DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Building & Construction (
-   -   Insulating cape-cod style roof w/attic room (

darin_i 11-03-2010 05:19 PM

Insulating cape-cod style roof w/attic room
Here's another one of the 'how do I insulate X' questions...

I have a 60+ year old 2-story home (in Pittsburgh, PA) with a cape-cod roof and a (soon-to-be) finished attic space. The finished space is a 12x25 area down the center of the attic area, there is a crawlspace on either side. I gutted the thing to the rafters/joists (2x6's spaced 16") to remove old flattened (i.e. useless) rock wool insulation (in the crawlspaces) and hopefully provide better insulation for the upstairs room in the summer.

My insulation work plan is as follows:
  1. R19 in the 2x6 joist areas in the crawlspce
  2. 2' piece of Durovent inserted next to the fascia (there is a 'gap' in the fascia for 'venting' -- old-style vents, I guess)
  3. No insulation against the roof deck in the crawlspace
  4. Durovent against the roof deck over the living area
  5. Faced R19 insulation under this (yes, compressed slightly against the Durovent)
  6. Durovent ends about a foot short of the peak in a little plenum area
  7. Ridge vent and power fan in the peak for air flow
My question is this:

I had a roofer (friend of a friend) come out to inspect the roof deck and approve my insulation idea. He said that I SHOULD NOT put the R19 in the rafters over the living area, and that I should use R13 (3 1/2" thick) insulation instead. He was concerned that insulation touching the decking under the roof deck would be 'a bad thing'.

I called the Durovent manufacturer and they said to stuff in as much insulation as would fit. However this would mean that insulation would contact the roof decking in the areas beside the Durovent in the rafter bays.

Both approaches seem to make sense to me, which one should I follow?

Thanks for any help anyone can offer here, winter is coming!

RemodelGA 11-03-2010 05:29 PM

First off - in the peak, do a ridge vent or a powered fan, not both. With both, the powered fan sucks in hot air off of the roof through the ridge vent.

Also, I would be very hesitant compressing the R-19 insulation to fit inside the rafter cavities while using the baffles. It's not such a concern of touching the roof decking as it is that compressed insulation loses insulating value.

In this instance we always fir down the rafters on the angled portion of the ceiling with 2x2s and then use the R-19 insulation and the baffles.

Consider a reflective barrier on the underside of the roof deck as well.

darin_i 11-03-2010 05:50 PM

Thanks RemodelGA,

The power fan is installed already, the peak vent -- kind of. What I think the guy who had this place before me did was open the slates at the peak (yes, a slate roof), but he did not cut the decking underneath. In other words, I believe there is little to no airflow out the peak - useless.

Airflow into the crawlspaces is augmented with a pot vent on each slope of the roof. I had considered the fir strips as you suggested, but the room is already vertically cramped as it is.

The reflective barrier would have to go underneath the baffles, right? If I recall, the Mylar radiant barrier needs an air gap to work correctly.

RemodelGA 11-03-2010 06:08 PM

Good to hear the ridge vent is only a faux ridge vent.

So for your choice between R-19 and R-13, I would go with the R-19. Make sure that when installed, it's not pushing below the bottoms of the rafters too much as this can hinder the drywall install (gravity plus insulation trying to expand is not a good combo for drywall). If it's too thick, peel off the outer most layer of fiberglass and re-install.

For the reflective barrier, I would install this against the bottom of the roof decking prior to installing the baffles. This can be a PITA if there's lots of roofing nails sticking down, but sometimes that's how it goes. There are lots of ways to install reflective barriers - at the roof decking, drooped between rafters, stapled to the bottom of the rafters, etc. All depending on the situation. The goal is for the reflective barrier to block radiant heat from getting to the insulation which then creates less work for the insulation.

One other option to consider is spray foam insulation. This is by far the most pricey option but would also yield the best results. I read somewhere that they are coming out with DIY foam install kits but I don't have much info on those.

darin_i 11-03-2010 07:03 PM

OK, I'll check on the insulation fill factor for the pillowing effect you mentioned. The drywall is 1/2" (tried to strike a balance between rigidity and carrying the cut pieces up 3 floors).

I did look at the spray foam. The only people I found in Pittsburgh to do it would coat my attic space 1" thick with foam for $1700. I further read about old houses that were 'foamed' having rot problems in the decking because moisture would get trapped in the wood of the decking instead of having the original, breathable situation. I couldn't tell you if the stories were true or a load. People tend to say things to get you to buy their services, I find.

The DIY kit looked like something I could use to evenly coat myself with the foam ;-)

Thanks for your advice.

Gary in WA 11-03-2010 08:54 PM

R-49 in the attic, R-38 in the cathedral ceiling:

Use some rigid foam board to get the R without losing too much space. Stay away from fiberglass. Use f.b. or house wrap on the back of the knee wall insulation to prevent wind-washing from the attic ventilation there. Leave 1" space between ends of baffles as per manufacturer: Run f.b. up to baffles and down between floor joists to keep air from over living spaces drawing heat away: Or use SPF. Radiant in your location is not, table 4: You will probably draw a negative number: When you furr the rafters to get the R required, add some sill sealer between woods for a thermal break. Since 2006, SPF and some substitutes are Code acceptable for unventilated cathedral ceilings:


darin_i 11-03-2010 09:47 PM


Thanks, that's a lot of good information all in one spot. My situation/plan looks almost exactly like the pic on pg 2 of the "Simple Savings" doc. I have foamboard air dams installed in the joist cavities (friction-fit & caulked against air movement). I am installing fanfold foamboard in the crawlspace to deal with the windwashing.

I doubt that I can approach R-38 in this case, unless I stack a pair of 2" polyisocyanurate boards (2 x R-13) on a spacer from the roof deck. I really like the idea, but it more than triples the cost of the insulation for that part of the roof, and increases the time. I have to think a bit about this....

Gary in WA 11-04-2010 12:36 AM

A lot of it depends on going "Code" or not. I encourage it, structurally and just for the minimum safety requirements. The sloped ceiling will lose a lot of heat there, causing ice dams with a snowfall... Use the ADA, airtight drywall approach, to help slow the heated air: Seal the air leaks from below, first:
Another trick is installing a rolled up fiberglass batt in a poly bag to block the cavities below the knee wall from air movement, f.g.batt alone won't. If you go SPF, do the attic lid also and omit the attic floor insulation, especially if heating ducts are there.


darin_i 11-04-2010 01:34 PM


Ok I read the ADA stuff in the document. One question: gluing the drywall to the joists/framing after putting in the insulation essentially means gluing the drywall to the vapor barrier, not to the studs themselves. Is this the intent of the document, or am I mis-reading it?

Also, this is a balloon-framed house, and it lacks a bottom plate on the outside walls of the house (there are fireblocks between floors though). Should I toe-nail some pieces of 2x4 in these areas at floor level to help support the drywall there?


All times are GMT -5. The time now is 04:24 AM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1