Upgrading Insulation
My question specifically relates to the slant area of my story and half house. I have an opportunity to upgrade the insulation in this area. I live in Zone 6. Access will be gained from the outside as were getting a new roof. Unfortunately as is the case in most houses in my neighborhood the rafters are of 2×4 construction. Currently the insulation is balsam wool insulation in the slant area. The plan is to expose the slant portion when getting the new roof by removing the roof sheathing in the slant area and then removing the insulation and installing 3.5’’ of XPS insulation in the stud cavities with all seams sprayed with canned foam although this is not enough insulation, it definitely is an improvement over the current situation.
Here is a link which closely resemebles my situation. This about the size of my knewalls and the slant portion in my house is only about 4’
http://thumbandhammer.com/projects/attic-3.html
Close up
http://thumbandhammer.com/projects/images/attic/guestfiredamage.jpg
I clearly understand this is not enough insulation in this area. And to do this correctly I will need to remove the sheetrock on the interior and increase the framing depth to allow for more insulation. However at this time money does not allow for this as it requires a complete remodel. I would like to at least use the opportunity of the roof being removed and having access to this area WITHOUT having to disturb the interior living space.
My questions are:
Would using XPS in these stud cavities be acceptable?
Would polyisocyanurate rigid insulation be a better choice?
I understand SPF closed cell is an alternative however because I don’t have enough depth framing wise to achieve the ideal R Value this would be a waste of money as SPF is the most expensive.
On slants wall like this….What is the recommended R Value……I would suppose the ridge area would require more insulation. Are these considered walls or ceilings or both?
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Replies
MS,
The energy code does not differentiate between flat and slant ceilings - they are both ceilings - and the requirement for zone 9 is R-49. However, if there is inadequate rafter depth (your rafters must be deeper than 2x4), you can have R-30 for no more than 500 SF or 20% of total ceiling area, whichever is less.
If you're stripping the roof, I would suggest leaving the Balsam Wool (cellulose batts) in place and installing 4"-6" thick rigid foam nailbase panels (usually EPS with OSB on one side) over the existing sheathing and then re-roof. If you'd like a vented roof (always a good idea), there are self-venting nailbase panels made with polyisocyanurate called Cool-Vent II.
This will bring you to or close to IECC standards and won't require any interior renovation.
M.S.,
Although Robert wrote "zone 9," I think he meant to write "climate zone 6." In climate zone 6, you need R-49 ceiling insulation. That means about 14 inches of EPS or 7 1/2 inches of polyiso.
In your case, you should install rigid foam on the top of your roof existing sheathing -- covering your entire roof. Then you should install your new roof.
Thank you for the responses. I had investigated the idea of XPS over the orginal sheathing but its not feasible for a few different reasons. One of which is whether the 2x4 rafters could support the additional weight of strapping and another layer of sheathing
Would insulating between the slants with XPS cause any issues besides lack of R Value? I revert back to the simple premise that I should at least make an attempt to do something while I can access this area. Although your suggestions are what ideally I would like to do they are simply NOT in the budget.
I was considering adding the XPS between the slants as I originally propsed….Then down the road in the next year or two remove the interior drywall in the slant portion add 2x2’s to the face of the studs (which will allow for more depth to add another layer of XPS in the slants) and then add a final layer of XPS horizontally across the face of the studs like this.
http://www.hammerzone.com/archives/energy/insulation/ceiling_sloped/rigid_foam.htm
This would get me apprx another 3.5’’ to 4’’ of XPS on the inside.
Would this be acceptable? This would be around 7’’ to 7.5’’ of XPS total…Is there a limit to how much XPS you can add from a perm perspective?
Yep, I was standing on my head when I wrote "climate zone 9".
"Would insulating between the slants with XPS cause any issues besides lack of R Value?"
Potential rotting of the roof sheathing by trapping moisture in an unvented roof that cannot now dry to the interior. Penny wise and pound foolish. Do it right the first time and you'll end up saving money.
I really feel some alternative exists vs. simply leaving my inferior balsam wool insulation intact or trying to detail the roof by adding XPS on top of existing sheathing and then re-sheathing over that. That alternative is what I came in search of here
Robert your comment on ventilation is fair a one. I assume your basing it off the assumption the slant area would be completely full of insulation thus no communication could occur between the sofit area and the ridge, but don't know as you didn't specify.
One alternative I was considering was if I have 3.5'' of space in the slant area why not only fill the slant portion with say 3.25 of XPS . This would leave a .25 gap for ventilation. I would run but the ventilation chutes from the sofit up to the slant area
Then when budget permits remove sheet rock, add 2x2’s to bottom of studs for depth additional layer sheet of XPS in the slant bays with a final layer sheets of XPS horizontally.
This way I get my ventilation from sofit to ridge. I upgrade the insulation in the slant cavities over the current inferior balsam wool. Thus when budget allows can get up to necessary R value by adding insulation from the interior.
I understand this is somewhat unconventional. But it would provide 2 blatant improvements over my current situation
Improve R value in the slant area
The gap in the slant area would allow for ventilation from sofit to ridge
Thoughts?
MS,
You must have at least 1" gap for adequate roof ventilation. However, any gap is better than no gap in terms of drying potential of the sheathing.
But, unless you have some other reason to pull off the roof decking in order to access the slant ceilings, then it's a waste of time, effort and money. The balsam wool is no worse and probably better insulation than fiberglass batts. There is no good reason to remove it until you're ready to do the job right.
However, if you don't now have good soffit and ridge vents, then re-roofing is the time to cut those in.
There may, however, be a reason to remove the roof sheathing. Unless you're in some unknown part of zone 6 in which there is no snowfall, 2x4 rafters are woefully insufficient and removing the roof deck would allow sistering those with properly sized rafters. That would create the roof cavity depth necessary for a continuous vent channel and decent insulation. But, in that case, I would recommend dense-pack cellulose.