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

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:
  4. Image #4: Martin Holladay

Apr 3, 2015 11:24 AM ET

Great article, thanks Martin.
by Peter Rogers

Great article, thanks Martin. Will be using the un-"vented-over" method when we build our addition, probably with the EPS buffered polyiso option for the foam.

Have you ever heard of someone installing 2x4s over the first half of the foam, parallel to the rafters, with 1.5" foamboard between the 2x4s to fill the gap, then installing the rest of the foam over top? Seems like you could avoid a lot of the frustration associated with using 10" screws. Plus, the tiny thermal penalty would be more than overcome by avoiding metal fasteners traveling directly through the assembly.
Do you see any drawbacks wiyh this method? Does it seem impractical?

Apr 3, 2015 11:33 AM ET

Response to Peter Rogers
by Martin Holladay

Yes, I've heard of the method you describe. You'll end up with a roof assembly that isn't quite as well insulated as it would have been without the 2x4s in the middle of the sandwich. But in some cases, ease of installation may trump performance. The decision is yours.

Apr 3, 2015 11:43 AM ET

Edited Apr 3, 2015 11:45 AM ET.

A couple of comments...
by Dana Dorsett

The amount of exterior-R needs to rise above the IRC prescripitive minimums if the total-R is going to be above code min. The whole point of the prescriptive minimum is to keep the average temperature of the roof deck sufficiently above the average wintertime dew point of the conditioned space air that it doesn't accumulate too much moisture. If you increase the interior-side R value such that the total R goes higher than code min, you MUST raise the exterior-side R proportionally or the roof deck will run colder.

For example, in climate zone 5 the code minimum is R49, and the minimum exterior-R prescription for splitting that R is R20, or about 41% of the total. To reach code min you would then have to install R29 on the interior side. But if R38 is installed between the rafters, the total R would then be R58, which is great, but the exterior-R is now only about 34% of the total- the roof deck will run substantially colder. To hit the same average roof deck temp as the code-min prescriptive with R38s on the interior would require R26 for the exterior foam bringing the total center-cavity R to R64. (R26 / R64= 41%)

On another note, regarding the derating of polyiso for temperature it's the average temperature through the foam, and not the exterior temperature that is the relevant parameter. At an average temp through the foam of 25F the performance drops below R4, true. But if polyiso is the only insulation and the interior is kept at 70F and the exterior temp is 25F, the average temp through the foam is 47.5, or about +9C. Looking at Karagiozis' curve ( ), at +9C the performance of polyiso at a mean temp of +9C is comparable to that of XPS at +9C. The performance of XPS at a mean temperature of +9C/47.5F is substantially above it's R5/inch labeled performance, which is tested at a mean foam temp of 75F, with the cold side at 60F, the hot side at 90F.

So, in the all-polyiso insulation stackup when the outdoor temp is +25F, the performance is still well above R5/inch, which is still pretty good.

But in the climate zone 5 prescriptive-mininum situation with R29 on the interior, R20 polyiso on the exterior, the warm side of the polyiso is colder than 70F. At an interior temp of 70F and an outdoor temp of 25F, the roof deck is colder, about 40% of the temperature difference above the outdoor temp, or about 43F. With an outdoor temp of 25F and roof deck temp of ~43F the mean temp through the foam is now about 34F/1C. Looking at Karagiosis' derating curve the performance of polyiso at a mean temp of 1C is about half that of XPS at 1C, which.

The performance of XPS at +1C is no better than R5.5/inch, which means that in a 40% of labeled R on the exterior stackup the performance of the polyiso is LESS THAN R2.8 / inch, not R4.0inch (!).

What this means for the average roof deck temp is complex, since during the warmer part of an average winter day in climate zone 5 the performance is substantially higher than during the coldest part of the day, but still performing at less than the labeled-R. But during the shoulder seasons it will perform substantially BETTER than it's labeled R as the mean-temp through the foam crosses through the +15C knee in Karagiozis' curve, which means the roof deck can dry faster.

To compensate for the plummeting performance of polyiso in those stackups it's useful to split the thickness using equal thickness of EPS to polyiso, maintaining the prescriptive minimums. EPS performance rises in performance at low temps, and with EPS on the exterior it keeps the polyiso layer wamer, warm enough to improve it's average performance. In the zone 5/R20 case, 2" of EPS (R8) and 2" of polyiso (R12) would meet the prescriptive minimum. But now rather than having a mean temp of 34F through the polyiso when it's +25F out it will be running about 38F/+3C through that layer, and would be performing at about R4/inch, or R8 @ 2" , while the EPS would be running about R9, for a net performance of R17.

