# Ceiling insulation using polyiso board and FG batts



## ddsrph (Nov 23, 2013)

My original plan was to use only polyiso board but due to the very labor intensive process am considering a combination. Here is the situation so far.

House is in southern Tenn, new construction, with well ventilated attic using 
Perforated vinyl soffit and ridge vent. 

The roof trusses are 6/12 on 24 centers with lower part that forms ceiling a 2 by 6. The first thing I did was nailed two by fours on two foot centers at right angle to trusses on the attic side of the truss bottom then cut and filled these spaces with 1 1/2 inch poly ISO board.

Next I cut and laid two layers of 2 1/2 inch poly ISO with staggered joints on top of the 1 1/2 poly ISO. This now gave me 2 1/2 plus 2 1/2 plus 1 1/2 for total of 
6 1/2 inches of poly ISO on top of the truss 2 by 6 that forms the room ceiling.

The plan then was to fill the two by six cavity down to where the sheet rock would be for the ceiling with more cut poly ISO. 

Instead I am considering having a insulation company fill the two by six cavity with craft paper covered R 19 fiber glass insulation.

This would mean from Sheetrock ceiling going up six inches of fiberglass and on top of that 6 1/2 inches staggered joint polyiso board. ALL The Poly ISO Board Is
Covered with the black paper (No Foil)

I would appreciate any and all opinions.

Thanks
JM


----------



## ddsrph (Nov 23, 2013)

Please note the photo shows the center section which has only the 1 1/2 and one layer of 2 1/2 but shows the two by fours like I described above.


----------



## ddsrph (Nov 23, 2013)

ddsrph said:


> Please note the photo shows the center section which has only the 1 1/2 and one layer of 2 1/2 but shows the two by fours like I described above.


My main concern iis will the use of the fiberglass batts be more likely to cause condensation problems inside the insulation? The total R value will be about 58 with 19 coming from the fiberglass and 39 from the polyiso. I plan to make sure the craft paper on the fiber glass forms a well sealed vapor barrier.


----------



## Bud9051 (Nov 11, 2015)

Hi dd, I apologize if I sound frustrated, but I'm not doing well at following your explanation.

I see standard trusses with your polyiso approach going down the middle. Somewhere under there are 2x6 bottom chords where you have filled that cavity with r-19 fiberglass. Then, you have 6.5" of polyiso down the middle (from the picture) with a floor built on top of it.

Question, what is the floor for? 
Question, what are you installing for insulation on the sides of that center floor. I can see trusses with no insulation.

R-58 is a bit above the minimum required for TN, but only hurts the pocketbook.

In TN a vapor barrier is less important than air sealing. Are you installing any ac or heat ducts in that attic?

Bud


----------



## ddsrph (Nov 23, 2013)

Bud
The picture shows the center section which is similar to the two sides except on the sides no OSB and the sides have an extra layer of 2 1/2 inches of poly ISO.
The center section has only the 1 1/2 plus one layer of 2 1/2 inch poly ISO. The photo was taken early in the process and is just the beginning. I posted it just to show the two by fours running down at right angles to the trusses. My biggest worry is warm moist air will rise thru ceiling and condense in the insulation. I don't know if the craft paper covered R 19 fiberglass will be a help or a problem.
The house will be heated with heat pump but all ducts will be exposed inside the house about one foot below the ten foot ceiling. I am using commercial spiral ducts.
Thanks
JM


----------



## Bud9051 (Nov 11, 2015)

To prevent any condensation you need to look in both directions, up to the cold and down to the heat and moisture. Having installed your poly on the cold side you now need to be sure the bottom of that poly or any other framing remains above the dew point in case any moisture or moist air seeps up from below.

I don't know the specifications or covering of your polyiso, but I doubt that it is going to offer any drying potential towards the attic. Thus, if any moisture gets in the only place is can go is down. That presents the conflict, that to allow drying downward you also open the door for moisture to move upwards.

Unfortunately, I don't have a traditional solution for you. Perhaps some of the pros here can help.

Bud


----------



## ddsrph (Nov 23, 2013)

Bud
I could go with original plan and go polyiso all the way. That would mean filling the ceiling two by six cavity with two more layers of polyiso instead of the r19 Fiberglas. I had already done that on the two end bays as you can see in this photo. I got all the poly ISO very cheap and have a lot of it.


----------



## Bud9051 (Nov 11, 2015)

LOL, I could say "looks perfect" but who in the H*** am I? I have no specifications on that polyiso, open nature or closed. I suspect it was manufactured to be used on a roof or other commercial application, just a guess. Does it meet residential fire codes? Does it have any air quality issues? 

As a builder, I could not use a product I don't have complete knowledge of and one that has a long track record for being used as I intend. Home owners have the flexibility to reinvent the process and only the local inspector is there (maybe) to keep them on track. I could provide a 98% confident answer and you could get really burned by something in that last 2%. 

Check your local code authority, since they always have the final word anyway. Get their ok, it they say ok, in writing and keep that piece of paper. If they are real smart, they will want your design reviewed and signed by an appropriate engineer so the responsibility shifts to them.

I do wish you the best and want you to know I'm not trying to be a hard a** just to make your life difficult. I'm just not the correct person to give you the ok even if I had all of the specifications.

Bud


----------



## BigJim (Sep 2, 2008)

Hey Jim, I wish Keith would pop in, he is very knowledgeable about insulation. There are several fellows here who are real experts with insulation also. I am not much help, when I was in business I depended on my subs.


----------



## ddsrph (Nov 23, 2013)

I have decided to have all the two by six ceiling space done by insulation company with the craft faced R19. That will mean removing the one bay that has the polyiso. The polyiso on top of ceiling two by six is just cut and laid in place which can be easily removed at a later date if any problems arise. The above laid in polyiso is in three eight foot runs down the attic separated by areas of what will be only fiberglass. This could help any potential retained vapor problems. If looking across the width of house between any two trusses will be:

Vented soffit--R38 min fiberglass--eight foot width polyiso--2 foot gap filled by fiberglass-- 8 foot wide center section polyiso--2 foot gap filled by fiberglass--8 foot width poly ISO--R38 min fiberglass--vented soffit.

Thanks
JM


----------

