insulating rafters

I have a less than 100 sq ft room off the back my house I am finishing. The roof is gently sloped and ties into the back wall of my house well below the rest of the roof line. I want to insulate it when I finish it and to be sure I am doing it the right way.

The rafters slope from about 9 ft down to 7 inside so i am just planning on sheetrocking to them. There is no soffit vents and there is no venting at the roof line either. Do i still need to leave a gap between the fiberglass and the roof or is that unnecessary since there is no venting?? is fiberglass a bad option?
2x6 rafters 24 oc about 8 ft long
I live in NC Mountains
I would love to do spray foam but its too pricey .

I appreciate it


Shed roofs should still be vented; there are several ways to vent it be it box vents, turbine bents or even smart vents…failing to vent a roof can create condensation and roofing issues. I personally would do a soffit and smart vent to create a natural air flow (leave gap between insulation and roof decking). As to type of insulation…I love cellulose however for such a small area I would install fiberglass, at least an R36 or better.



dont think the smart vent would work. it an old room with roof etc on already…

I could easily cut it some soffit vents underneat but the rafters are sealed in between. would drilling several small holes say 1/8 -1/4 inch holes, provide enough flow??

Also same deal with the top. The room is sort of lean to style and the roof ties into the back of the house. If I put a box vent vent or even side vents in there there is no way for air to flow between the rafters??? would again drilling some holes provide enough flow??

will i have issues with a box vent being the upper roof overhangs this one a few feet? Better off with side venting??

I really appreciate the info

oh yeah and i only have 2x6 rafters so i can really fit more than r 19 in there. I know thats way under done but the room is strictly a laundry room and noone spend much time in there.

To be clear, the roof is slanted and acts as a ceiling too?

Do you have any pictures you can post?


Roof to wall vent.

Install 1" nailer strips against the rafters and underside of the roof sheathing.
Install a 2" Foil faced [FONT=Univers-CondensedBold][size=1][FONT=Univers-CondensedBold][size=1][size=2]Polyisocyanurate Foam Sheathing[/size][/size][/FONT][/FONT] pressure fitted in between the rafters and push against the nailers strips. R-13. [/size]

If your rafters are 2x6, you will need to add 1" furring to the bottom of the rafters so you can install a 3-1/2" High Density fiberglass insulation which gives you an addittional R- 15.

This gives you a total of R-28.

The foam sheathing can be decreased in thickness and rafters sistered so as to increase the rafter depth for a thicker batt insulation to achieve an R-30.

Now the eave has to be vented using either round vents in the wall, if no soffit, or a vented roof drip edge.

Hope this helps, and pictures would have made it easier. :slight_smile:

R806.1 Ventilation required. Enclosed attics and enclosed
rafter spaces formed where ceilings are applied directly to the
underside of roof rafters shall have cross ventilation
for each
separate space by ventilating openings protected against the
entrance of rain or snow. Ventilating openings shall be provided
with corrosion-resistant wire mesh, with 1/8 inch (3.2
mm) minimum to 1/4 inch (6 mm) maximum openings.

R806.2 Minimum area. The total net free ventilating area
shall not be less than 1/150 of the area of the space ventilated
except that reduction of the total area to 1/300 is permitted, provided
that at least 50 percent and not more than 80 percent of
the required ventilating area is provided by ventilators located
in the upper portion of the space to be ventilated at least 3 feet
(914 mm) above the eave or cornice vents with the balance of
the required ventilation provided by eave or cornice vents. As
an alternative, the net free cross-ventilation area may be
reduced to 1/300 when a vapor barrier having a transmission rate
not exceeding 1 perm (5.7 × 10-11 kg/s ⋅ m2 ⋅ Pa) is installed on
the warm-in-winter side of the ceiling.

R806.3Vent and insulation clearance. Where eave or cornice
vents are installed, insulation shall not block the free flow of
air. A minimum of a 1-inch (25 mm) space shall be provided
between the insulation and the roof sheathing and at the location
of the vent.

Joe, ventilation would not be required if the cathedral ceiling were insulated with spray foam or high density cellulose.

