# Rigid XPS to block wall



## bluefoxicy (Nov 5, 2012)

I want to insulate a block wall, formerly a firewall but the house there will soon vanish. The city has demolition scheduled for early 2014. That wall gets ridiculously cold inside, and I want to apply rigid XPS to the exterior.

Problem: XPS typically attaches with a nail pattern of 6 inches.

Not into block.

I am curious if there are established design patterns for this. Adhesive? Fasteners into the wall?

I want to do a 1 inch R5 XPS layer (R6.8 ISO costs twice as much), offset 50% and place a second 1 inch, then place 1 inch fir stripping, radiant barrier, 1 inch fir stripping on top the first set, then attach siding. I imagine somewhere along the way I need to drill through the XPS into the block and attach some kind of scaffolding system to hold the fir stripping and siding in place.

The radiant barrier will be big in the summer. In the winter it will be negligible, since it's outside and most of the heat migrating through will have to migrate back through the insulation to return to the block; the bleeding around the edges will be faster, and the material itself will probably heat up faster and pass heat effectively. A radiant barrier on the inside would be huge in the winter (in contact with 1 inch of spray foam covering the wall), but require a 2 inch false wall in the most inconvenient place possible. I'm relying on the XPS in the winter.

The wall is an entire side of the house and it will exist at exterior temperatures (or MUCH hotter in the summer) until I somehow attach XPS to it effectively. It will also likely be stucco coated, as the city will probably treat the wall when they remove the house; it may simply have cracks repaired after demolishing the house next to me.

Two layers of XPS, a radiant barrier, and siding is simply the best way to do this. It will provide R10, plus in the summer the sun will shine directly on siding placed in front of a radiant barrier--this is huge. In the winter, the wall is essentially R10 in this configuration; the exterior radiant barrier is useless.

If it's not enough in the winter, I can look into an additional R7 per inch from spray foam on the interior. That of course raises questions: A radiant barrier would be effective here in front of 1 inch spray foam, since the 5/8 drywall would be backed by a hollow cavity with no ventilation, plumbing, electric, etc, and thus the drywall would primarily radiate heat at the radiant barrier. On the other hand, it's not particularly a lot of heat; if conduction across the 1 inch air gap to the radiant barrier is bigger than radiation, then just a 2 inch fill of spray foam would provide better winter insulation. Conduction from envelope air flowing behind the wall would give advantage to 2 inches of spray foam; but that will NOT happen, so the drywall may actually radiate more heat than is conducted to the barrier (and then radiated across air in bubble wrap, reflected from another radiant barrier, and conducted back across...).

Either winter configuration is vastly inferior to the summer configuration in the summer (i.e. R14 from the spray foam is less than R10 + radiant barrier outside, and putting the radiant barrier inside is going to heat the block wall and carry a lot more heat by conduction).

I'll probably need to use both strategies together to achieve fully effective insulation, leveraging some R17 plus a very carefully planned manipulation of the thermal exchange the wall is subject to and the localized envelope to get something around R30-R50 performance depending on conditions. It will always perform poorly in the winter compared to the summer.

What a mess. The big problem is I don't know how to physically get all the material attached to the exterior of a block wall.


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## Windows on Wash (Aug 30, 2011)

Where is "here"?

You can attached foam to a block wall with screws and plates and then have your vented wall design with the furring strips over top of the foam.

Depending on where you are and what type of siding you are proposing, the radiant barrier is probably overkill.

Foam to the inside is fine but why not just use more rigid foam or stick frame out your wall and use batts. 

By the time you get and R-10 into the wall and the block, a 2x4 interior wall with R-13 batting will work quite well and better than the insulation values indicated because of the thermal mass of the wall and rigid foam.


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## bluefoxicy (Nov 5, 2012)

Here is Baltimore. Last summer I was taking in 4kW of heat in one room; I had a 12000BTU air conditioner in a 200sqft room and the room would just raise temperature 10 degrees during the day anyway, got a $500 electric bill.

The radiant barrier isn't so much "overkill" as it is application-specific: that wall is sun-facing. We've had 106F summers and -10C winters (not sure where that is on the F scale).

2x4 + R13 might work. The wall is coming up a stairwell abutted against the block, so any structural wall there is severe. Also, at the front door, 4 inches would be over the door and it wouldn't open; I have 2.5 inches to work in there. That's why I want to avoid it, or only do a 2 inch depth wall by attaching two fur strips. (Fur? Fir? Isn't it a tree?)

I'm trying to maximize performance in the winter and summer with minimal space removal in the interior. Interior radiant barrier is an uncertainty: two inches R14 is well-understood, while a radiant barrier in front of one inch R7 is going to perform based on a lot of factors. If the drywall heats up to house temperatures and mainly radiates heat at the radiant barrier, it'll perform better; the big question is if it'll perform better than just an extra R7 of insulation there. Of course, if the drywall wasn't there, air flowing across the radiant barrier would conduct and be replaced with convecting warm room air, and performance would be terrible. Radiant barriers are VERY application-specific.

Foam block to the wall with screws and plates. Okay, That's a start. The R-10 XPS and siding are the most important part of this.


