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jefsboys 03-02-2012 08:54 PM

Insulate Attic. No soffit vents. Roof vents yes.
I put a very general multiple level question in remodeling.
I am going to post the insulation part here in hopes I can get some feedback.

Recently purchased 1904 house. It has a hip style roof. The floor in the attic is rough 2X8's on 16" centers with blown insulation the depth of the 2X8. It has 1X4 T&G hardwood floors.
There are no soffit vents. The roof has 4 shell vents, one on each portion of the roof.
The rafters are 2X6.
I am fine with extending the 2X6's to a greater depth for more insulation.

After a couple/few days of reading numerous articles it appears I may have my work cut out.

I have had an architect by and structurally I am good to do this. The peak of the ceiling is almost 13' feet high.

I am going to frame out just 1/4 of the attic over a bedroom. This portion of the attic has a dormer in it already.

My reading has led me to possibly using rigid foam as the middle option on costs.

This attic really isn't "vented" as their are no soffit vents. The soffits hang over past the side of the house by 2'. The soffits when opened up will go right into the attic.

I will be putting plaster ceiling up in here. 2 coat plaster over board.

I have numerous things running through my head from all the reading. As you know when researching it can be a challenge to find a scenario that matches your own....
With that in mind I am to present my situation in hopes I can get some focused input.

As I see it a few of the things that need addressed....
The venting.
The type and depth of insulation on ceiling.
The type of depth of insulation on walls that will divide this conditioned portion of the attic to the rest of the attic.

Here are a few pics....
By jefsboys at 2012-03-02
By jefsboys at 2012-03-02

jefsboys 03-05-2012 02:36 PM

Here is what I am considering doing. I actually think that due to the old installation of the cedar shingles this is in my favor and may work ok.

The old roof that was attached to the rafters only had 1 X 6's perpendicular to the rafters and then shingles on top of those. The new roof simply had plywood nailed on top of the old cedar shingles.
This left 2" gaps between each of the 1 X 6's which create cross ventilation between rafter bays, so now we have 2" long by 3/4" deep cross vents. This is very good since the jack rafters need ventilation to rafter bays that extend all the way down to soffit vents and up to upper vents.

Using 1/300 for vent requirement.....
The attic is 900 sq. ft requiring 3 sq. ft of venting, if my research and math is correct. I currently have 1.4 sq. ft of roof venting from 4 slant fin vents, one on each side of this hip roof.

Again, if my math is correct I am shooting for at least 1.6 sq. ft. of soffit intake to get the 3 sq. ft. and from my reading 50/50 is recommended but to have a little more on the intake is better than the other way for good reasons.
Will be adding soffit venting to each rafter bay. I currently have 1X3 T&G soffits, so I will be able to remove a section and install the vents.

Furring out the existing 2X6 rafters with another 2X6.
Considering making my own rafter baffle spacers.
Put 2" rigid foam between rafters and against spacers to create vent channel.
9" of insulation.
3/4 rigid foam on ends of rafters to prevent bridging.
Wall board on top.

The R value of the insulation will be R51 based on research for the materials I can get.

Any thoughts and or input would be greatly appreciated.

Please shoot holes in this where necessary.

shazapple 03-07-2012 06:08 PM

Looks like you are on the right track. Make sure you are calculating the Net Free Vent Area and not just the area of your vents (you can get this from the manufacturers website). I would go for continuous soffit venting, as any other type doesn't give very much air flow. I might also consider switching the 3/4" rigid and the 2" rigid, as too thick of XPS will act as a vapour barrier.

jefsboys 03-07-2012 09:24 PM

Thank you very very much for the feedback. It is greatly appreciated.
I probably have ~30-40 hours of reading on this and my mind is like spaghetti now.

Concerning the slant fins...
I checked the manufactures website and they have NFVA of .35 sq. ft. per unit. I have 4 of them on the roof, one on each side of the roof.
How about the following sandwich? It saves me about 4" of ceiling thickness.
This would yield R38....
  1. 1" Baffle spacer
  2. 6.5" Rigid - between rafters.
  3. 3/4 Rigid on bottom side of rafters.
  4. 1X3 strapping - seriously considering this for the strength/integrity it will bring by screwing that to rafters then having the wall board tightly attached to the 1X3's
  5. Wall board.
I am still trying to figure out the moisture barrier. I know this needs to be on the inside(heated) area.
If my 3/4" rigid that was on the inside face of the rafters was faced would that take care of it?
Here is what I was thinking...

shazapple 03-08-2012 07:06 PM

That may save you 4", but certainly won't save on your wallet! You could look into making your vents out of plywood and then sprayfoam, might be cheaper and get a similar R value. They will try to tell you ventilation isn't needed but I think you've done enough reading to know better. Also make sure your rafters are equal spacing, as cutting that much rigid board wouldn't be fun.

