafters can I use Siga wigluv or should it be sealed with some spray foam or silicon?
Also If can fit it flush with 1" polyiso on top of the first plywood layer I can put the 2" on top? I guess there is no issue with putting first 1" polyiso and then 2" polyiso on top vs. start with 2" and then add 1"?

Thanks,

/Mats


88.
Apr 29, 2017 6:27 AM ET

Response to N/A N/A
by Martin Holladay

First of all, I advise you to change your user profile so that your screen name isn't "N/A N/A." Here is a link to an article that explains what you need to do: How the GBA Site Displays Readers’ Names.

Second: You are making a big mistake by installing recessed lights in your insulated ceiling. These lights (a) introduce air leaks even when carefully installed, (b) take up volume that should be devoted to insulation, thereby lowering the R-value of your assembly, and (c) create hot thermal chimneys that accelerate the stack effect and lead to hot spots on your roof.

For more information on this issue, see these articles:

How to Build an Insulated Cathedral Ceiling

Ban the Can

Q. "With a depth of 7 inches, the recessed lights will go through the first plywood layer but I should still be able to cover them with 1 layer of 1 inch polyiso (R6) plus 1 inch square cut piece for the rectangular piece. I believe this is enough?"

A. It's not. You're wrong.


89.
Apr 29, 2017 8:32 AM ET

lights
by Charlie Sullivan

The good news is that you have lots of options for other types of lights to install. Not only are there now low-profile LED lights that look the same as recessed lights, but in a high-ceiling room, you don't need them to be low profile anyway.


90.
May 3, 2017 8:07 PM ET

Climate Zone 3
by Sarah Deeds

R30 Insulation not R38 required......


91.
May 4, 2017 7:01 AM ET

Response to Sarah Deeds
by Martin Holladay

Sarah,
Your comment is cryptic. I'm not sure if it's an observation or a question.

If it's an observation, you're right: many Zone 3 locations in the U.S. have not yet adopted the 2012 IECC, or have adopted a modified (watered-down) version of the 2012 IECC. That means that some Zone 3 homes can get away with R-30 roof insulation instead of the R-38 insulation mandated by the 2012 IECC. (GBA often advises builders to check with their local code authority concerning code questions like these.)

If it's a question, here is the answer: If you are installing R-30 insulation in Zone 3 (following the method discussed in this article), at least 13% of the total R-value of the roof assembly must come from the rigid foam layer. That means that you'll need a minimum of R-3.9 of rigid foam (1 inch of EPS).

For more information on this topic, see Combining Exterior Rigid Foam With Fluffy Insulation.


92.
Jul 14, 2017 2:35 AM ET

1930's commercial building update
by Sepp Spenlinhauer

Hi Martin,
I've just read every word of this article, almost every comment, and joined the website too... Great information here.
I've got two complex issues that have me concerned and we have already hired the roofer and have product on the roof waiting to be installed. Scheduled work to start in two to three weeks, but I fear they will start sooner.
General project, 'restore' and update a 1930's building purpose built for a newspaper, offices & printing plant, plus a 1970's addition. The 1930's building is a steel 'superstructure', wood truss building with brick walls and limestone façade.
The 1970's addition is a steel building, steel decking with cinderblock walls.
We are doing our best to fix years of bad conversions and neglect and bring the building into the modern age. After the newspaper left, it was a school, a boxing gym, artist lofts, and warehouse....
There are two very different roofs to contend with:

Roof 1> 1930's building, 2nd floor office space.
Original roof (still intact) is hot tar over 7/8" tongue & groove deck. We believe according to a roof core that there is only one layer of torch down over the hot tar. At most two layers. Engineers have certified that we do not have to tare the existing roof off. Since the original plaster ceilings were not salvageable, we have exposed the roof trusses and wood roof deck. The intent is to leave them exposed along with the brick walls. There are some small intermittent leaks that appear during heavy rain, but not light rain.
We are Zone 4, which I understand is R38. The roof is a flat roof.
The current stackup is:
>Solar System with racking & ballast - Minimal screws into the OSB layer TBD.
>PVC SlipSheet
>Durolast PVC White Roof
>Densdeck 1/2" (or equal) (Screwed to the OSB Layer)
>4" Polyiso (2"/2")
>1/2" OSB (Screwed thru the polyiso into wood roof trusses)
>4" Polyiso (2"/2")
>Existing Torch Down & Hot Tar
>Tongue & Groove
>Roof Truss

One other item of note is that because the interior tongue & groove ceiling is fully exposed, the contractor has agreed to align the screws from the OSB into the roof trusses so we don't have a ceiling riddled with screws. And in order to ensure enough pull out resistance, the screws have to land in the roof trusses, not just the wood deck.

