Hello everyone.
Writing from Hastings Highlands, Ontario. We have very cold winters, hot summers. Our planting zone is 4a if that is helpful at all.
Working on a first time renovation project and trying to formulate a plan for our attic ventilation. Our home was built sometime between 1850-1870. It’s old but sturdy although it does come with many layers to uncover. We heard baffles are pretty important for our attic ventilation to prevent ice damming, minimize moisture problems and to increase the roof’s lifespan.
The home is a single gabled construction. The roof pitch is quite steep and the roof extends down a little over halfway on the second level (attaching a photo of the exterior for a better idea). We’ve brought it down to the studs in most areas as there was quite a bit of water damage. Half of the roof was replaced with steel on top of the old shingles and the first thing we did was to match that on the other side (also on top of the old shingles). The attic has a fairly decent layer of blown in cellulose covering it (approx. 7 inches) however the cellulose goes all the way up to the edges of the roof with no gap allowance. The home has a unique “double sheathing” type situation happening. There are boards that run the full interior perimeter. Between these boards and the actual sheathing are patches of old cellulose that have sank and compacted over time (this is present in the walls) and what looks to be blown in yellow fibreglass insulation (between the roof sheathing and the slanted ceiling on the second level). So that’s kind of laying the land for my question.
We wanted to install baffles but we knew it would be basically impossible to extract the yellow insulation from the attic. We decided to remove one of the horizontal interior boards on the slant so we could push the insulation from above and have it fall onto the floor below. When we removed the board what we found was instead of a nice air gap that went down to the soffits like we expected, a layer of concrete that in theory prevents any kind of airflow coming from the soffit and additionally any potential baffle from getting down there.
With all of that said – wondering should we remove the concrete so we can move forward with our baffle installation plan? Or should we scratch the original plan and do something else? Would love some advice here.
Attaching photos for reference of the situation. The very dark photo is from inside the attic showing the current insulation situation and a very old pole beam!
Thanks in advance and apologies if my info above isn’t 100% clear. Like I said, we are newbies in the home reno department.
Replies
How much space is between the inner and outer boards?
and what are your plans for this space?
There is 3.5 inches between the roof deck and the inner boards. This is the top floor of our home so our bedroom, office and guest room.
If your efforts to remove what looks like concrete between the slanted top plate of the wall and the roof deck, you can plan to open up the wall below that plate. ( I would drill holes to preserve the structural integrity.) The top of that wall should be within the soffit enclosure.
You can lower the slanted part of the ceiling, bring in the inner surface of that wall, or both, or some other way make room for air circulation on the inside edge of that top plate.
It also is possible to have dense pack cellulose blown in without leaving space for venting baffles.
Thanks for the notes above. To the last point -- do you see any major downfalls in simply blowing in cellulose and not including any baffles for ventilation? (The attic has a whirlybird FYI).
Moisture is the problem. The metal roof (or any waterproof roof) will not let the sloped part dry.
You might get away with having the sloped part of the roof dense-pack filled, along with the outside walls, but it may lead to problems over time.
Your situation is not as bad as a full cathedral ceiling, but any moisture accumulated within the sloped cavity will either have to work its way sideways up a few feet of insulation, or wait for summer to work its way back into the interior space. In the mean time, it can feed mold and rot in the timbers and boards.
My son has a house a few hundred miles south and east of you in New York State. His then-83 year old house had no insulation, shingle roof, and plaster walls. After replacing knob and tube wiring in the outside walls, we had dense pack cellulose blown in, including the sloped part which is similar to your situation, except the bottom part of the slope is open beyond a knee-wall. The rafters also are quite a bit taller than yours are. Ten years later, there are no obvious issues.
For you, the bottom the slope being closed off adds to the challenge, and you would also end up with about half the required R value.
If you are not up for the above the roof option,
you could look at leaving the sloped part of your existing structure empty, and putting 7 or 8 inches of rigid foam board inside. (covered with drywall). Probably have to box in a gap at the top corners of those windows. You should be able to vent the bottom through the outside wall into your soffit, and pull insulation away from the roof deck above the outside of the flat part of the ceiling. (where baffles will be required).
The exterior walls would be a good place for dense pack cellulose. This needs proper equipment, and not the DIY blow-loose in the attic machine you might be able to rent.
Thanks Mike.
This makes sense. Lots of options I suppose and it's very hard to know which route to pursue. But we will take your advice and look into the options you've listed here.
You wrote: "You should be able to vent the bottom through the outside wall into your soffit, and pull insulation away from the roof deck above the outside of the flat part of the ceiling. (where baffles will be required)."