A mean daily temp of +25F is pretty typical for a climate zone 5 location in January, but it's (usually) substantially higher in December & February, and the performance of polyiso will rise rapidly at those higher temperatures, reaching the peak performance knee in the curve in March in the 2" + 2" stackup. So even though the roof deck moisture accumulates faster in January than anticipated based on just the labeled R values, the drying season begins sooner, so it's still a moisture-safe and decent thermal performance stackup, a stackup that really will indeed average about R20 over the entire heating season.

Of course going a bit higher than the prescriptive minimums gives you more moisture margin, but it's still worth splitting the thickness 50/50 with EPS to compensate for the derating curves. For an all-foam code-min solution that can be reduced to about 35 / 65 in climate zone 5 (4" of EPS + 5.5" polyiso) and it would meet or beat R49 throughout the season.

Apr 3, 2015 12:03 PM ET

Response to Dana Dorsett
by Martin Holladay

Thanks for your very helpful comments. I have edited my article to reflect your points.

Apr 3, 2015 1:59 PM ET

Martin, Great article. If you
by Lucy Foxworth

Great article. If you refer to this article in QA answers will non-GBA prime members be able to read it? It's really good information.

Apr 3, 2015 4:36 PM ET

First time in 14 years I did not have ice dams!
by terry grube

Live in zone 5 NH. 200 year colonial.
1.screwed old boards down, had to replace some, skinned with plywood
2. Ice and water as air barrier
3. 3 inches of polyiso, 1 inch sheets staggered and taped
4. 3 inch thick layer of Eps on top of polyiso , taped seams
5.screwed 2x4 on flat every 16, screwed plywood through 2x4 to deck and beams
6. Second layer ice and water over that.

Had 4 feet of snow on roof and did not sniff an ice dam. First time in 14 years. Great way to go if anyone is interested. Even with all this believe it or not the roof is lighter because I started by stripping 4 layers of shingles. Got to love old houses.

Apr 3, 2015 5:03 PM ET

I looked at the links in
by Eric Habegger

I looked at the links in Lucy's comment (good links Lucy!) and was taken by an idea in the third one using SIPs on a roof. SIPs really are a good idea here because of the thickness and the nailer being included. The big disadvantage is that you don't need more than 1 layer and then it wouldn't be staggered, like two individual layers of foam would be.

Here's an idea for the enterprising types. (Not me) Shouldn't there be a standardized type of SIP that is T&Ged on all 4 sides just like hardwood flooring? I'm not talking T&G on the nailers only but on the whole thickness of the SIP. If SIPs could be made like that in a standardized way it would eliminate all the fussiness in using 2 individual layers of foam. You have all the advantages of staggered seams by using that method, perhaps with some canned spray at the joints.

Apr 3, 2015 5:08 PM ET

Attaching Metal Roofing
by Matt Brewster

If I am planning to install a standing seam metal roof, is it possible to attach it directly through the rigid foam? Or, if a top layer of plywood is necessary, what's the thinnest plywood that you'd recommend?

Apr 3, 2015 6:02 PM ET

Edited Apr 3, 2015 6:11 PM ET.

Response to Lucy Foxworth (Comment #5)
by Martin Holladay

Taunton Press has decided to put all my blogs (those written after late January 2015) behind the new paywall. These blogs can only be read by GBA Prime members. However, Taunton is still offering a 10-day free trial membership.

Apr 3, 2015 6:05 PM ET

Edited Apr 3, 2015 6:12 PM ET.

Response to Terry Grube (Comment #6)
by Martin Holladay

Thanks for sharing your story. I'm glad that following this approach solved your ice dam problem.

Apr 3, 2015 6:07 PM ET

Edited Apr 3, 2015 6:12 PM ET.

Response to Eric Habegger (Comment #7)
by Martin Holladay

I'm confused by your reference to links in Lucy's comment -- which comment? which links? -- but my article does discuss the use of SIPs as one approach to adding rigid foam above the roof sheathing of an existing house.

Apr 3, 2015 6:10 PM ET

Edited Apr 3, 2015 6:13 PM ET.

Respose to Matt Brewster (Comment #8)
by Martin Holladay

As far as I know, standing-seam metal roofing can't be installed on rigid foam; it has to be installed on plywood, OSB, or (possibly) purlins. If you want to be sure, talk to your roofer or the manufacturer of the roofing panels.

I imagine that you could install it on 5/8" plywood, but of course 3/4" would be better.

Apr 3, 2015 7:13 PM ET

Links for roofing over foam
by Lucy Foxworth


I wrote a response to Dirk Gently's question about installing foam on a roof. It's this post. That's where the links are.