I don't believe that there is a vapor barrier in the current stackup. Is this a concern?
I read comment #3 several times and I understand the concern, my question is, because we are basically exposed to the ceiling, how much should we worry about not having EPS on the top layer? (We are on the very northern edge of Zone 4, as in the next town is Zone 5).

Roof 2> 1970's building, steel deck
Again, the original roof is intact. The building was purpose built for a high speed web-offset press with huge ink drying ovens & motors generating heat, so the idea was to get the heat OUT of the building, not trap it inside. There is also a 10 foot x 30 foot skylight in the middle of the room, and the ceiling height is 18 feet. The skylight is single pane glass, a whole separate issue I'm sure!
Existing building construction is a heavy gauge steel deck, fully exposed on the inside. There are NO screws holding the roof on. It is beautiful to look at and we are doing our best to keep it free from screws. Again, the structural engineer has determined we do not have to remove the existing roof.
This space will be used as a residence, again with a fully exposed roof structure.
Current Stackup:

>Solar System with racking & ballast - Minimal screws into the OSB layer TBD.
>PVC SlipSheet
>Durolast PVC White Roof
>Densdeck 1/2" (or equal) Screwed into the 1/2 OSB.
>1-1/2" Polyiso (new)
>1/2" OSB (new)
>Existing roll roofing layer - possibly a torch down, (original roof)
>1-1/2" Polyiso - Glued directly to the steel deck. - (original)
>Steel Deck. - (original)
>Screws up into OSB. (new) Rather than screw down into the steel deck, the contractor has agreed to drill holes in the steel deck and screw up INTO the OSB so that we can have a fixed pattern of screw heads on the exposed ceiling so that it doesn't look like a warehouse with screws sticking out into the space.

Concerns are many here. Because of the huge skylight and the structural setup, we can not add more than 2" of additional height to the roof or Durolast will not permit the design because the curb heights become too small.
Also, the Steel deck is a commercial 'deep channel' heavy gauge deck. The channel 'pockets' are open air with only the 1-1/2" polyiso above it.
With the low total R Value and these air-channels, I'm worried in the winter that the roof deck will be cold enough to literally condense moisture on the inside.
Note, we have had the building for two years, but it has only been heated to 55° with a scorched air blower heater, to keep the sprinkler pipes from freezing, and we have NOT seen a problem, but the space was vacant and I assume ambient moisture levels were not high enough.
We considered doing a tareoff but the consensus is that the Polyiso in place now is still effective and removing it won't allow us to increase the overall R value, so it was not worth removing it.
I have also thought that perhaps we should figure out a way to fill the steel channels. One idea is to drill holes from the roof down into the channels and fill them with foam and then cover them with the OSB. (Any recommendations on the foam to do this?)
Any thoughts or other advice?

I hope I made it as clear as possible!
If you're interested, we started a facebook page to collect the history of the building and there are many photos showing it over the years, two of them show the 1970's roof as it overhangs the car park. It was built for the Peekskill Evening Star newspaper.
www.facebook.com/peekskilleveningstar/

I hope my post isn't too long winded for you!
Regards,
Sepp.


93.
Jul 14, 2017 6:08 AM ET

Response to Sepp (Comment #92)
by Martin Holladay

Sepp,
Q. "I don't believe that there is a vapor barrier in the current stackup. Is this a concern?"

A. The old hot tar (asphalt roof) is a vapor barrier. Relax.

That said, you need an excellent air barrier at the bottom of this stack-up. If any exploratory holes have been drilled in the old roofing, or there are obvious seams or air leaks, these need to be carefully repaired and sealed before the first layer of polyiso goes down. Alternatively, the first layer of polyiso needs taped seams. Even better: tape the seams of the OSB layer in the middle of the sandwich. Use a high-quality tape like Siga Wigluv.

Q. "I read comment #3 several times and I understand the concern, my question is, because we are basically exposed to the ceiling, how much should we worry about not having EPS on the top layer? (We are on the very northern edge of Zone 4, as in the next town is Zone 5)."

