How to insulate to R49 attic truss?

[ELCouz]

Hello,

My attic truss are made of 2x4 on the roof support. How can I install R49 batts on a 2x4?
It is 14 inch thick and I am wondering how to hang this... Do I really need to expand the 2x4 to 2x12 with wood or there is another way?

I will install roof baffle as well to let air flow from the soffit to the roof vent.

See attached image

Thanks a lot!
PS: I am in Quebec, Canada it gets cold

 

[The Cobbler]

basically you just need something to hold the insulation. and I would insulate the knee wall and the floor joists rather than the trusses at that point. just build out the walkable attic area .
do you plan to use the attic? does it need to be insulated or can you insulate the floor & leave the attic uninsulated space?

 

[billconner]

The 14" batts between trusses is not ideal because of wind washing. Cold air will circulate in the 1 1/2" gap between them and reduce insulation. Loose fill - I prefer cellulose - would do a much better job at the top.

I'd also recommend keeping insulation at roof deck. By coming down kneewall and across floor, more area, more heat loss, more money. Especially recommended I'd this space has any ducts in it or accessible storage.

The sloped section is tough. I'd look at rigid foam which is about highest R and gives you the baffle automatically.

 

[cmalinowski]

The sloped section is tough. I'd look at rigid foam which is about highest R and gives you the baffle automatically.

Sorry to hijack, but I'm in just about the same boat. Slightly larger (28ft wide). The knee wall and the ceiling I should be able to insulate just fine with 24" batts. I don't need R49 which helps. I'm in Northern NY and will probably go with just R19. I don't plan on insulating the sloped roof area behind the knee wall. I may do something to insulate in that area, but as of now, don't plan on it.

I plan on putting in rafter baffles from knee wall to roof on that sloped section. So, that makes it all that much worse to insulate there. Thinking that I may have to just extend those slopes with additional 2x4 (20 joists, so 40 2x4s ) and spray foam that sloped area.

I plan on mine being usable space for people to be in, like an apartment.

Do I really need to expand the 2x4 to 2x12 with wood or there is another way?

I would think on the sloped section you may need to do that for sure.

Such a tough call on the knee wall and ceiling, but slightly easier. Could put in a batts and fill those 1.5" gaps with additional batting "scrap" or 2" rigid foam board. I guess you could also do thinner batts that fit 2/4 and then stack thicker batts or blown in fill (at least for the ceiling section) on top. Of course, could just do blown in fill in the ceiling area completely. Then that just leaves the knee wall as the biggest problem. Could just do rigid foam board behind it. Will **** putting in that thick, and would be spendy.

But I'm feeling your pain for best way to do it.

 

[thammel]

I have attic trusses and fully insulated the attic room. I stapled poly insulation baffles all along the angled section of the ceiling for airflow up to the ridge vents from the soffits. I used r38 in the ceiling and r19 in the walls. The 28x32 x12 garage was built using 2x6 studs so I have r19 in the walls. I did all the insulating and sealed all gaps and holes and taped all insulation cuts. The garage is much better insulated than my 2000 build house!

 

[billconner]

I have attic trusses and fully insulated the attic room. I stapled poly insulation baffles all along the angled section of the ceiling for airflow up to the ridge vents from the soffits. I used r38 in the ceiling and r19 in the walls. The 28x32 x12 garage was built using 2x6 studs so I have r19 in the walls. I did all the insulating and sealed all gaps and holes and taped all insulation cuts. The garage is much better insulated than my 2000 build house!

Did the sloped part get ceiling or wall insulation?

 

[ELCouz]

basically you just need something to hold the insulation. and I would insulate the knee wall and the floor joists rather than the trusses at that point. just build out the walkable attic area .
do you plan to use the attic? does it need to be insulated or can you insulate the floor & leave the attic uninsulated space?

I need to maximize attic space since this will be an office, I can't leave it uninsulated.

The 14" batts between trusses is not ideal because of wind washing. Cold air will circulate in the 1 1/2" gap between them and reduce insulation. Loose fill - I prefer cellulose - would do a much better job at the top.

I'd also recommend keeping insulation at roof deck. By coming down kneewall and across floor, more area, more heat loss, more money. Especially recommended I'd this space has any ducts in it or accessible storage.

The sloped section is tough. I'd look at rigid foam which is about highest R and gives you the baffle automatically.

How do you fill the angle section with cellulose? I mean I only have 2x4 as something to staple on a barrier.
I am trying to find out how to hold 14" inch thick insulation on 2x4 rafters without doubling them with 2x12. The cost increase would be insane just to hold insulation let alone the weight increase on the trusses.

EDIT: I am thinking to double the 2x4 rafters to 2x8 total and go to R30 instead. Doesn't met new home building code but meh .... doesn't apply to garages.

 

[billconner]

I may have misunderstood some of the details. I was suggesting cellulose for above the highest horizontal ceiling.