Apr 4, 2015 5:56 AM ET

Response to Lucy foxworth
by Martin Holladay

Thanks for clearing up the confusion. If other GBA readers are interested, here are the links that Lucy shared:

Insulating the roof with polyiso foam boards

Retrofitting an Insulated Cold Roof

Ventilated Cold Roof

Apr 8, 2015 9:38 PM ET

Washer-head extra long screws - up to 18 in. long
by Mike Guertin

FastenMaster's HeadLok screws work great for screwing down 2xs or sheathing over thick foam. The washer-head style has good holding power and planes out flush without the need to overdrive and bury the hex head on many other screws.

Best part is they're available up to 18 in. long

Apr 9, 2015 3:42 AM ET

Response to Mike Guertin
by Martin Holladay

Thanks. I have edited the article to include your FastenMaster link, and the information on maximum length.

Apr 16, 2015 6:39 PM ET

Great information, Martin, as
by Michaela Riley

Great information, Martin, as always! I am so happy to have found your advice through this website.

Just confirming that I understand correctly: Through-fastened metal roofing can be attached through 1x3 purlins over foam sheathing, correct? I am about to start building an 8x20 "tiny house" and am sold on this method of roof insulation, but I'm struggling to keep my build weight low enough for my trailer. So if possible, it seems best to use purlins on my house, rather than another layer of (heavy) sheathing. (I plan on using 2" of polyiso on top of my structural sheathing.)

What do you think? Any concerns? Thanks in advance!

Apr 16, 2015 7:20 PM ET

Response to Michaela Riley
by Martin Holladay

Yes, you can install through-fastened metal roofing to purlins. I would suggest 1x4s rather than 1x3s. You can install them 24 inches o.c., screwed through the foam to the rafters below.

Of course, you still need solid sheathing (usually OSB or plywood) between the rafters and the rigid foam.

May 6, 2015 2:10 PM ET

Edited May 6, 2015 2:11 PM ET.

Interior Air Barrier Question
by Andy Kosick

First, this article (and commentary) are incredibly thorough and very helpful as I am working with a customer interested in just such a roof retro-fit. If I can still get a question answered it is whether an interior air barrier is necessary on the inside of fibrous insulation under the roof deck when using the "Option 2" roof. It seems like it should be. Assuming it is, do you think dense-packing cellulose behind something like Mento-Plus would work. The house has vaulted and attic portions. Roof deck insulation is being chosen for continuity, the attic areas have limited accessibility, and cellulose will already be on site. To avoid getting into spray foam or having to get rigid material into the attic, I'm wondering if this is a possibility?

May 6, 2015 2:35 PM ET

Response to Andy Kosick
by Martin Holladay

It is always important to make sure that you know where your air barrier is, and to seal all leaks in your building envelope.

Many builders establish the air barrier at the sheathing level. It is also possible to use well-taped rigid foam (above the roof sheathing) as your air barrier. Any air leaks at the eaves or ridge must be located and sealed.

Once you've done this, a second air barrier under your fluffy insulation isn't strictly necessary -- but it certainly can't hurt. If you want to establish your primary air barrier on the interior side of the fluffy insulation rather than at the roof sheathing layer or rigid foam layer, that's certainly possible.

Q. "Do you think dense-packing cellulose behind something like Mento-Plus would work?"

A. I assume that Mento-Plus is an air barrier. Most installers of dense-packed cellulose use an air-permeable membrane like InsulWeb for dense-pack jobs. Because InsulWeb is air-permeable, it allows air to escape during the installation process. For more information, see How to Install Cellulose Insulation.

May 6, 2015 5:04 PM ET

Thanks for the quick response.
by Andy Kosick

I see your point about installation issues, duly noted. Although it still makes me nervous, I feel reasonably reassured that if an air barrier is established at the roof deck and an adequate percentage of R-value is outside of that, some air permeability of the fluffy stuff is acceptable. Besides it seems that an open cell foam installation wouldn't be much different than net and blown cellulose under these conditions.

Aug 4, 2015 3:58 PM ET

Percentage of rigid above plywood at unvented cathedral ceiling

Climate Zone 4C Portland Oregon: I want to ensure that I have the right percentage of r-value above the roof deck relative to below the roof deck for dealing with condensation at the roof deck. In comments, Dana Dorsett commented particularly on this subject. She talks about the exterior side needing a minimum of 41% of the total r-value. What/where does this comes from and backup for this?

Aug 4, 2015 4:03 PM ET

Edited Aug 4, 2015 4:09 PM ET.

Response to Daniel LaJoie
by Martin Holladay

The relevant code requirements are explained in the article on this page, as well as in How to Build an Insulated Cathedral Ceiling.

