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Fix 1.5 story roof insulation/venting

EngineerNate | Posted in General Questions on

Hi all, quick background: 1925 craftsman home. 1.5 stories. Climate zone 4a mixed humid. Closets built into the “wings” of the 1.5 story space.

Two of three closets are getting demoed due to unrepairable damage to the plaster from a bad roof install and chimney flashing, since fixed. The roofer put in a ridge vent.

When we demoed the closet, all of the cavities were packed with blown in, and there were some old school rockwool batts here and there. But that’s mostly gone now.

Some photos for reference here:


1. Can I reasonably expect to get a solution here that vents adequately? There is currently no air path between the addition’s attic and the original attics. They built the addition on top of the old roof without cutting the eaves off and the original roof was not vented.  So the addition has soffit vents but no ridge, and the ridge has no soffits feeding it air. I had thought to sawzall/drill some big openings between these two roof spaces while I have this closet demoed. Worth it or folly?

Some previous discussion of the addition roof situation here:

At that point in time I was not fully understanding the added complication of the “lower” and “upper” existing attic spaces created by the 1.5 story design.

2. Is there a way to get some insulation in the ceiling of this closet with a vented assembly without rotting my roof deck? (Roof is decked with osb over the top of the old solid wood decking.) The roof is 4X construction so there’s not a lot of depth to work with.

3. The walls of the rooms upstairs have their double top plates notched into the roof 2x4s. I assume to move them as far outboard as possible. This leaves very little room for airflow. I could possibly bridge the spaces with 1″ pvc pipes jammed through to keep some clear airflow. Currently old blown in is definitely hampering any flow.

A photo of the above notched top plate situation here:

The house would have been a perfect candidate for insulation above the roof. But I didn’t know any better a year and a half ago and had a brand new, pricey standard roof with a ridge vent put on.


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  1. Expert Member
    Akos | | #1

    You can generally dense pack short sloped ceiling sections. Provided the mini attic above and bellow is vented, any moisture can move through the cellulose and dry to the other sections.

    Looking at your picture, you can install some gable vents into your siding for the rest of the attic spaces. Do your best to seal up the ceiling there, more important to fix the air leaks than to get perfect venting.

    1. EngineerNate | | #2

      You mean the ceilings inside the house, to keep moist air inside right?

      I had an engineer out and he recommended gable vents as well. He also recommended adding vents up under the gable to each of the lower attic spaces. I had thought gable vents weren't thought well of anymore.

      Would it be prudent to do a thin layer of xps spaced 1" from the roofline above the dense pack to leave a clear air channel? That seems relatively easy to do.

      Would rockwool bats be considered appropriate as dense pack? Or does it need to be something blown in tight.


      1. Expert Member
        Akos | | #3

        Gable vents work, there is nothing wrong with using one. As long as you size the free area based on the sqft and slope of the roof it serves, you should have no issues.

        Provided there is some reasonable amount of venting, most roof issues are caused by house air leaks, important to deal with those first.

        Normally you dense pack the sloped section to not have to remove the ceiling. If the ceiling is off, you can go with batts. Adding a vent space is even better. With older real 2x4s I have used the acoustic insulation batts before for vent space as they are slightly thinner than the regular batts. This gives you a 1" vent space, no need for any fancy baffles. If you are going to make some baffles, make sure it is out of unfaced foam. You don't want the baffle to be a vapour barrier.

        1. EngineerNate | | #4

          Got it. It's totally demoed at the moment. Aside from the limited space the world is my oyster so to speak.

          I like mineral wool in general because it doesn't hold water like fiberglass.

          The house had terrible air sealing at the second floor attic. I built a new attic access with full weatherstripping etc to help address that. I'm having all the electrical boxes etc replaced concurrently with this work so I should have ample opportunity to seal all the ceiling boxes up.

          1. EngineerNate | | #5

            One other followup, is unfaced rockwool a reasonable solution for the exterior wall? It looks to me like there is some kind of foam board under the siding outside the original wood sheathing.

            My biggest concern is causing rot with insulation that traps moisture where it shouldn't be!

  2. EngineerNate | | #6

    All, I've made a diagram to clarify this question. Hopefully it's helpful.

  3. Expert Member
    BILL WICHERS | | #7

    I have some of the same 1.5 story issues as you in my own home. What I did was to make baffles with 1/2" polyiso (sometimes thicker, which I'll explain), that I used to make vent chutes to tie the upper and lower attic sections together at the "corner" where the knee wall (which is full height in my home) ties into the rafters. My rafters aren't notched though, so I have more room to work with -- and they're 2x8 rafters too.

    In some areas I used thicker polyiso cut into wedge shapes so that I could glue it over the top plate of the knee wall between the rafters. I did this in areas I didn't think blown cellulose would get into well, or other areas I thought were suspect in terms of getting in enough insulation. It was not fun installing these pieces, working on an impovised catwalk in a sand dune like mass of blown insulation in the lower attic.

    Regarding mineral wool in walls with exterior rigid foam, that will work fine. Try to determine what the type of exterior rigid foam is, and the thickness. The "what kind is it" question is best answered by looking at it with an expierienced eye. If you're not sure what it is, post some GOOD pics of it and some of us here might recognize it. Once you know what it is, you'll know how many R per inch it is. You can measure the depth by poking a wire or nail through it and keeping track of how far in you go.

    Now that you know what kind of rigid foam you have, and how thick it is, you can use the normal tables for your climate zone to see how much exterior rigid foam you need to be "safe". If you have enough, or more than enough, you can go ahead with mineral wool and not really worry about it. If you have less than you need, you need an interior side vapor retarder (I've used MemBrain for this before). If you want to play it safe, just install the interior side vapor retarder and don't worry about it -- the smart vapor retarders used like this can only help you, not hurt you here.


    1. EngineerNate | | #8

      Thank you!

      I've now noticed that they put some of the pink foam under the siding but that in some areas it looks like there's just osb. I am unsure of what exactly they did. I will have to investigate further.

      I will for a start, open up the addition roof to the knee roof and then make some allowance for air to get from the knees to the attic. I have full access to 3/4 of the knee wall attics and will have partial access to the fourth. That one I may end up having to simply shove some PVC pipes all the way through the slanted finished ceiling space (2.5' it so) to bridge the two, I do not want to tear out that closet.

      Would it be a bad idea to sandwich rockwool with a layer of XPS on the interior that covers the roof beams in the "cathedral" part of the ceiling? I was thinking of this to get a higher r value vs just furring the 2x4s down.

      Assembly would be xps baffle, rockwool, xps full coverage, furring strips for sheetrock, sheetrock.

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