I don't understand how you are getting 14" batts on the sloped ceiling section. They look like 2x6? Or what are you attaching a ceiling to if there are 14" Batts there?

And I misunderstood around kneewall, you are keeping insulation at roofline. Good.

If basically you want deeper roof rafters, some plywood gussets - maybe 6 x 16 x 1/4 - suspending 2x3s or 2x2s - would give you something to hold the insulation.

 

[cmalinowski]

I am trying to find out how to hold 14" inch thick insulation on 2x4 rafters without doubling them with 2x12. The cost increase would be insane just to hold insulation let alone the weight increase on the trusses.

Maybe use 2x4s instead of 2x12s and just space them down further. There would be an ~8" air gap between the 2x4s, but it would be "closed off" helping with wind washing in that 1.5" gap. You could stuff them with scrap to help.

Or you could **** the 2x4s together and just spray closed cell foam in that sloped section. I read that closed cell spray foam is R6-R7 per inch.

Apologies if what I'm suggesting is silly. I'm ignorant on this stuff and really just tossing out ideas because I have the same situation and really need to figure out a solution as well. Not quite as cold a climate as yours, so I don't need it that thick, but that sloped section really has me perplexed.

 

[thammel]

Did the sloped part get ceiling or wall insulation?

That area wasn't thick enough for ceiling so I used wall insulation.

 

[ELCouz]

Thanks guy for your feedback... I've been brainstorming a lot of scenarios in the last days.
I think my best so far would be to ignore the baffle and do my own using 2x2, 2" rigid foam R10 over it , then put 5.5" R20 batts and finally a polyethylene vapor barrier over the batts. This way I know the gap won't be crushed by the batts and save cost to skip the premade baffles.

To recap:
-2x4 rafters doubled with a 2x6 total depth would be 9".
-2x4 cut in two give me 1.75"x1.5". I will use the short side as spacers for the air between roof and rigid foam. 1.5" air gap is sufficient for a 8/12 roof slope.

-remaining depth is now 7.5"
-install R10 2" thick rigid foam, remaining depth is now 5.5"
-install R20 batts for 2x6 walls remaining depth is 0"
-install polyethylene barrier over everything.

This will give me R30 total with a nice air 1.5" gap between roof and insulation. I checked code and R28 is minimum for cathedral roof.

What do you think about this plan?

 

[ELCouz]

I may have misunderstood some of the details. I was suggesting cellulose for above the highest horizontal ceiling.

I don't understand how you are getting 14" batts on the sloped ceiling section. They look like 2x6? Or what are you attaching a ceiling to if there are 14" Batts there?

And I misunderstood around kneewall, you are keeping insulation at roofline. Good.

If basically you want deeper roof rafters, some plywood gussets - maybe 6 x 16 x 1/4 - suspending 2x3s or 2x2s - would give you something to hold the insulation.

Sorry I may also been not clear as well, I'm not an expert in insulation.
For the horizontal section it make sense to use cellulose. Thanks for that idea.

See my previous message, 14" inch wont be necessary since I was confused that R49 to R60 insulation code is for ceiling and in my case it's more like a cathedral roof application with insulation in rafters.

I learned a lot in the past day.

Apologies if what I'm suggesting is silly. I'm ignorant on this stuff and really just tossing out ideas because I have the same situation and really need to figure out a solution as well. Not quite as cold a climate as yours, so I don't need it that thick, but that sloped section really has me perplexed.

Search for cathedral roof insulation, this is what I found out. This is our scenario.

 

[cannuck]

To do this in fiberglass is a complete waste of time and money. R values for glass batts are a mere pipe dream - in as-built conditions no where near R3/inch. To get your target R50 the thinnest you can get would be 8" of closed cell foam - best done with board. By just doubling up your 2x4s you are making a thermal bridge that will once again defeat the entire purpose. To do walls with glass batts you need to build staggered stud arrangements of 2 x 6 offset half spacing to get a theoretic R38 - maybe R 30 for real.

Are the roof trusses already built?? cuts you options dramatically. What are the rafter dimensions to allow for the 2" air gap? If 2 x 6 you could fit rigid Urethane board in 3 1/2" between studs I would run a set of 2 x 4 boards at 90 degrees minimizing your thermal bridging and board that layer with 3 1/2 rigid PU board (NOT styrene, nowhere near the insulation and incredibly flammable). This would give you 7" that in fact would approach mid R 40s range. If you want to actually hit R50 do the second layer in 2 x 6 with 5.5" polyisocyanate rigid board (with foil face on inner surface). You can do that with parallel second layer studs as well, but as with glass, offset them between the outer layer to break the thermal bridge.

I have been struggling with this question for my new on-farm shop - that I want an R50 roof on. I want to go with MIP (metal insulating panels) but the thickest anyone makes is 6" where I really need 8. My second option is go with a built up roof with 8" PU board. Friend of mine did that and pays less for heat for double the floor space of his older building with 4" PU.