In Climate Zone 4, the code requires at least R-10 of rigid foam above the roof sheathing for the type of roof assembly that you are contemplating.

Aug 17, 2015 12:34 PM ET

Estimate for foam over rafters
by Tom Woodman

Hi folks,

First, an heartfelt "thank you" to all the professionals on this site for the building science education I'm getting!

I have a typical American Foursquare in zone 5. I just got a quote from a roofing company to tear-off the original cedar shake plus two layers of asphalt, install nailbase over the rafters, and then re-install the fascia and gutters, new asphalt shingles. The quote was $26k. Is that reasonable?

My neighbor has the identical home next to me. Last year, he paid $7500 for the tear-off and new asphalt shingles. I was hoping I could get the nailbase done for a total of around $13k, but that's just speaking with my wallet and the practicality of my spending.


Aug 17, 2015 12:41 PM ET

Response to Tom Woodman
by Martin Holladay

There is no way for us to determine if the quote is reasonable. We don't know the size of the roof; how many feet off the ground it is; whether the roof has good access for ladders or difficult access; the clearance to adjacent houses; and whether there is room to set a dumpster or to stage materials and equipment.

We don't even know the R-value of the nailbase.

If you don't like the estimate, call up a few more roofing contractors and get a few more estimates.

Aug 19, 2015 8:31 PM ET

Edited Aug 19, 2015 9:19 PM ET.

SIP vs. Rigid Foam in Z2 Hot-Humid
by Andrea W.

I have a small uninsulated concrete block home in Clearwater, Florida. (Added stucco and elastomeric paint to exterior, would have added rigid foam if I knew then what I know now.) There also is no insulation in the wall between the attached garage and the house. Now the simple gable roof needs replacing. I can see it buckling in a couple small areas and there has been leaking, which has been fixed. AC handler and ducting are in the attic and probably leaky. Also, I'd like to use the attic for storage and possibly cathedralize the living room (which is the center of the rectangle). I read your other article, "Creating a Conditioned Attic-Five questions to consider before insulating a sloped roof" and this. I have seriously thought about using SIPs. Innova Eco is in Miami, not too far, and their roof SIPs are 10", with EPS and MgO panel on top side. They spray a coating on top of that. I have requested a quote. However, it sounds like your rigid foam concept might be preferable.

Keep in mind that it rains a LOT here. It has rained heavily almost every day for the past 2 months. The temperature never gets below 32-F and I'm not worried about heating the house. I do not set the AC below 80 in the summer, and let it get up to 86 during the day, running it mainly to reduce humidity. (It is undersized for the current condition of the house, so it will always run long enough to remove humidity.)

If this were your house, what would you do? Is there a specific formulation that would be preferable in Zone 2 hot-humid-rainy?

EDIT: I was thinking about removing the sheathing, at least in the small damaged areas, but I just saw the photo of the installation over existing roof. Would this work?

Thank you for your always-excellent articles.

Aug 20, 2015 6:48 AM ET

Response to Andrea W.
by Martin Holladay

Q. "If this were your house, what would you do?"

A. If I could afford to do the work, I would install roof insulation that meets minimum code requirements. In your climate zone (Zone 2), that means R-38. Since your air handler and ductwork are in your attic, this insulation should be installed above your roof sheathing, so that you create an unvented conditioned attic. In other words, you should follow the advice given in the article posted on this page.

You shouldn't do any of this work without first exposing enough of your roof sheathing to make sure that the sheathing and rafters are sound. Repair as necessary before proceeding.

If you can't afford the work -- welcome to the club. Do the best you can with the money you have. Good luck.

Sep 4, 2015 5:45 AM ET

Edited Sep 4, 2015 6:09 AM ET.

Above-deck insulation under aluminum, on an unconditioned house
by Robert Johnson III

Aloha--relieved to just find this site, and impressed with the level of expertise in evidence here! I've not yet found a treatment of our particular simple unconditioned building scenario--forgive if I missed it. Excellent article, and dialogue, here.

We live in a screen-windowed 1965 tract house (no AC or heat year-round), in Kane'ohe on the island of O'ahu, which is rather humid and warm, and rainy for much of the year. The house is unsealed and unconditioned. About a third of the 2,500 sf low-pitch roof is over a crawl-space attic; the rest is cathedral-style with 3.5" x 7" beams (only about 12' at peak), and nothing but about 1.5" x 8" brushed cedar T&G and Ice & Water Guard between us and the sky post-tear-off. (We did the tear-off just in time for flash flood rains today--but that's a different issue.)

