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325_man 12-06-2012 07:19 PM

Fire Blocking Basement
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I am beginning to frame my basement. I'm planning to do the ceiling level fireblocking first, before starting to frame.

The builder for our home installed blanket insulation on the foundation walls of the basement. I am planning to push the framed walls up against the blanket insulation, which I think it will take care of the horizontal fire blocking requirements.

I attached two different options of fire blocking.

Which one of the two options is the best way to fire block?
I am thinking to use sheetrock vs plywood. Is one material better than other for fire blocking? Is there a required thickness on the fire blocking material?
Any input are appreciated.

Background Info:
Location is Prince William County, VA
Project is an unfinished basement framing, single family home
Framing for bedroom, 1 full bath, and entrainment room.
Home is 7 years old, foundation walls already have blanket insulation attached


TheCamper 12-06-2012 07:49 PM

The method in either diagram will work, I have done both. Usually the framer takes care in setting the sill plates as it makes the rest of the frame easier to keep straight and square. So when you place your fireblocking tight up against the sill plate and frame your wall to the fire blocking material, whether it be that your top plate butts up to it or your inner edge of your top plate is flush with the inner edge of the fireblock, it helps you keep a straight and square frame. I don't believe that there is a required thickness but the material needs to function as intended e.g. dimensional lumber or plywood no less than 1/2". Check with your local Code official about keeping the batt in place. I think that if it is a rock wool he should say yes and if it is fiberglass he should say no, as fiberglass is not a satisfactory fireblock. The main purpose of the fireblock is to prevent the vertical to horizontal movement of a fire in the wall. Check with your local code official about if the fireblock every 10' of wall is required. Some believe it is better to have some air movement for ventilation. good luck.

Trucon01 12-06-2012 08:14 PM

Where in PW county? I'm in Manassas

325_man 12-07-2012 07:24 AM


Originally Posted by Trucon01
Where in PW county? I'm in Manassas

I'm in Manassas too.

325_man 12-07-2012 08:14 AM

Remove the Insulation?
Do you think I should remove the insulation, look for cracks, and repair them, before I frame the wall?

I am hoping to not disturb or to reuse the existing insulation.

jonhayden 12-07-2012 02:17 PM

I'm in the same situation you are right now. I'm about to start framing and my town gave me the exact same image you have. They also said that the blanket insulation DOES serve as the wall blocking. But I was still confused on how to do the top. I finally found the following image. Since I have to install wood blocking to attach my walls too, I figure I'll also attach drywall pieces on top of the wood blocking.

Gary in WA 12-07-2012 06:51 PM

Fiberglass batt or mineral wool batt is acceptable as fire-stop, though I would use solid wood or drywall= not air permeable;

I would not risk using batts without a layer of air-tight foam board first;

Leaving a gap for air movement is not suggested as the concrete wall will get wet (follows the room's RH) unless covered air-tight =f.b.;

Excellent how-to guide from a fellow moderator:

Batts with a poly facer or sheet poly did very poorly, especially above grade; pp. 8, 26-29, 47-51, and a summary on page 69, case #2 and #3;


325_man 12-08-2012 06:49 AM


Thanks for all the input, I really appreciate them all!

Since it is a code requirement in Prince William county ,VA that all lumber in contact with masonry, i will leave the existing insulation blanket on.
I will however cut open areas with cracks, seal them, then place back the insulation blanket. I do have a patio on one full side of the house up against the house foundation which goes out 15'. I'm wondering if fixing the cracks on that side of the house is necessary.

I will use 1/2" gypsum board (a.k.a. Sheetrock) for my fireblocking. The reason being sheetrock is less expensive than plywood. Also, it would blend nicely (thickness wise) with the ceiling sheetrock when it is time to board everything up.

Please do chime in if there is any flaw on my plan.


Gary in WA 12-08-2012 09:26 PM

So you will framing a wood stud wall against the insulation blanket draped on the concrete wall by the builder to pass minimum energy code?

Did you get a chance to read pages 6-11 of the second link I posted earlier?


325_man 12-09-2012 07:30 PM


Originally Posted by GBR in WA (Post 1069225)
So you will framing a wood stud wall against the insulation blanket draped on the concrete wall by the builder to pass minimum energy code?

Did you get a chance to read pages 6-11 of the second link I posted earlier?


Thanks for pointing this out, Gary!

It states that the builders' insulation blankets are "unsuitable for use by the home building industry due to serious problems associated with mold, decay and odors."

While I am not disputing what the paper says, I am pondering how many people found that the statement is factual (i.e. found mold,decay, and odors by using builders' blanket insulation). My concern is that the statement is made by companies who made these insulation.

How about if I puncture enough holes on the blanket, would this make the my existing insulation breathable, hence becomes acceptable? How about if I remove the bottom half of the existing insulation like their acceptable solution (e.g. half wall insulation) and puncture holes on the upper half, would it be better?

My understanding is that there are two 2 options:
  • Half Wall Insulation with Fire-rated Foam Sheathing
  • Full Wall Insulation with Foam Sheathing Covered with Gypsum Board

I am still puzzled with how the diy-ers install their proposed options.

325_man 12-10-2012 07:06 PM

I apologize for sounding to negative on my previous message.

I look into Pactiv 2-in x 8-ft x 4-ft Extruded Polystyrene Insulated Sheathing at Lowes for $30.86. It has an R value of 10.0

Let me know what you think.


Gary in WA 12-11-2012 01:33 PM

First, I didn't think you sounded negative. I give short answers because I detest typing, lol. Easier for me to state a fact and leave a link rather orate about the answer. Very hard to read facial expressions on a forum, other than the obvious sticky faces in "advanced reply". No harm, no foul.

Simply slashing a vapor barrier does nothing, an air-barrier- yes!;

Problem is letting indoor humidity get to the cold basement concrete wall to condense;
Sounding repetitive, I know....

Many AHJ accept that foamboard, check locally with yours as they have the final say for your safety. Just don't allow air behind the board, even from the adhesive pattern, it's that important;

I have other links on f.g. w.o. f.b. if needed but using foamboard will also raise the dew-point of the wood frame/insulation cavity to prevent rot in the frost depth and above-grade portions of the basement. Minimum code energy for your Zone 4;

R-10/13, footnote "c"; That will work, mastic/tape the joints and canned foam the top/bottoms.


spring3100 12-11-2012 08:00 PM

Save yourself the time of cutting that wood and cut strips of ROXUL Safe and sound mineral wool insulating,easy to cut with a sheetrock saw,you could put a flamethower to it and it wont burn.

Gary in WA 12-11-2012 10:14 PM

Yep, Roxul is good stuff, not as air-permeable as fiberglass, though both will let indoor vapor through to condense on the concrete wall (hence the f.b.)
Use ff PIC and Roxul, win-win.


325_man 12-22-2012 12:58 PM


Originally Posted by GBR in WA (Post 1071430)
Use ff PIC and Roxul, win-win.

Gary, sorry - I am not familiar with 'ff PIC' abbreviation. Can you please clarify? Thx!

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