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Warm roof design

Happy new year!

This site an amazing help! I have a question regarding warm roof construction.

I'm in the process of making a low pitched warm lean to(mono pitch) roof. After watching the videos and reading the articles on this site, I feel that I have a design I'm happy with.

1/2 ply sheathing
weather proof barrier
2 layers of XPS(readily available in my area)
2x4 battens secured to framing
Metal roofing secured to battens.

If possible, I want to avoid laying another layer of ply on top of the battens. Is there any reason this is needed? I have attached a simple illustration(pdf) of the design.

Any info would be greatly appreciated!!

Cheers! Paul

roof section.pdf160.06 KB
Asked by user-6877107
Posted Jan 1, 2018 10:50 AM ET
Edited Jan 2, 2018 11:32 AM ET


8 Answers

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Where are you located? Does the roofing manufacture's instructions allow the type of installation you are considering?

Answered by Steve Knapp CZ 3A Georgia
Posted Jan 1, 2018 11:15 AM ET


Minimum R-values for roofs range from R-38 in Climate Zone 2 and 3 to R-49 in Climate Zones 4 through 8.

To achieve the minimum R-value requirement in Zones 2 or 3 requires 8 inches of XPS.

To achieve the minimum R-value requirement in Zones 4 through 8 requires 10 inches of XPS.

For more information, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

As Steve Knapp points out, some metal roofing manufacturers allow their roofing to be installed over strapping (purlins), while other manufacturers require solid sheathing (plywood or OSB). The pieces of strapping (purlins) need to be installed parallel to the ridge, not perpendicular to the ridge as shown in your sketch.

No matter what you decide to do, you need a high quality air barrier between the bottom layer of roof sheathing and the first layer of rigid foam. Remember as well that building codes require roofing underlayment (for example, asphalt felt) directly under the roofing layer.

Answered by Martin Holladay
Posted Jan 1, 2018 11:33 AM ET
Edited Jan 1, 2018 11:36 AM ET.


With continuous layers of insulation, unbroken by rafters it only takes about 75-80% of the prescriptive R-values to hit code performance on a U-factor basis. eg:

Code min in zones 4 & higher is R49 if thermally bridged by rafters, but on a U-factor basis it only needs to be U0.026 (= R38.5 "whole assembly"). With credit for the R-values of the structural roof deck, the ceiling gypsum interior & exterior air films etc that usually doesn't take more than R36-R37or so of continuous insulation layer, which is ~75% of the prescribed R49 between joists or rafters.

To get there in zones 4 & higher with the labeled R of XPS that would take 7-8" of foam.

In zones 2 & 3 it would usually take ~6"-6.5", in zone 1 ~5" -5.5".

Add 20% if using EPS, or designing at the fully depleted performance of XPS.

Answered by Dana Dorsett
Posted Jan 1, 2018 12:14 PM ET


For more information on the U-factor method mentioned by Dana, see this article: Three Code-Approved Tricks for Reducing Insulation Thickness.

Note also that green builders try to avoid the use of XPS, since XPS is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see Choosing Rigid Foam.

Answered by Martin Holladay
Posted Jan 2, 2018 6:00 AM ET


Thank you for your answers! Martin that article was super useful, thank you.

I'm located in Japan and working with an older post and beam house. This has presented some challenges.

-In my area the energy efficiency code calls for roughly 4 inches (115mm) of 2nd/3rd generation XPS

The XPS I'm using is a Japanese brand called "kaneraito" According to their website - sorry it is in Japanese (http://www2.kenzai.kaneka.co.jp/kanelite/faq/index.html#qa06) it is manufactured without freon gasses. I need to look into it some more. Here XPS is a cheaper alternative to EPS, if it turns out that it uses other non-eco blowing agents I will find an alternative.

-Thank you for the advice on the strapping. Initially I planned to use a type of roof metal roofing that is installed horizontally( comes in rectangles and is filled with hard insulation- I realize the insulation would be pointless above and air pocket, but again, this is very cheap and easy to install.) However after reading your advice I have found metal sheet roofing that can be installed directly on strapping that will run perpendicular to the roof line.

##- I guess the ply above strapping would make it possible to create an overhang?

So I plan to have
-ply sheathing with taped seams
-tyvek house wrap or tyvek roof liner(expensive) if house wrap is unacceptable
-foam to code specified thickness
-asphalt roofing liner
-metal roof

Thanks again!

Answered by user-6877107
Posted Jan 2, 2018 9:30 PM ET


Your mention of the fact that your brand of foam insulation is "manufactured without freon" implies that you are confusing two problems. You are confusing (a) the problem of blowing agents that deplete the ozone layer with (b) the problem of blowing agents with a high global warming potential.

The use of freon (CFCs) as a blowing agent to manufacture foam insulation was phased out decades ago, in compliance with the terms of the Montreal Protocol. Worries about global warming potential are a newer concern -- one that is not yet addressed by an international agreement.

In Europe, manufacturers of XPS use a blowing agent that is more benign than the one used to make XPS in the U.S. I'm not sure what blowing agent is used by XPS manufacturers in Japan.

Here is a link to an article with more information on this issue: Avoiding the Global Warming Impact of Insulation.

Answered by Martin Holladay
Posted Jan 3, 2018 5:14 AM ET


I believe European XPS is all blown with CO2 at this time, and has a performance of ~R4.2/inch for 1.5lb density & up (the same as EPS.) HFO1234ze has been approved for use in XPS in Europe which would give it higher performance, but I don't know of any manufacturers (if any) are using it (yet.)

For blowing XPS "In Japan, iso-butane is also widely used." but propane, butane and HFO1234ze are approved blowing agents for XPS there, according to this docuement:


You'd have to consult with the manufacturer to know for sure, but it's probably a hydrocarbon (propane, butane, or iso-butane), and would also have performance comparable to EPS of similar density & thickness. At 115mm that would be about R19, in US terms. If it's blown with HFO1234ze it will be higher.

Answered by Dana Dorsett
Posted Jan 3, 2018 11:24 AM ET


Martin and Dana,

Thank you. Sorry I did read the article and found it very useful, I think the confusion is in the terminology and translation.

KANELITE XPS is a product of kaneka.co. The third generation is rated at under U 0.028. Their website states that process is "non-freon" but written in Japanese as "nonfron". However the term is used differently here, combining both the Montreal accord and the Kyoto protocol to include ozone depleting and global warming substances- CFC, HCFC and HFC. In order to use the 'NONFRON' label companies can not use HFC134a.

Dana I think you are correct in that the process uses hydro Carbons. The only foams that use 134a that are manufactured in Japan are urethane based(according to the ministry of environment env.go.jp)

Answered by user-6877107
Posted Jan 3, 2018 9:04 PM ET

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