I've not yet been able to find building science addressing above-deck insulation for such an open house. What we just tore off was the original pitch and gravel and two layers of modified bitumen; what we'll lay down is aluminum standing seam. Here's what I (know I) don't know.

Q1: For such an unconditioned house, would one or two inches of above-deck insulation help somewhat reduce the transfer of the tropical sun's heat into the living area?

Q2: If so, should we resort to XPS? Does this look approach to the above-deck insulation look functional--

g. aluminum standing seam
f. tar paper
e. plywood (1/2"?)
d. XPS (1"?)
c. tar paper
b. ice and water guard
a. wood deck

Q3: Can we get away without venting in the above-deck roof, given our unconditioned house?

Q4: I don't know how the roofers will be able to attach the standing seam once the roof is built up in this way--the screws must somehow artfully pass through the above layers to stop within the old 1.5" brushed cedar T&G deck without being visible from below in the open beam ceilings. Probably can't just anchor to the beams, which are spaced about 4' apart. (Hurricanes rarely actually hit us, but every once in a while, they will. We're somewhat inland, close to the Ko'olau mountains, but storms are possible.)

Q5: I've read in a couple of places that "whirlybird" attic fans (we're about to install a solar one to cool our attic) can cause attic wood rot--is this a concern under our unconditioned scenario?

Mahalo Nui in advance for your help as we take this path to a "permanent" and cooler roof!

Sep 4, 2015 6:02 AM ET

Edited Sep 4, 2015 6:07 AM ET.

Response to Robert Johnson III
by Martin Holladay

Q. "For such an unconditioned house, would one or two inches of above-deck insulation help somewhat reduce the transfer of the tropical sun's heat into the living area?"

A. Yes, although you can't expect to be cool and dry when the windows are open all day, and the weather is hot and humid. Your expectations need to be low enough to be satisfied with the approach you have chosen. If the humidity is beginning to get to you, you should install window glass and an air conditioner.

Q. "If so, should we resort to XPS?"

A. To insulate above your roof sheathing, the usual approach is rigid foam. However, XPS is the least environmentally friendly option (because it is manufactured with a blowing agent with a high global warming potential). Either EPS or polyiso would be more environmentally friendly.

Q. "Does this look approach to the above-deck insulation look functional?"

A. Yes, your stack-up will work.

Q. "Can we get away without venting in the above-deck roof, given our unconditioned house?"

A. There is no need to vent the roof you describe (especially in a hot, humid climate, where venting is counterproductive).

Q. "I don't know how the roofers will be able to attach the standing seam once the roof is built up in this way--the screws must somehow artfully pass through the above layers to stop within the old 1.5" brushed cedar T&G deck without being visible from below in the open beam ceilings."

A. First of all, a little bit of math should help with roofers choose a good screw length. (I think that 2 3/4 inch screws would work fine.) That said, I think that standing-seam roofs are routinely fastened to 1/2 inch plywood, without being attached to rafters or roof framing. Talk to your roofer.

Q. "I've read in a couple of places that "whirlybird" attic fans (we're about to install a solar one to cool our attic) can cause attic wood rot--is this a concern under our unconditioned scenario?"

A. In a hot, humid climate, you definitely don't want to install a powered attic ventilator. For more information on this topic, see Fans in the Attic: Do They Help or Do They Hurt?

Sep 4, 2015 1:56 PM ET

Edited Sep 4, 2015 1:57 PM ET.

Above-deck radiant barrier and aluminum, on unconditioned house
by Robert Johnson III

Thank you again very much for your help! May I ask what you think might be the relative effectiveness at slowing heat transfer using a radiant barrier (TechShield) board under aluminum standing seam as follows (instead of ~1.5" of polyiso), on a year-round unconditioned, unsealed house in hot/wet climate?

e. aluminum standing seam
d. high temp underlayment
c. TechShield (1/2")
b. water & ice peel & stick
a. wood deck


Sep 4, 2015 2:42 PM ET

Edited Sep 4, 2015 4:16 PM ET.

Response to Robert Johnson III
by Martin Holladay

A radiant barrier won't be effective if you sandwich it between roofing underlayment and Ice & Water Shield. The radiant barrier only works if it faces an air space.

If you are willing to build your stack-up this way:

f. aluminum standing seam roofing
e. high temp underlayment
d. 1/2-inch-thick radiant barrier roof sheathing, shiny side down
c. 1x4 furring strips, 16 inches on center, installed from soffit to ridge (parallel to the rakes)
b. water & ice peel & stick
a. wood deck

...then you would get a benefit from the radiant barrier.

