Replies

1. 
   Bill_King | Oct 05, 2001 01:01am | #1

   *
   Jim,

   I don't have an answer for you, but I have a similar situation. Can you site the reference about the rigid insulation gassing off over time?

   -Bill

1. 
   Gabe_Martel | Oct 05, 2001 01:11am | #2

   *Nothing wrong with the rigid insulation in the space between and behind the 2x4s. I wouldn't worry about the off gassing either. Like most products we use in construction, when properly installed is quiet safe.You will be installing drywall all around and over the room anyways to give it some degree of fire separation. With kids in the attic bedroom, spend extra on detection and maybe a residential sprinkler system. The enemy in most fires is the furnishings.The SM will be encapsulated behind the drywall, so not to worry.Gabe

1. 
   piffin_ | Oct 05, 2001 01:24am | #3

   *Agree with Gabe, once encapsulated, it's safe. By the time fire gets to melt it the kids are out or it's too late anyway. Provide them with egress windows/alternate routes regardless of insulation type.If you fall a little short of R-30, so what! Any thing is better than what you've got now.Other options: Use 1" Thermax over framing and fibreglass, behind the sheetrock. Use longer screws to put the SR up.

1. 
   Mongo_ | Oct 05, 2001 03:03am | #4

   *Jim, Here's what I'd do...First I'd pull down the 2x4's. Are you cussing yet?Then, one of two options...I'd either put R11 unfaced friction-fit FG batts in between your rafters, or if you have the money, my first choice would be to friction fit 2" pieces of rigid foil-faced polyisocyanate in between the rafters, the face flush with the bottoms of the rafters. Fit them tight to the rafters, but gap and foam the panel-to-panel butt joints (see below). The 2" polyiso will give you about R14.4.Regardless of whether you used FG or the polyiso between the rafters, I'd now attach another layer of 2" polyiso 4' by 8' sheets to the faces of the rafters. Attach them with 10d nails and caps. You only need a few to attach them, essentially you just want to tack them up, as the furring strips will lock them in. Don't tuck the sheets tight against each other, gap them by about 1/4-3/8th of an inch. As you lay each sheet, make a tick mark on the foil face to show the location of the rafter behind it. It'll help when you put up the furring strips. Once all the polyiso boards are all up, use those cans of spray insulating foam to fill the gaps between the boards. This will give you one heck of an air barrier.Now I'd attach furring strips. You can either use real furring strips, 2" ripped strips of ply...or any other 3/4" product. Attach them with 4" screws into the rafters, using the tick marks you made earlier for reference. You can still use your 2 by 4's, though the 2 by 4's would be overkill by now. Personally, I'd save them for another project. Your choice.Place any wiring before you drywall...then run the sheetrock.The FG and polyiso would give you around R27, the double polyiso wuld give you about R31.If you don't want to rip out the 2 by 4's, then I'd run the FG batts behind the 2 by's, friction fit between the rafters. Then run the polyiso between the 2 by's. It won't be as tight, but it'd do. I much prefer polyiso to a beadboard type of product.If you use a beadboard-type of product, don't sweat the off-gassing. As others have noted, if there is any, it'll be behind the drywall and will go to free atmosphere via the roof venting.Long post...tired fingers...my apologies.

1. 
   piffin_ | Oct 06, 2001 01:17am | #5

   *Good job M.Of course the very best is to get it all sprayed.That'll save wrestling four foot pieces up through a three foot stairwell. LOL

2. 
   Mongo_ | Oct 06, 2001 04:28am | #6

   *Best invention ever when it comes to working in attics......Boom trucks!Have a good weekend, Mongo

3. 
   Rob_Susz_ | Oct 07, 2001 06:37am | #7

   *Leave the furr-down intact, sheet it with foam as mongo prescribes, then densepack that baby with cellulose.-Rob

4. 
   Gabe_Martel | Oct 07, 2001 03:19pm | #8

   *...and sit back and watch your roof assembly rot.Gabe

5. 
   Steve_Zerby | Oct 07, 2001 03:54pm | #9

   *Would you care to explain why this would cause the roof assembly to rot, Gabe?Steve

6. 
   Rob_Susz_ | Oct 07, 2001 04:46pm | #10

   *while you are at it, tell me why mine is not rotting. I did foil faced on the inside, polyiso on the outside, no venting, and densepack in the rafter bays.I left a small part of my gable end UNINSULATED and OPEN to the interior for the last 3 years for observation. The OSB looked a little damp once, but a 2x4 never looked damp or had condensation. The moral of the story....R.5 will condense, but R2 will not.By the way, the indoor RH never goes above 45% in the winter.-Rob

7. 
   Gabe_Martel | Oct 07, 2001 05:08pm | #11

   *How would you know?Leaving a small openning only allows any moisture to dry before forming any droplets.It's what happens behind and inside the assembly that counts.Kinda knew I'd get a rise out of the FreddyL twins.Gabe

8. 
   Gabe_Martel | Oct 07, 2001 05:12pm | #12

   *I'll give you a hint. The house is 80 years old, how would it have been framed?Gabe

