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House in Japan: EPS vs Urethane

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Hi! First of all, let me say that I am a total beginner and am not even confident I can explain my question using the proper technical words. I live in Japan and am about to have a passive type house built. Reaching the real passiv haus standards would cost a fortune here so I am doing my best to match my budget.

Before choosing between 2 house builders, I asked them what kind of wall insulation they recommend in order to reach a 0.3 U-value withing the house. Provided they use the same windows, ceiling and floor insulation I would like your advice on 2 kinds of wall insulation.

1. 105mm glass wool as interior insulation, plus 120mm EPS exterior insulation.


2. 105mm urethane board interior insulation. Each company is assuring their material will not loose any of their insulation power even after 50 years, but I find that hard to believe. Also I have heard bad things about chemicals in EPS. ] All things equal (including prices) which is better? Which is more reliable in the long run?

Thank you, David
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Welcome aboard!

I can shed some light on your questions.

All foam insulations off gas over time. EPS loses the least r-value, but it's not as high to start with...so there's that.

Extruded polystyrene (often pink board or blue board) has better resistance to water damage in the real world, and is physically more durable than bead board (EPS)


Urethane foam insulation comes in a couple varieties, sprayed in place and rigid board. The modern versions are way waaay waaaaaay better chemistry and durability than urethane foams from 20 or 30 years ago. They get used in some of the toughest installed conditions anywhere, commercial roofs. Read about it here:


Urethane foams have pretty much the highest r-value per centimeter of any insulation, and the gasses they use for foaming agents to make the board are pretty benign, both to you and to the environment.

I will throw a caution in there about any kind of glass fiber insulation. It performs according to the specifications, pretty much only in the lab. In the real world, when the going gets tough (very cold conditions, wet conditions) it can lose 1/3 to 1/2 of it's rated insulating power.

Here's a comparison of mineral wool and glass fiber insulation:


I personally really really prefer blown in loose cellulose, keeping in mind, it will settle 3-10% in a year. I just plan the construction so I can go back and top up the wall a year later, and I have great insulation, and cheap, and it words as advertised ESPECIALLY when conditions are tough.

Here's a comparison of those two products:


Have fun and ask questions!
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David, to answer you question since you posted it in "Natural Building" a good Natural Builder would use neither options. Below are the current long standing issues with polyurethane still under investigation by the EPA. EPS depending on what the manufacture is using for blowing agents and/or fire retardants will perform the same. As far as sustaining u-values for 50 years, I've seen SPF last less than 12( you can find in my blog in my signature about half ways down), when the chemicals break down and toxic outgas so does it usually around heat and moisture. The other danger both options present is since they are both zero perm vapor barriers drying cannot occur nor is promoted by surrounding layers. Natural Builders build foam and chemically free homes not sick ones.

I'm a net zero Passive House Architect just about ready to break ground here in the US but, I don't follow all the hype. I'm using high perm inert mineral wool interior batts and exterior board to thermally break my studs/trusses. I'm using BEOPT for simulation software, but I'm positive WUFI would fail your two options for microbial growth. I'm using blown/MW hybrid in my roof from Greenfiber which is intert, the settling issues are mitigated by installing it wet per mfg. If someone has not run that model for you they are selling you an unfounded sales pitch. I find it an interesting question which leads me to believe it is sales hype, since your simulation model should answer your questions and determine all energy and electric loads, and provide cost options. The insulation choice and placement depends on many variables nobody out here will be able to answer accurately. If some builder is telling you they can design to Pasivhaus standards (which is the standard used world wide and derived from Germany) then they should at least be using PHPP. In the US, we now have PHPP_US both of of which are embedded in WUFI.

I'd warn you against this trend of air sealing in toxic materials to ACH .5 US/.6 GPH without understanding the ventilation and air change requirements. As far as I know, ASHRAE 62.1 does not account for Indoor Air Qualify or reference toxic materials WUFI and BEOPT, etc has limited simulations for therefore it takes a trained eye to interpret outputs. The last EPA bullet addresses this concern, there is alot of controversy around those rates and sizing HRV/ERVs. The best way to avoid a sealed sick home is to use inert materials and understand the chemical and physical reactions they have to each other. I have taken a less layering is more approach. It starts with a knowledgeable Architect that has decades of experience with material science and models such as WUFI. You might want to talk to ASHRAE and WUFI about ASHRAE 160 and these foam material options under consideration. Besides those issues none of the foam options compare in compression and deflection to mineral wool boards for foundations.



Another good resource,


Hope that helps.

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David Taquet
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Thanks a lot. Actually after I posted this message last night, the architect sent me the first draft of the floor plans and told me it would be an option to substitute blown in rockwool for the blown in glass fiber, for a price which I don't know yet.

I think both companies use vapor barrier (sheets I guess). I need to check more on that too.

As far as the EPS vs polyurethane, I agree I am kind of iffy for both. I will push them further and see what other options they have. Being a foreigner, and having a slightly original design (as far as "regular" Japanese clients is concerned) they really want to build my house and use it as an open house to promote their companies. That should give me some leeway to negociate better materials for the same price.
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