According to ASHRAE Fundamentals, a 3/4-inch-thick air space bounded by such a radiant barrier will have an R-value (resistance to downward heat flow) of between R-2.1 and R-2.3. That is less effective than 1.5 inch of polyiso, which has an R-value of about R-8. So the polyiso wins.

Sep 4, 2015 3:54 PM ET

Edited Sep 4, 2015 4:01 PM ET.

A few comments for Robert Johnson III
by Dana Dorsett

On a low-pitched roof in the tropics you may actually exceed the ~160F operating temp limit for polystyrene (EPS or XPS). I've measured roof temps about that high on dark membrane flat roofs even at 42 degrees north latitude, on a 70F early afternoon under a clear sky(!). Polyisocyanurate would be a much better choice, since it's good to 300F. For reference:

The solar reflective index(SRI) of the finish coat of your aluminium roof also matters- it makes as much or greater difference than a radiant barrier under the roof. The aged SRI of various roofing materials can be searched on the CRRC site:

White finishes tend to have higher infra-red emittance compared to bare metals, and a somewhat higher SRI lower solar spectrum reflectivity compared to bare metal. Take a look at this vendors' published SRIs:

Note that the zinc plated aluminum SRI is only 72, due to the ultra- low 0.08 emittance. The white (probably titanium oxide) finishes from that vendor are comparable or better than the much shinier bare zinc. This is pretty typical.

Mounting metal roofing on 2x purlins giving 1.5" of air between the roof and foil-faced polyiso gives you the thermal benefit of the radiant-barrier facer on the polyiso, as well as the full R-value of the polyiso. The exact R of the polyiso isn't constant with temperature and it falls off with rising temp, so it depends on just how hot that purlin-cavity gets. Any convective cooling you can get through that purlin gap is going to help, but it will vary with location on the roof, the greatest benefit being near where the convection air is entering.

(edited to add)

In that climate, the thermal mass of an inch or two of concrete on top of the roof deck plus an inch of polyiso on top of that does quite a bit for thermal performance, if the structure can take the load. (Reinforcing thin concrete with steel wire would be important to keep it from breaking up and becoming a hazard in an earthquake. Presumably local building codes cover concrete roofs.)

Oct 25, 2015 3:38 PM ET

Preexisting rafter-mates and vapour barrier in retrofit planning
by Peter Zwarich

We have a vaulted ceiling in our old cabin-like home. it is a storey and a half. upstairs we have knee walls and a front porch overhang with no insulation. straight forward roof - moderate to highly sloped. we have the usual intrusions plus a wood stove chimney and 2 light tubes. We need a better roof as we currently have both ice damming with some water making its way inside and likely not enough insulation.
The vaulted ceiling profile from the interior to the exterior has the following layers: drywall, vapour barrier, extended rafters with 5.5" of Roxul (R22?), rafter-mate baffles, original T&G roof deck, roofing paper/felt, shingles.
I am interested in adding insulation to the roof. EPS overtop the existing shingles & 2x4's for air channels, 1x3 strapping across the channels for a vented cold metal roof overtop the added foam.

1. Are the rafter-mates problematic? If so, would their effect be negated if I filled the gap with spray foam from above the deck down into the void? I would need to drill access holes down through the shingles and decking - what distance between holes would be needed?

2. Is the vapour barrier problematic? need to be removed? (if so then I would forgo the approach in question 1.)

3. Should the porch overhang be insulated from underneath? (has a t&g Ceiling).

4. What happens at the peak? Currently there is a ridge vent - does it just get ripped off? along with careful mating of rooftop foam to follow?

5. We live in Sudbury, ON. What zone are we in??

Answers here can help me wrap my around around the best course of action moving forward and will help determine whether there is work to be done on top of the existing roof or both beneath and on top of the existing roof.

Many thanks! Great and helpful articles on this subject that I wish existed but 16 years ago. Time to get it done right.

Nov 17, 2015 10:29 PM ET

Zone 3 Roof Plan
by Patrick Cronin

Thank you in advance for providing your expertise here!

I am building a 4/12 roof in Zone 3 (Charlotte area North Carolina). I'm planning on open cell spray foam on the underside of the roof deck.

Given that it seems that the ideal roof deck build-up would consist of:

1. OSB sheathing
2. 1/2" rigid foam board
3. OSB sheathing
4. Moisture barrier
5. Asphalt shingles.

I am curious as to whether the first layer of OSB could be omitted if the foam board did not react to the spray foam?

And considering my climatic zone, what would be the optimal roof and wall construction method?