Faced insulation would need to be taped at the seams but should be a vapour barrier, depending on the perm rating.

The product you linked (polyiso) wouldn't act as a vapour barrier itself. Typically you see it faced with kraft paper, which isn't quite a vapour barrier.

Msradell 03-08-2012 08:02 PM

Have you considered using spray foam insulation directly on the roof (hot deck installation)? It would certainly be much simpler and probably wouldn't cost any more than doing all the other steps you are discussing.

Gary in WA 03-08-2012 11:04 PM

I wouldn't use plywood for the baffles, any condensation in that air chute would grow mold. 1" is minimum per code, anything more is better. The cross-air channels may give you more resistance/turbulence than a straight shot, I'd go with 1-1/2" or 2":

The R-max link is a vapor barrier (0.03 perms because of the foil facing), though it really doesn't matter with that great amount of solid foam board. No diffusion will get through.

The exhaust vent are questionable. 1.4 sq. ft. = 202 4 = 50 OR a 9" turtle-back vent. The one pictured appears to be a 6" vent, unless it was "super-sized" later.
BTW- R-38 is required per your location if under IRC: unless you have your own State energy code as we have.

Install the soffit continuous next to the fascia board for positive wind pressure rather than against the house wall.


jefsboys 03-09-2012 08:02 AM

Thank you all for your time and input.

Concerning spray foam insulation.....
When reading about the spray foam insulation (hot roof) I was leaning away due to the importance stressed that it had to be perfect or growth could happen and that it is hard to recognize any failure in it until it is out of control.
My thinking on avoiding this maybe wrong and input is appreciated.
Also, this seemed as the most expensive way to go. I thought significantly more.
Concerning my rigid foam layout....
So the way I currently have the foam boarding thought out I would not have to apply any special vapor barrier?

Concerning the exhaust vents....
They are 8" at 51 sq. in. NFVA.
I did just go look and the roofer does have part of the 8" diameter opening covered by part of the roof sheeting. I will open that up to get the full venting.

Greatly appreciate the direction on where to better locate the soffit vents.

shazapple 03-09-2012 09:49 AM

No need to avoid spray foam as a material, as long as you are aware of the pros (airsealing, high R value) and cons (sensitivity of application, cost). It certainly cannot magically bypass code and best practices and not require ventilation.

With proper ventilation and airflow I can't see a plywood being more likely to grow mold in this situation than in a normal unconditioned vented attic. What else would be recommended in order to support the sprayfoam?

edwardh 06-27-2012 08:32 PM

Need some help. My attic looks exactly like the one in the photo. I want to do the same thing as the thread author - remodel into a room.

One question: how will venting with soffit vents and baffles help if the ridges are not all vented. Does that make sense? Each baffle is a channel, and if i had four ridge vents running up the peak, it would vent all the spaces between my rafters, but all I have are two shell vents.

edwardh 06-27-2012 09:01 PM

How about this (answering my own question?): leave an open space at the peak below the shell vents where air can gather from the rafter bays and escape. Eh?

edwardh 06-27-2012 09:28 PM

That would be great if all the rafter bays converged at the peak, but they don't. They end at their respective ridges. So the question still stands.

edwardh 06-27-2012 10:26 PM

And drilling a hole into each rafter to create a channel of air flow seems like it would compromise integrity.

edwardh 06-29-2012 11:23 AM

I'm still hoping for some guidance. Summary of the question: in a finished attic, how do I vent hip style roof with no ridge? The rafter bays "dead end" into the hip rafter, so airflow gets blocked.

I looked at notching the rafters, but I don't see how the insulation doesn't block that.

There's drilling holes in the rafters, but again how does the insulation not also block those too (drill in the top 1" of the rafter)?

Any help is appreciated. Thanks.

shazapple 06-29-2012 11:40 AM

soffits are useless without venting at the ridge. Leave a little space at the peak as you mentioned so air can flow between rafter bays and out the gravity vents, or install a ridge vent.

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