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Old 12-30-2010, 07:17 AM   #1
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fireblocking and xps


alright, I am very confused about this... maybe someone explain this in detail as opposed to just "fireblock every 8ft vertical and 10ft horozontal"...

I am doing basement construction and the walls are insulated from the inside with XPS because the builder for some reason didnt do it on the outside... I've read just do the whole wall seal it up then frame a stud wall infront of it... then I also looked at insulpink, the XPS with slots in it for furring strips... which is still a continious sheet of XPS... but then after all that research and no one including building science saying anywhere during this people say make sure you fireblock it, but everyone is vague on how... just "fireblock it" well I know the top, thats easy put drywall to block the air flow... but people say firebock it every 10 ft left to right... and this is where no one really will explain it, some vaguely say to the concrete but no explination on why.... but then others say that defeats the purpous of putting in XPS then because its not a complete seal...

but then I was thinking how this also seems weird to fireblock to the concrete, because we have XPS sheething instead of plywood, how is that any different? it makes a complete insulation blanket around the house and is never interrupted with a fireblock or anything, if one end caught on fire wouldnt the whole thing also just go up since nothing is there to stop it?

I just need someone to actually in detail explain this to me, because vague answers isn't helping and contractors I've talked to around here look at me with a dazed and confused look when I ask about how to fireblock with XPS

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Old 12-30-2010, 08:17 AM   #2
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fireblocking and xps


I am not a professional and more importantly, I am not your professional

The vertical fire blocking deals with vertical walls with a continuous open cavity (think balloon framing). The vertical fire blocking is as simple as separating the cavity with either fire resistant foam or traditionally, a 2xN where N = depth of the studs to completely block the cavity.

For horizontal fireblocking, this *usually* means the plenum space in an open truss floor system and/or the separation between living units in a multi-unit dwelling. Some inspectors like to see it every so many joists in the are between a basement and the living area above.

I can see where some inspectors will consider horizontal fireblocking necessary if there is a continuous air gap behind your stud wall. A simple stud running completely from floor to ceiling should suffice as a horizontal fire block but the local AHJ always has final say.

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Old 12-30-2010, 11:06 AM   #3
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fireblocking and xps


Quote:
Originally Posted by muthian View Post
I am not a professional and more importantly, I am not your professional

The vertical fire blocking deals with vertical walls with a continuous open cavity (think balloon framing). The vertical fire blocking is as simple as separating the cavity with either fire resistant foam or traditionally, a 2xN where N = depth of the studs to completely block the cavity.

For horizontal fireblocking, this *usually* means the plenum space in an open truss floor system and/or the separation between living units in a multi-unit dwelling. Some inspectors like to see it every so many joists in the are between a basement and the living area above.

I can see where some inspectors will consider horizontal fireblocking necessary if there is a continuous air gap behind your stud wall. A simple stud running completely from floor to ceiling should suffice as a horizontal fire block but the local AHJ always has final say.
wouldn't wrapping your house in XPS though make your house more prone to completly going up in flames since there is nothing to stop it from spreading from sheet to sheet? that's the part I'm most confused about because some say you have to stop it some say that defeats the purpous of XPS insulation if you put a block between sheets
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Old 12-30-2010, 12:46 PM   #4
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fireblocking and xps


The purpose of fireblocking is to slow the spread of fire to allow people inside time to escape. On a large open area, nothing slows the spread of fire. The large open air plenum of open web trusses pretty much allows the fire to go whereever it wants as quickly as it wants endangering those people upstairs that much more quickly. This is why a fireblock such as drywall or other similar material is required every so many feet.

Outside the house, it doesn't matter too much

I am not saying their are not codes regarding outside material flammability but the vinyl siding over the XPS doesn't have that much of a difference in flash point 500F vs 630F on the two products on my house.

In regards to your basement project, I'd put a piece of 2x material behind the walls between rooms if someone was picky enough to care. Otherwise, I would run a continuous perimeter of XPS on the outside covered by a rated material. By the time the flames got to the XPS it will be a raging inferno anyway and other things inside your home have probably off-gassed enough to knock you out.

Last edited by muthian; 12-30-2010 at 12:51 PM. Reason: Added last paragraph
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Old 12-30-2010, 09:35 PM   #5
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fireblocking and xps


My county requires exactly the 8' vertical and 10' horizontal fire stops in a stud wall as you describe. This PDF has some clear pictures. Breaking up a space, such as above a suspended ceiling or in a truss space, is referred to as draft stop, a different problem. None of this will answer your question about integrating insulation, which I am also trying to understand.

www.fairfaxcounty.gov/dpwes/publications/basements/details.pdf

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