9. 
   Steve_Zerby | Oct 07, 2001 08:12pm | #13

   *Gabe,I'm not going to engage in cat and mouse with you. I just think if you are going to make statements like that you could elaborate on your position.To dismiss those of us who are looking for better ways to build as "FreddyL" twins is to be diminish your own credibility. Do yourself a favor and engage in real dialogue rather than trying to "get a rise" out of people.To that end:A balloon-framed house is all the more reason to go with a non-vented solution. A vented roof assembly stacked on top of a balloon frame will only add to an already considerable stack effect. You could probably put a turbine in each wall bay and power the whole house off of the air-flow through the stud bays. Making the roof assembly airitight would be a good start at slowing down the exhaust of some of that expensive hot air. Of course you can argue that you can do that and still vent the roof by simply creating an effective air barrier between the walls and the roof assembly via poly and sealants and fiberglass. The way it's usually done. Either way you've effectively cut off the air flow between the house and the roof assembly.Of course any house that you start to tighten up--whether through poly and sealants or through high-density insulation--had better not have excessive moisture to start with or you will indeed create problems.But why would tightening up the roof in either fashion lead to rot in a house with indoor moisture levels that are within a healthy range? Care to postulate?Steve

10. 
    Gabe_Martel | Oct 07, 2001 10:06pm | #14

    *Dense cel is not air tight by any stretch of the imagination...but it does slow down the flow enough to cause areas of accumulated moisture coming from as far away as a damp basement, bath and shower area and the kitchen and laundry rooms, in a balloon framed house.BTW don't think that I need any credibility from the FreddyL twins to state the obvious.Besides I'm doing a 13 million dollar building envelope as we write so....don't think I need any help from the FreddyL school of Weatherization.GabeGabe

11. 
    Steve_Zerby | Oct 07, 2001 11:14pm | #15

    *Gabe,Glad you are doing so well in your business. I guess I should feel honored that you are willing to spend your advice so freely on this board when others will pay you so handsomely for it. Someone is paying me way too little for way too much work ;-)First things first. The house must be free of major moisture generation problems before tightening of any type is attempted. At least we can agree that a damp basement in a balloon framed house is a prescription for failure in any case. But I'm firmly convinced that a low-perm air barrier is more fail-safe than a high-perm air barrier.You are correct that dense cells are not airtight. Neither will a sheet of poly poked full of screw holes and myriad other penetrations be. Both will allow some moisture migration via air movement. But the poly, due to its very low perm rating, will not allow for any drying via diffusion, thus creating a greater moisture trap in the long run.IMVHO.Steve

12. 
    Gabe_Martel | Oct 08, 2001 12:20am | #16

    *Poly is not a moisture trap unless your silly enough to have a layer on the inside as well as the outside.Foam is a vb so adding it to the outside and having the exterior walls also insulated and then adding a vb on the inside, whether with a poly or just applying paint over a good drywall job will sometimes do it, could be a trap.Gabe

13. 
    David_Doud | Oct 08, 2001 03:17am | #17

    *to touch on the original question, having done four of these ceilings for myself over the past 20 years, my most satisfactory room is one where where I used foil faced with air space on both sides; it's not the highest R-value, but it is the most comfortable room - the rooms where I packed FG, furred with foam between the furring, and drywalled, get hot - the ceiling warms during the day in the summer and remains a radiant heater at night - the foil faced with air space cools quickly, but is also comfortable in cold weather - with air above and below, moisture is not a worry - my unreferenced 2 cents, zone 5, 6000 hdd, no ac -

14. 
    Rob_Susz_ | Oct 09, 2001 04:55am | #18

    *Gabe, the amount of money you make does not make you an expert.I was recently doing diagnostics on a multi-million dollar home of a very large builder in our area. It tested 9300 cfm50, he had never heard of "air sealing."I guess you ought not listen to me any more, I don't make enough money, but I bet Lstiburek does.-Rob

15. 
    Gabe_Martel | Oct 09, 2001 11:45pm | #19

    *I never said it does.Good for you.Couldn't care less.Gabe

2. 
   Jim_Belvin | Oct 09, 2001 11:45pm | #20

   *
   I have an 80 yr. old house, and am upfitting bedrooms in the existing attic space, without "popping" up. I will have some sidewalls up to 5'-0" high, with areas of sloped ceiling, which will give the rooms sort of an "atticy" feel, which is all right, but I don't want to furr down the existing rafters any more than possible. I have scabbed 2x4's onto the existing 2x6 rafters in order to give me a flat/level surface to fasten the sheetrock to. This leaves me with 7-1/4" depth to add an R-30 insulation between the drywall and the underside of the roof deck. This depth does not allow me to add regular batt insulation and have room for air circulation between the top of the insulation & the roof deck.
   One idea I have is to use rigid insulation. I find out that 4" of rigid insulation will give me the R-30 that I am looking for, and have 3-1/4" of airspace above the insulation. My concern is that I understand that rigid insulation has harmful chemicals that might "gas off" from it as it ages, and in case of a fire, might give off deadly smoke if it burns. Since I will be putting my children up in the bedrooms,I am particularly concerned about this. I am looking for any feedback on this, or alternate ways to insulate this space.