# Polystrene (DOW) Rigid Insulation board - fireproof



## BlueBSH (Oct 29, 2009)

I don't think that paint will give you the required firestop... XPS is flamable and will burn with toxic smoke... you do have to put something like drywall infront of it and encapsulate it basically... along with fireblocking ontop...

I just did a part of my basment and I had 1" XPS, half inch air gap, 2x4 stud wall with R-13 in the walls (unfaced) then drywalled...the fiberglass plus the drywall is a good firebarrier as fiberglass doesnt burn generally and the drywall gives you at least 15 minutes of firestoping or more depending on the thickness


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## AndrewF (Dec 29, 2008)

Thanks for the response.

The spray foam insulation I bought is fire rated, so I am covered there.

I could glue the drywall to the foam board, but if the basement ever had a water problem, I imagine it would be a nightmare to pull the drywall off and replace.

I'll proceed with my plan to stud out a wall in front of the insulation board.


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## Gary in WA (Mar 11, 2009)

Be sure to not leave a gap between the stud wall/foam board to degrade the insulation with a convective loop. http://oikos.com/library/insulating_framed_walls/index.html

http://www.buildingscience.com/docu...llation-of-cavity-insulation?searchterm=insul

Gary


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## BlueBSH (Oct 29, 2009)

GBR in WA said:


> Be sure to not leave a gap between the stud wall/foam board to degrade the insulation with a convective loop. http://oikos.com/library/insulating_framed_walls/index.html
> 
> http://www.buildingscience.com/docu...llation-of-cavity-insulation?searchterm=insul
> 
> Gary


no gap between the studs and foam board? *scratches head* I was told by a few people there has to be a gap to allow for drying... now I am confused.. since I haven't done the last 2/3rds of my basment yet, you are saying I should build the wood stud wall firm against the foam board? even if I stuff the cavities tightly full of R-13 unfaced?


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## Gary in WA (Mar 11, 2009)

“Performance of cavity insulations are degraded primarily by two factors:


*Absence* of insulation
Convective looping in *and around* the insulation
 The fist factor is a contributor to the second. *Gaps, voids, and incomplete* cavity fills leave spaces in which convective looping can occur.
An *omission *or degradation in any type of insulation will reduce the insulating performance of the overall assembly. 
One of the heat exchange mechanisms that cavity insulations are intended to control is convection. When convective looping is able to occur in *or around* the insulation, its function is subverted. Also, *air moving* through insulation because of wind or other pressures can also subvert its performance.” Sited above, previously. 
So you have an air leak under the p.t. bottom plate against the concrete floor because you omitted the poly sill sealer to act as a thermal/capillary break as well as an air barrier. Or interior air passing by the bottom edge/backside of drywall/sill plate (forgot to air seal the ADA way), or around the electrical box. With a gap behind the faming, the air can travel to the length of the wall, carrying your heated air away. F.g. batt* has* to be covered on all six sides to be effective. With the back of the f.g. against the f.b., you will be better to stop the air movement. Pages 37-51: http://www.buildingscienceconsultin...010-03-10_When_R-Value_Doesn t_Measure_Up.pdf

R-13 is *made for a 2x4* wall- 3-1/2”, how will that cover any gap in back without creating a gap in front for convective loop? Granted, the foam board (tempering the temp. inside the wall) should stop a lot of the loops along with the added density of the f.g., and the ADA will prevent moving air leaks.. If gaps are left *anywhere*, you will have them….. basic science- heated air will rise, cooler air will sink. The stud *wall is required* to be fire-stopped every 10 lineal feet, as per minimum safety code, to prevent a fire at that outlet from surfacing clear at the other end of the house in less than a minute.

Gary


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## AndrewF (Dec 29, 2008)

I am not putting any batt insulation in the wall cavities. 

According to the building science reports, if I do 2" insulation in this climate, batt insulation is optional.

The foam board is giving me R10.

The other reason I am not doing batt insulation is that if it were ever to get wet or there was moisture back there somehow, it can mildew/mold.

So, in my case, I have the 2" foam board glued to the wall, and then am building a 2x4 stud wall directly in front of it and then putting the green board on it. I decided on building the 2x4 wall instead of other methods because it would make it easier to replace any drywall should the basement ever flood, even after I dont live here anymore.


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## AndrewF (Dec 29, 2008)

Here are some pictures so far.


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## Rita2 (Sep 4, 2010)

AndrewF said:


> Here are some pictures so far.


Hi all!
bumping this thread to ask if this method is ok. I am having hard time
understanding all the insulation guidelines but I though that the dead space that will be created when the drywall is attached to the studs will take away any R-value? I am asking because thinking of doing same thing
to me garage only thing is it's an unfinished garage and I would just
be cutting the rigid foam to fit then tuck taping or sealing edges with foam? I live in the dry desert, Nevada and just checking here before I
get started..thanks for all help.


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