3" of SPF is = to about 19.5 and high desity celulose is about 22 @ 5-1/2"s.
Depending on the geographic area, most zones require a 30 r-factor or higher.

In his case, it would still be cheaper to go with the Spf rather than remodel to provide ventilation using standard products.

And no vapor barrier is required with either of the above products. :slight_smile:

Why does this sound like an enclosed porch?

And that is how we end up with crawlspaces Bob. :wink:


The only exception I see to ventilation is a conditioned attic assembly and I guess that’s what you’re talking about. I didn’t know they made an air-impermeable celulose insulation. (Air-impermeable is required, except for Zones 2B and 3B.)

Joe, conditioned attic or cathedral ceiling, same difference.

Here is the info.

SPF is about the most economical product to use with the most R-Values and air sealing qualities available for circumstances like was asked above.
Considering the material cost and labor envolved for all the other alternate methods of achieving an r-30 or better, SPF at a cost of $1.00 per board foot, is very appealing. At least, that is what it costs up here.

The poster has no pictures, nor does he show his location for others to help. Geographic location makes a difference. :slight_smile:

I have been using applegate insulation for 10 years.
My first home I built for myself was just over 2000 sq ft and I used fiberglass insulation, the next home I built for myself was over 4000 square ft with 10 - 20 ft ceilings, the power bill was the same as my 2000 sq ft. home. Since then, other then small additions, I use cellulose whenever possible.

Jeff, that is quite a few steps better than the wood shavings and sawdust that I used to use as a kid with the old Man. ;):):smiley:

Not to be mean, but are we builders now.
I am leery of giving advice over the InterNACHI MB.
I thought this was for HI Q&A.
Look really I am not being mean but MR.Letts , you are the example of the word free bee.
I see this all the time and wonder are you a HI or trying to get building advice, because I can see you surely do not know how to build.
You know no building terms like grade or roof slope ,or how to measure said slope but yet you are building ( or have built ) and coming here asking questions after the fact.
I see something funny here.
I am not being mean ,but some one is going to buy that home after you have built it.
Is it up to code?
See its your first statement that confuses me
( I want to insulate it when I finish it and to be sure I am doing it the right way.)
and the last statement
( I would love to do spray foam but its too pricey .)
All your questions should have been asked before you started building and would have been answered for free at your local builds supply shop.
That is what they are there for, to help you understand PROPER BUILDING PRACTICES…
I am not being mean but help full, for you would have saved money by asking all the right questions. Also learned a thing or 2.
I wish you the best of luck and Merry Christmas.

That is very interesting reading Marcel.
I am looking into seeing if they are in Canada.
I see Applegateinsulation are also Christians in there business thinking and life practices.
Another plus.

Are you taking your meds Robert?

Thanks Robert I agree the information given was great but could we or should we be opening our selves up for Concerns with some one broadcasting this information .
It might be a better idea for the posters to remove their posts or have NACHI remove this string.
He has been using us to tell him how to do electric work also .


This is one of the places not to get better information from. How many of the people working there are actually experienced/certified tradespersons or have worked many years in the field of construction. Are there any building specialists in these stores?

In an analysis of the building industry I did a few years ago, I found they distributed a lot of poor information especially since homes have become more energy efficient and airtight, needing better interior and exterior techniques to be durable in the long run.

I used to have a quote from the senior sales associate of a “Big Box” store on the home page of my website. Ten years ago, when I was consulting/working on raising a 112 year home with 14 corners, removing a partial stone foundation and then building a full depth permanent wood foundation (PWF) under the home, I went into the store looking for a product that I hadn’t used for about 10-12 years. He hadn’t heard of it so I described what it was being used for…our conversation went on for about an hour and wandered through various aspects of building/renovating. To quote him:
"I’d like to follow you around for a year or so!"

Any cellulose manufactured to recognized standards will do the job when properly installed. There are a number of cellulose manufacturers in Canada. One of the larger serving Quebec, Ontario and the Maritimes is headquartered in Vars, south of Ottawa; it has another plant in central Nova Scotia.

Support Canadian manufacturers, Christian or not.