Thank you again,


Nov 18, 2015 6:03 AM ET

Response to Patrick Cronin
by Martin Holladay

If you want to install rigid foam above your roof sheathing in Climate Zone 3, then 1/2 inch of rigid foam board is not "ideal." This rigid foam layer needs a minimum R-value of R-5, and that's impossible with 1/2 inch of foam. As my article states, you need a minimum of about 1.5 inch of EPS, 1 inch of XPS, or 1 inch of polyiso.

In your question, there's another issue: the difference between a minimum standard and an ideal roof. It's hard to design the ideal roof, but in general, thicker rigid foam is better than thinner rigid foam.

When building a sandwich as you propose -- OSB, rigid foam, and OSB -- you can't omit the lower layer of OSB, because roof sheathing is structural. It has to be fastened directly to your roof trusses or rafters to obtain the strength that engineers insist on.

Dec 16, 2015 1:33 PM ET

Fire considerations of air gap under shingled roof deck?
by John Charlesworth

Given that:
1) roof shingles are normally nailed into 5/8" plywood or OSB roof decking, and
2) a sloped ventilation channel is often needed directly under this wooden decking (and above the exterior foam) to prevent moisture accumulation,
what happens in the case of a fire? Seems to me that fire would spread rapidly through such a roof (much quicker than though a roof that had mineral wool in direct contact with the underside of the topmost roof decking).

Is there a less flammable roof decking material/stack up that you'd recommend?

I'm in Zone 4 Coastal.


Dec 16, 2015 1:51 PM ET

Response to John Charlesworth
by Martin Holladay

The vast majority of roof shingles are fastened to roof sheathing that has an air gap underneath. (Most sloped roofs are uninsulated -- these roofs cover a vented unconditioned attic.)

This type of roof (a vented roof) meets building codes and the expectations of the insurance industry.

It's possible that unvented insulated roof assemblies are somewhat safer in a fire than conventional vented roof assemblies -- but if that's true, I've never seen any data that shows that it's true. Perhaps some researchers have studied the issue and published a paper reporting on their findings -- but if they have, I haven't seen it.

In the absence of data, I hesitate to speculate.

Dec 16, 2015 1:55 PM ET

Second response to John Charlesworth
by Martin Holladay

Q. "Is there a less flammable roof decking material/stack up that you'd recommend?"

A. I wouldn't recommend it (because of the high expense), but you could adopt commercial construction techniques if you wanted to. It's possible to use steel framing and to install steel decking (or concrete decking) if you want. You can even install steel roofing on top of your steel or concrete decking.

Dec 16, 2015 5:14 PM ET

Edited Dec 16, 2015 5:15 PM ET.

The firefighters I know talk
by John Charlesworth

The firefighters I know talk about that, when flames enter the soffit vents and set the decking on fire, it's game over in a hurry.

Thinking of doing a "chainsaw retrofit" which (unfortunately) would connect the rainscreen gap on the walls to the roof's ventilation channel.

I can reduce the flame spread up the rainscreen gap by using mineral wool exterior insulation and fiber-cement siding (leaving the wooden furring strips as the only flammable item on the exterior walls) but wasn't sure how to do a similar risk reduction on the roof ventilation channel.

Is there a non-flammable sheathing material for roof decking that's suitable for attaching shingles to? (Thinking something like DensGlass)?

Dec 16, 2015 5:49 PM ET

Edited Dec 16, 2015 5:52 PM ET.

Third response to John Charlesworth
by Martin Holladay

It sounds like you are talking about a small subset of structure fires -- namely structure fires that are caused by wildfires. These are buildings that burn down after being ignited by burning vegetation near the house.

Structure fires caused by wildfires probably represent a small percentage of structure fires in the U.S. -- (and I welcome statistics on this issue from knowledgeable GBA readers).

The percentage of structure fires caused by wildfires ranges widely by location. Here in New England, this type of fire represents something close to 0% of all structure fires. In dry areas of California, the percentage is of course higher.

Dec 17, 2015 7:32 PM ET

Rigid insulation under a Cool Roof in Zone 3?
by Blan Holman

Appreciate this extremely helpful blog. I live in Charleston, SC, in an old (1858) 3-story wooden house. Plan to replace asphalt shingle roof with standing seam metal. Currently there is no insulation anywhere, and the third floor, basically an enclosed attic, gets crushed by heat in the summer.

Proposed project: strip off the asphalt shingles, apply 1" polyiso and then "cool roof" metal over that. Would then add cellulose in rafters, and on top of of the third floor ceiling (the third floor ceiling is low, leaving an A-frame of unfinished and partially vented attic above it).

My question is whether the polyiso is worth it if we are going with a very reflective/emissive metal roof, if heat gain on the third floor is the big energy issue.

Thanks for your thoughts!