A. Well, your 8 inches of polyiso will have an R-value that is at least R-40, and probably more. I wouldn't worry.

Q. "I'm worried in the winter that the roof deck will be cold enough to literally condense moisture on the inside."

A. For the roof with steel decking that has deep channels, the two biggest areas of concern are: (1) The interior perimeter of the roof, where the ceiling meets the wall -- you need to seal the air leaks where the steel channels meet the wall. (2) The exterior perimeter of the roof, there the steel channels may be covered by flashing that allows air penetration to the steel channels between the decking and the lowermost layer of rigid insulation. These channel openings need to be exposed and sealed with spray foam.

I'm not as familiar with commercial roofs as I am with residential roofs. If I were you, I would hire a consultant from Building Science Corp. in Westford, Mass. The consulting fee will be worth every penny you spend. They are experts at air leaks and corrugated steel decks like the one you describe.


94.
Jul 14, 2017 11:36 AM ET

Edited Jul 14, 2017 2:33 PM ET.

Much thanks!
by Sepp Spenlinhauer

Martin,
Thank you for the quick answers and help.
I will reach out to the company you recommended.
We have had a real hard time getting good advice in all areas of the building. From recommendations to spray foam historic brickwork (HUGE NO NO!) to putting vents in the historic limestone façade!
HVAC requirements has also been a big problem for mix-ed use, conversion of a purpose built historic structure.
And I look forward to reading more on the site in next few weeks. (Indeed I was up for two more hours after I wrote this email last night)

I hope you had a chance to see the photos on facebook, we are very excited to bring this building back to life.

Regards,
Sepp.


95.
Aug 3, 2017 4:51 PM ET

How long has this insulation system been used?
by Jack Halperin

I live in the San Francisco Bay Area in the east bay which is climate zone 3. I'm planning an addition and remodel. The roof structure will be 2x8 rafter and I have 2" I can add on top of the sheathing due to zoning height constraints. I'd like to install r30 mineral wool battsbetween the rafters and 1.5" of polyiso above the sheathing. My architect is unfamiliar with this and thinks closed cell spray foam between the rafters is a more proven approach. How long has this sandwich insulation approach been used? I'd rather use it but am concerned if it doesn't have a long track record.


96.
Sep 16, 2017 12:11 PM ET

Permeable or Unpermeable membrane under Metal Roof
by Rick Milne

Martin,
I am building an unvented roof in Marine 4 zone and following your excellent article above, including JL,s suggestion to put an impermeable peel n stick on the bottom layer of roof sheeting which I just completed. My quandary is I am putting down plywood above 7 inches of EPS for my SS metal roof and elswhere on GBA there has been a variety of opinion on whether a permeable membrane would have the ability to breathe out through metal. I am using a snap lock panel tha does have a few slightly raised flanges. Mr. RIversong suggests breathable in all cases, which was my original thought given the bottom impermeable layer. Most of the new synthetics are very low perms .06 or so unless I go to a polyolefin or similar breathable such as GAF deck armour....but these seem to be geared towards shingles. Both these high and low perm synthetics all indicate they pass the D4869 water shower test in Canada so I assume they stop some water penetration equally. HOwever am unsure which way to go as if the metal prevents any realistic breathability I am wondering if the impermeablemembranes would be better protection for the plywood as they may prevent trapped moisture between the metal and the membrane from coming back through the membrane into the deck.... thanks in advance.


97.
Sep 16, 2017 4:01 PM ET

Edited Sep 16, 2017 4:02 PM ET.

Response to Rick Milne
by Martin Holladay

Rick Milne,
There is no outward drying through a standing-seam metal roofing. The metal roofing is a vapor barrier.

If you build an unvented roof assembly -- a common approach -- the vapor permeance of the roofing underlayment is irrelevant.

If you want ventilation channels between the top of the insulation layer and the underside of the upper layer of plywood roof sheathing -- an unnecessary step in most cases, but useful in some climates to reduce the chance of ice dams (a problem that does not generally affect standing-seam roofs) -- the usual method would be to create a ventilation channel by installing 2x4s on the flat, 16 inches or 24 inches on center, from soffit to ridge (in other words, the 2x4s are perpendicular to the ridge). These 2x4s are installed above the top layer of rigid foam, and are fastened to the rafters below with long screws. The result is a ventilation channel that is 1 1/2 inch deep.

The top layer of plywood is fastened to the 2x4s.


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