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## Windows on Wash (Aug 30, 2011)

That certainly is a bunch of juice to cool a small room.

How well is the room sealed up?

If you have more space, additional rigid foam to the exterior can help and then you can just run the rigid foam, furring strip, and drywall combo to the inside.

If you want the radiant barrier, you can add one to the exterior wall but as you noted, you will need an air space and some venting for convection on either side. 

If you are using vinyl siding, there is plenty of air movement behind that to make it work.


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## bluefoxicy (Nov 5, 2012)

That room heats up in the floor. Judging by the burned down house next to mine, I can seal a major draft into the floor if I rip the ceiling out of the porch and apply spray foam. Spray foam that entire wall, radiant envelope that segment of the roof, (it's on the side of the house facing the sun all day), stuff with stone wool batting, and it'll be ready to enclose the porch should I ever choose. That room in particular is a major source of heat ingress in the summer.

The block wall is the big offender in the winter. One entire side of my house is a big cooling panel. You're probably right about the radiant barrier being less than useful there; downward heat loss is where radiant barriers work well in the winter, along with the roof or ceiling in the summer.


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## Windows on Wash (Aug 30, 2011)

Sounds like you have a plan.

Post up some progress pictures.

WoW


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## Gary in WA (Mar 11, 2009)

IMO, radiant is NA for your location, just add more insulation. You will only heat up the vinyl from both sides with it.... You know the vinyl is slot-punched" to allow for thermal expansion from the sun (on only one side); "
*Q: How do I fasten vinyl siding to a wall?* 
*A: Choose aluminum, galvanized steel, or other corrosion-resistant nails, staples, or screws. Aluminum trim pieces require aluminum or stainless steel fasteners. Remember, as temperatures change, vinyl siding can expand and contract ½" (12.7mm) or more over a 12'6" (3.81m) length." *From; http://www.vinylsiding.org/installation/faqs/


Add most of the insulation outside to control the convective looping in the block cavities. R-10 for mass wall:http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Maryland Add wood furring inside for a wiring raceway (fire-block every 10' and top/bottom) with no insulation so the HVAC controls the RH; http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/Master%20Architect%20Binder/Technical%20Bulletins%20and%20Case%20Studies/Commercial%20Technical%20Bulletins/Tyvek%20with%20Exterior%20Insulation%20Building%20Science%20Bulletin.pdf


The City applied vapor barrier on the block is fine, the fb on the exterior will be your drainage plane/air barrier/vapor barrier, *fig.1*; http://www.buildingscience.com/docu...rol-for-new-residential-buildings?full_view=1


That is a lot of insulation you have planned.... after R-20- (95-96% stopping heat loss) the gains are minimal vs. cost; http://www.google.com/url?sa=t&rct=...VjgAkW8XzKgQkGg&bvm=bv.44442042,d.cGE&cad=rja

Gary


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## bluefoxicy (Nov 5, 2012)

Gary in WA said:


> That is a lot of insulation you have planned.... after R-20- (95-96% stopping heat loss) the gains are minimal vs. cost; http://www.google.com/url?sa=t&rct=...VjgAkW8XzKgQkGg&bvm=bv.44442042,d.cGE&cad=rja
> 
> Gary


The EPA now says attics must have a minimum of R50 and homes must be insulated to R30. o.o


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## Gary in WA (Mar 11, 2009)

My whole post addressed the wall. If you start adding fb and insulation to both sides of a wall, you better know the dew-point location or paint may peel as it can't dry out. You may also have minimal capillary draw from the footing/CMU to add moisture to the cavities.... though a big IF.And the wood studs will hold some moisture. 

Page 39- code required for mass wall = R-8? http://www.google.com/url?sa=t&rct=...iE_taktUeCDYytA&bvm=bv.58187178,d.cGU&cad=rja

I had a hard time understanding the attics/roofs mentioned so I wasn't commenting on that. I was using the 2012 IECC your State is under; http://www.reedconstructiondata.com/building-codes/maryland/
not the EPA.

Gary


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## bluefoxicy (Nov 5, 2012)

Interesting. I think everyone is quoting the insulation business quoting the EPA; a lot of Googling turns up "EPA SAYS YOU MUST HAVE HUGE R VALUE!!!!" these days.

Foam panels on one side should be sufficient is what I'm getting from this discussion.


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## Gary in WA (Mar 11, 2009)

Call your local AHJ and ask them.... I'm not a betting man, but.... lol. Good thing you came here. You need to meet minimum code. They sell insulation?

Per your location (per code), you should use a foam thick enough to prevent any interior condensation on the blocks; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements Check this in the other link I gave...

Cross-reference code required; http://austin-green-builder.com/2012-international-residential-code-irc-changes/

If they don't vapor/air seal the block you may need to mastic/mesh tape the fb joints rather than use tape that could leak as the fb shrinks; http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks

ADA the drywall to help meet code; http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_par052.htm

http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Foam board the rims, if any; http://www.diychatroom.com/f103/how-insulate-rim-joists-simply-190336/

Gary


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## Gary in WA (Mar 11, 2009)

Questions? Sometime my answers are not quite clear.... so what is the plan for the wall and pictures would help me understand the ceiling better.

Gary


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