Could you please clarify this? Having a hard time understanding this part of your response.
Thanks a lot!
I made a little sketch (ok, two)
You have that slanted top plate at the top of your wall cavity, which you have said is topped with some concrete stuff that blocks the vent path from the soffit to the slanted part of your ceiling/wall.
If you cannot clear that concrete type stuff from over the top plate, you can drill some holes on the outside part of the top of your wall, below that slanted top plate, which it seems to me would be within the soffit. ( Of course you would want to drill one hole and verify before going mad making holes in the outside wall)
If you then left part of the boards you have removed absent, (couple inches would do), it should be suitable to have air from the soffit pass below that top plate, over sheet insulation you install on the slanted part, and up the slanted part of the roof.
where the flat attic floor is, you would need to pull the existing insulation away from the roof deck and install a baffle to keep it (and more which I would hope you put in) away from the bottom of the roof deck.
Seeing how little space there is between the sloped ceiling and roof deck, you may want to consider building the roof up with exterior insulation.
To do this, remove the metal roofing ( will be reused) and strip the roof deck. Repair the deck as required and add rigid foam board insulation to the required R-value (in Ontario: R-31 for roofs without attic space R60 with attic space: http://www.mah.gov.on.ca/AssetFactory.aspx?did=15947.)
Apply ice shield and roof underlayment, and then double strap for the metal roofing, first layer inline with the rafters, second layer horizontal. Re install the metal roof.
This will solve the venting delima, get the required insulation without loosing any interior space, and allow you to level/even out the roof deck & ridge
Yep. With 2x4 roof rafters/ceiling joists there is no room for baffles and proper insulation. Have to lower ceiling which makes for crappy living spaces or insulate on top like recommended here.
Thanks for the info here. We are tight on budget so I'm not sure if we'd be able to pull the roof off and add the ventilation/additional insulation from above...
Do you think if we simply void the venting and blow in densely packed cellulose we would find ourselves in a pickle in the long run? Perhaps we will need to reevaluate the budget! Appreciate your insight.
The reason the existing structure has lasted is the leaky nature of the previous insulation efforts, allowing enough air flow through to keep the structure dry. Dense packing that space will eliminate the air flow.
With dense pack R values ~ 3.5/inch, and the limited space, there won't be enough insulation to prevent condensation unless there is a complete vapour barrier on the inside. There is no such thing as a perfect vapour barrier in building materials, so there needs to be some open area to dry.
The shingles on the roof are also a vapour barrier, so any moisture that does get into the roof assy will be trapped. This is the pickle you want to avoid. If you pull the metal roof, strip the shingles, and then double strap before reinstalling the metal roof, there will be a ventilation channel between the roof sheathing and the metal. That would allow the roof to dry to the outside. If you add insulation on the roof deck when doing this, it should be thick enough to prevent condensation occuring in the building structure.
Also, with the limited installed R value, the energy cost will be high, and those rooms may be uncomfortable.
Are you in an area requiring permits / inspections? If so, you will need at least R31 in the sloped part of the ceiling.
Yes, I see. Moisture here seems to be the biggest concern.
Ok so let's say we decide to go with removing the sheet metal, stripping shingles, double strapping, adding insulation to the roof deck solution - we would most certainly need to wait until spring/summer to do so as the roof currently has 3 ft of snow on top of it. What should we do in the interim with that specific area? Should we: a) keep the blown in loose fibreglass as is; b) remove all of the blown in fibreglass and re-blow in dense packed cellulose as we will be doing this with the walls and adding more to the attic floor; c) remove everything and leave it empty; or d) something else? We want to be able to progress on the renos and not have this item stop us in our paths.
We are in an area that seems to be fairly relaxed re: permits and inspections so we are not overly concerned but would definitely like to have a warm and comfortable space to live.
Our heating plan is as follows: primary - wood burning stove; secondary: energy efficient heaters (2 of them) in basement, in floor radiant heating in the bathroom and a mini split heat pump upstairs.
https://www.finehomebuilding.com/membership/pdf/7319/021228066.pdf
https://www.finehomebuilding.com/2012/05/17/insulating-a-cathedral-ceiling
You could clean up all the old insulation and install dense pack in all the cavities, and in bags up to the peak of the roof. The installation folks attach a fabric to the rafters to make a space to fill with the dense pack cellulose.
https://www.greenfiber.com/uploads/documents/823201350021PMCathedral-Ceiling-Applications.pdf
This would let you use some of the attic space if you like, and simplify the interior details. (like lighting in the ceiling and use of this space to run communication cables and plumbing for your split systems)
Would not put a vapor barrier in any of the ceiling, but be sure to get six inches of foam outside as soon as possible.
you would end up with a little over R12 from the cellulose, and R33 from six inches of polyiso. R45.
