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Old 02-23-2015, 12:13 PM   #1
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Roxul Insulation


I want to install R30 Roxul insulation in the ceiling of my garage. My joists are 6-3/4". Roxul is 7-1/4". Do I need to add 1/2" to the bottom of my joists to make up for this 1/2", or can I install 4'x8' foam board over the Roxul and compress the 1/2"?

-Paul
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Old 02-23-2015, 08:48 PM   #2
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in my limited experience with roxul (did most of my house with it). i would say you would be pushing it. though it may work, well enough.
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Old 02-24-2015, 07:05 AM   #3
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Okay... Thank you. I was hoping to get away with NOT having to install joist extensions. I thought maybe 1/2" might be pushing it, which is why I asked.

I'm not seeing 1/2"x2" boards. I do see 1"x2", which I can either cut down or install as is. What do ya'll recommend? Is it okay to install 1"x2", which would leave a small gap between the foam board and the bottom of the Roxul insulation. Or is it best to get everything even and hugged up against the foam board?

-Paul
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Old 02-24-2015, 08:17 PM   #4
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i was just thinking about it. i was using 3 1/2" roxul = fit into 2x4 cavities. yours is much thicker. so i would say you have a good chance of doing it. why don't you just try it and see what happens.
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Old 02-25-2015, 12:17 PM   #5
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Originally Posted by Fix'n it View Post
i was just thinking about it. i was using 3 1/2" roxul = fit into 2x4 cavities. yours is much thicker. so i would say you have a good chance of doing it. why don't you just try it and see what happens.
Ayuh,.... I wouldn't shim it,....

Just let the foam board panels float on it,....

Yer not usin' the space above this as storage or anything are ya,..??
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Old 02-26-2015, 07:08 AM   #6
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Ayuh,.... I wouldn't shim it,....

Just let the foam board panels float on it,....

Yer not usin' the space above this as storage or anything are ya,..??
The space above the garage is living space: three bedrooms, two bathrooms, and a hallway. All of these are like living in a refrigerator in the winter. The other half of the house is heated by a gas stove in the lower level. The heat radiates up through the floor and the stairwell. Celling fans whisk the heat around, and it heats the upstairs really well! The two bedrooms that we use, we heat at night with little space heaters, then turn them off during the day. This method is cheaper than running the furnace.

We set this up because using the furnace was so costly. Heating the house above the garage is sooooo expensive! Cold air blows for five minutes or so, then it turns to warm, never hot air. The heating ductwork in the garage is poorly insulated, and it's possible I might find it's not insulated at all, when I remove the sheetrock in the garage ceiling.

Having the Roxul hang down 1/2" below the joists concerned me as far as installing the foam board. The foam board would be 1". I'm not sure how rigid it will be to compress the Roxul down. I also don't know if compressing Roxul reduces its R value. I know with standard insulation, you don't want to mash it into its space. Fluff and stuff is the rule of thumb with Fiberglass. Is Roxul the same?

-Paul
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Old 02-26-2015, 10:17 AM   #7
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If you really had to you can shave the roxul down. Very easy to cut with a bread knife. I don't know if squishing the roxul would affect it more than cutting a little off. It might not rely on air like fiberglass batts do, but you should email them to make sure.
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Old 02-26-2015, 10:35 AM   #8
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Originally Posted by premington View Post
The space above the garage is living space: three bedrooms, two bathrooms, and a hallway. All of these are like living in a refrigerator in the winter. The other half of the house is heated by a gas stove in the lower level. The heat radiates up through the floor and the stairwell. Celling fans whisk the heat around, and it heats the upstairs really well! The two bedrooms that we use, we heat at night with little space heaters, then turn them off during the day. This method is cheaper than running the furnace.

We set this up because using the furnace was so costly. Heating the house above the garage is sooooo expensive! Cold air blows for five minutes or so, then it turns to warm, never hot air. The heating ductwork in the garage is poorly insulated, and it's possible I might find it's not insulated at all, when I remove the sheetrock in the garage ceiling.

Having the Roxul hang down 1/2" below the joists concerned me as far as installing the foam board. The foam board would be 1". I'm not sure how rigid it will be to compress the Roxul down. I also don't know if compressing Roxul reduces its R value. I know with standard insulation, you don't want to mash it into its space. Fluff and stuff is the rule of thumb with Fiberglass. Is Roxul the same?

-Paul
Ayuh,.... R-value comes from depth, not so much the material,....

Inches of complete dead air space, is the best insulation, short of a vacuum space, lackin' even air,....

Whether you trim, or compress the roxul, yer loosin' R-value,...

My minds picture, was lookin' at yer project differently, sorry,....

Now that I think I see it better,....

I still say, stuff the roxul into place, 'n cover it with yer foam board sheets,...
Screw 'em up, 'n go far enough to tighten against the roxul, but not compress it to the wood 2x's,...

There'll be a 1/2" to 3/4" of air space, 'tween the 2x's, 'n the foam board,...

Tape it off after, 'n it'll be just fine,...

Or you could use furin' strips to batten over the 2x's, for added Strength,...
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Old 02-26-2015, 12:02 PM   #9
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Yep, furring the joists would take no time at all.
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Old 02-27-2015, 08:07 AM   #10
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your not compressing much of anything with just the xps. AND you can't/shouldn't leave the xps expossed = fire hazard. put drywall over it. this will help to compress what little is allowed.
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Old 02-27-2015, 09:06 AM   #11
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You have too many problems with this whole scenario to even think about squishing a bit of roxul insulation.

Your floor joists for the living above the garage are less than 2 x 8s?
Maybe this is just a one car garage and only spans abut 12 max?

You are going to replace the Drywall correct? With 5/8" type "X" I hope?

No penetrations in the ceiling for light fixtures or anything like that right?

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Old 02-27-2015, 09:16 AM   #12
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Fix'n it: Yes... The idea is to install the XPS foam board, foil-side down, tape the seams, then install 5/8" fire-rated sheetrock over the foam board. I assume the Roxul and the taped foil on the foam board will provide dual vapor barriers.

I called Roxul and talked to a support rep. They do not recommend compressing Roxul at all under any circumstance. Roxul should be installed without and excess pressure on the material. Compressing it degrades the R value of the material and compromises its integrity over time.

123pugsy is correct: they recommend installing furring strips on the underside of the joists to make up for the 1/2" that Roxul extends below my joists. Anyone else reading this thread that is dealing with what I'm facing should do the same thing.
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Old 02-27-2015, 09:17 AM   #13
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Oooops... My first paragraph should have been directed to AndyGump, not Fix'n it... Sorry 'bout that! :-)
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Old 02-27-2015, 09:20 AM   #14
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May as well forget the foil backed stuff.

The foil needs about 1 1/2" of air space for it to be of any value.

Where are you located anyway?

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Old 02-27-2015, 12:27 PM   #15
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May as well forget the foil backed stuff.

The foil needs about 1 1/2" of air space for it to be of any value.

Where are you located anyway?

Andy.
Really? I didn't know that (the 1-1/2" foam board thickness). I thought the idea was to get a nonconductive surface against the joists and keep the conductive side (foil) off the joists. The extra 1/2" really makes that much of a difference?

I'm in COLD and blustery Rochester, NY. It's been wicked-cold here the last two to three weeks. Up to -35f wind chills at times!
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