Dec 18, 2015 6:58 AM ET

Response to Blan Holman
by Martin Holladay

Q. "My question is whether the polyiso is worth it."

A. If you are planning an unvented insulated roof assembly -- one in which the insulation follows the roof slope -- the polyiso is not only "worth it," it is required. The polyiso performs an essential function: it keeps the roof sheathing above the dew point during the winter, preventing moisture accumulation in the sheathing.

If, instead, you are considering a vented roof assembly, then the polyiso would be unnecessary.

Dec 18, 2015 10:29 AM ET

Rigid insulation under a Cool Roof in Zone 3?
by Blan Holman

Thank you! Wondering if this is not an "insulated roof assembly" -- cellulose would go on top of the conditioned third floor living space's ceiling, which is a good four feet below the rafters that will support the new metal roof. The space above the ceiling could be thought of as uninsulated attic; if so, would answer be the same? Charleston SC is very warm and humid.

Dec 18, 2015 10:38 AM ET

Response to Blan Holman
by Martin Holladay

If you have a vented unconditioned attic, then installing polyiso above your roof sheathing is a waste of polyiso. If you want to install a thicker layer of insulation, then all of the insulation belongs together -- in your case, on the attic floor. Take the money that you would have spent on the polyiso and use it for a thicker layer of cellulose.

Of course, I hope you don't have any ducts up there. Installing ducts in an unconditioned attic is always a mistake.

For more information on these issues, see:

All About Attic Venting

Creating a Conditioned Attic

Keeping Ducts Indoors

Dec 18, 2015 11:01 AM ET

Rigid insulation under a Cool Roof in Zone 3?
by Blan Holman

Thanks! Last followup I promise: house was built in 1858 -- attic space above the third floor ceiling is not intentionally "vented" (so gable vent, no soffit vents) but it is leaky, w gable ends constructed out of lapped siding with no sheathing.

Also, to complicate things, because the third floor living space is mostly an enclosed attic, the third floor walls follow the roofline. So while it has a flat ceiling (creating an 5 ft wide by 4 ft tall A-frame "attic" above it), the 4 ft of sloped walls follow the roof line, then terminate into a knee wall.

I could *probably* get cellulose in the rafters between the sloped wall and roofline. Did I mention there are five dormers? Nothing easy with this house. My concern from the thread was having cellulose in those rafters without insulation above roof decking. Then again it is very warm here so maybe water concerns are lessened. We get about 20 freezing nights a year total.

Charleston is a lovely town w beautiful old houses. But they are very strict about changes to rooflines and features...

Dec 18, 2015 11:04 AM ET

Rigid insulation under a Cool Roof in Zone 3?
by Blan Holman

*no* gable vents, and no soffit vents

Dec 18, 2015 11:25 AM ET

Edited Dec 18, 2015 11:30 AM ET.

Response to Blan Holman
by Martin Holladay

If you have a section of your sloped roof assembly that is insulated (or needs to be insulated), then of course you have to follow the usual rules for these types of insulated assemblies. You still have the usual choices: you can create a vented insulated roof assembly or an unvented insulated roof assembly. If the assembly is an unvented insulated assembly, it would need at least some rigid foam on the exterior side of the roof sheathing in your climate zone.

If these concepts are new to you, you can read more about them in these two articles:

How to Build an Insulated Cathedral Ceiling

Insulating a Cape Cod House

Jan 9, 2016 5:32 PM ET

novice: want to insulate over asphalt shingle cottage roof
by Amy D'Ancona

Any help gratefully received: 1953: northern Michigan near Traverse City, my parents had a 900 s f tongue and groove cottage built. Asphalt shingle roof. The underside of the roof, the supporting beams, all are visible and beautiful to me. So I want to insulate, but on top of the roof. No insulation at all now. I am trying to learn from this site. Would anyone advise me further? I have subscribed to GBA and have begun to read articles. Don't know what "zone" I'm in, there. Thank you

Jan 10, 2016 6:28 AM ET

Response to Amy D'Ancona
by Martin Holladay

Traverse City, Michigan is in Climate Zone 6. Here is a link to GBA's Q&A page -- the Climate Zone map is on that page. (If you click the map, it will get bigger):

For information on installing rigid foam above roof sheathing, read the article on this page (the page where you posted this question). If you have further questions, you may want to post them on GBA's Q&A page.

Jan 11, 2016 3:38 PM ET

question for Martin on air barriers
by David Hicks

You wrote, "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)." Is it acceptable to use a layer of taped foil-faced polyiso foam immediately above untaped sheathing as the air barrier?

[Editor's note: To read the answer to this question, as well as other comments, advance to Page 2 by clicking on the 2 below.]

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