You could clean out the old insulation and run dense pack cellulose in the walls, slanted roof, and all the way up the roof rafters, and then get as much polyiso as you can stand on the outside asap. it would be important not to have an interior vapor barrier anywhere along the roof/ceiling surface, with the foam on the outside.
https://www.greenfiber.com/uploads/documents/823201350021PMCathedral-Ceiling-Applications.pdf
the dense pack folks can install a netting along the bottom of the rafters, and fill with cellulose.
this would let you use the open attic space for utility routing and perhaps some light storage.
I'll second UncleMikes idea for dense pack the complete rafter space, and leave the attic space alone.
Quick sanity check on amount of external roof insulation required to prevent condensation. Need to keep the roof structure dry.
Outside design conditions ( Don't have exact location, I used Peterborough): -26°C
Indoor design condition: 22°C 30% RH
Dew point: 3.6° C (http://www.flycarpet.net/en/PsyOnline)
Temperature Ratio (dew point-outdoor) / (indoor-dew point) = 29.6/18.4 = 1.6
Inside Insulation: Dense pack R value: 3.5/inch x 4 in = R14
Outside insulation required: Inside x Temp Ratio = R14 x 1.6 = 22.4
If less external insulation is used, need to reduce the humidty level on the extreme cold days to prevent condensation. It may be difficult to maintain 30%RH on cold days anyways.
Catmandeux - wow - really appreciate these calculations. Even though I doubt I will ever be able to read that chart I think I understand what you've calculated here. So to confirm - the external insulation R-value that we would need to make up for based on our location's temp ratios and interior R-value is an R value of 22.4 - correct? (We are a little bit colder than Peterborough - closer to the Bancroft region).
Endless thanks to you and UncleMike for your help here. It's so generous and really appreciated by two people who feel moderately in over their heads!! :p
Yes, the external insulation R value is directly proportional to the temperature and dew point ratios. The calculation is a sanity check, to get a estimate of the minimum required. So, if polyiso, at least 4" thick. If XPS, ( the common pink or blue board) at least 5". There is also the relatively new GPS (graphite expanded polystyrene) similar R value to XPS, but not as strong/stiff.
When you decide what the interial insulation plan is, check the ratio again using the actual thickness of the insulated space, and add in the R value for the interior wall/ceiling, and the roof deck for the total inside R. Wood is 1.25 per inch , and 1/2" drywall is .45
Weather data for any location in the world is available here:
http://ashrae-meteo.info/v2.0/
Look under the Stations tab for the nearest weather station. The -26°C design temperature is good for the Bancroft area.
Ok, makes sense. Thanks for this. Will double check once we fully understand the interior insulation plan. Feeling like we are coming to a finalized plan thanks to your help!
Hi catmandeux,
I'm having a hard time understanding your math for temperature ratio and what outdoor dewpoint has to do with it, do you have any links that explain this method in more detail? I would love to understand how to use this method as it seems much quicker than what I did.
I used indoor air conditions (temp and RH) to find the dew point temperature for that (indoor) air, then found where in the wall that temperature would occur. I did this for 7 different configurations:
https://drive.google.com/file/d/1VoX3j1pPsgTU1_LlQdUiuPfvKwQi99gp/view?usp=sharing
Can you take a look at what I put together and let me know if you agree with the logic? The "Charts" tab has some visuals.
If my math and assumptions are good, then I think the three best options are:
1. Add 5 inch XPS on exterior and DO NOT add any dense packed cellulose inside. This will result in a total R value of 27.5.
2. Add 10 inch XPS (Actually 9" and up will do) and add the dense packed cellulose inside (dense packed cellulose not even necessary for moisture prevention purposes though and you can get above R50 without, see option 3). This will result in a total R value of 64.75.
3. Add 10 inch XPS only - NO CELLULOSE. This will result in a total R value of 52.5.
A few other points:
- it appears the R value of polyiso drops substantially (from R6 toR2 at -9.4°C) as temperature drops (http://www.owenscorning.com/NetworkShare/EIS/10019949-FOAMULAR-XPS-vs-Polyiso-Tech-Bulletin.pdf). This would drastically affect temperature profile, which is why I went with XPS for this design.
- Can you comment on my use of 50% RH in the design? I think 30% is way too low (wouldn't it be so uncomfortable that you'd want to run a humidifier? Itchy dry skin...), I went with 50% as it seems extra safe but what are you thoughts on using 40% or 45% as a "middle ground"?
- There's also the added load on the roof to consider. I'm not sure it's significant though. I added a tab to that spreadsheet and considering the snow load of Ottawa (which I believe is ~50psf), the insulation will only increase the max load by ~1% to 5%. This is all assuming the structure was built to (current Ottawa) code.
Sorry for the long post, I'd love to hear any thoughts on this, the google sheets page I linked has comments enabled, please feel free to add any.
Thanks,
Nick.
Hi catmandeux,
I just took another look at your math and now realize that I was mistaken and you never used outdoor dew point temperature.
I understood
"Temperature Ratio (dew point-outdoor) / (indoor-dew point) = 29.6/18.4 = 1.6"
as
"outdoor dew point temp / indoor dew point temp"
but now I see that it is
"(indoor dew point temp -MINUS- outdoor temp) / (indoor temp -MINUS- indoor dew point temp)".
Sorry about that, didn't mean to tell you that you were doing anything wrong :). I'd still love to see how/why this method works if you have links/additional info.
Also, the spreadsheet linked and other comments in my previous post are still valid if you're available to comment.
Thanks,
Nick.
The calculations are just different approaches to the same problem. My quick check is solving "Given X amount of insulation inside the vapour barrier, how much exterior insulation is required to ensure the dew point is outside the vapour barrier". Your spreadsheet is set up to solve "Given a total insulation of Y, what is the temperature profile through the insulation, and is the dewpoint outside the vapour barrier". Both of them are using the ratios of inside, dew point & outside temperture to establish the dew point location in the insulation, but using a slightly different calculation procedure.
The quick check is just that, a sanity check on a design. The spreadsheet is a better design tool, as it is easy to compare multiple options and factors such as cost to come up with the best design.
50% RH at the design temperature will likely have condensation on the windows and doors. Unless the house is extremly well sealed, it would be difficult to get the RH up that high, as any humidity you add ( and all the latent heat) is lost in the air exchange. I think 30%RH is reasonable design value. The RH can be increased at more moderate temperature.
The external insulation will weight less than those shingles that are up there now. The metal roof is also considered slippery, so the effective load on the roof is less than with singles.
Regarding type of insulation, XPS is probably the more cost effective, and readily available. The relatively new to North America GPS insulation board is similar in performace to, and generally less expensive than XPS. It also does not use green house gases in its manufacture if being green is a factor. Could be the topic for another thread.
Hi Mike. Thanks for your sketches above. Helped to clarify what you were saying immensely.
Explored a bit more today by drilling a hole as suggested - looks like the top of the wall does lead to the soffit area - and by removing the concrete between two of the rafters. Looks like there is not only concrete in there but also wood blocking as well. See photos below.
Some additional questions moving with the idea of insulating from the outside:
1) If we are insulating from outside and dense packing the walls & slanted roof do you see a downside of leaving the cellulose that is covering the attic floor currently? We can pull it away from the edges and place baffles there to make sure it doesn't sit up against the roof deck. Or is having insulation along both roof decks from above as well as the attic floor a bad idea? If so we can look into the option of removing it all from the attic floor and using the netting to contain it against the roof deck as suggested.
2) If we do blow in the cellulose into the slanted wall on the 2nd floor and insulate from above, should we try to maintain the air gap using baffles/air holes into the soffits or is that redundant since the double layer of strapping from above will create the roof venting? Basically in simple terms is it a bad idea to have insulation (cellulose), vent gap (w baffle/holes), insulation (rigid foam), air gap (double strapping)?
Additionally, and I probably should have included this above but we are/were hoping to leave the interior boards exposed, repair/caulk the gaps and holes and then paint. Is this insane or should we just give up the idea and go with strapping and drywall instead? We feel fairly attached to the idea so my hope is that you say it's a relatively sane idea :)
On your second photo, with the hole, I want to guess the outside wall board sheathing runs all the way up to meet the roof outside board.
I think planning to leave the boards exposed is a wonderful idea, if it is coupled with the dense pack cellulose, and outside insulation. Talk to whoever you will have install the cellulose. You likely will need to do some repair and caulk to the boards before the insulation is blown in. (so the moving air that carries the cellulose into the cavity does not try to fill the second floor space) You will have a unique and interesting texture on the walls.
You would not need any air gaps and baffles, except as a temporary step to allow for access to install the dense pack cellulose. With the sealing and insulation on the outside, the best use of the space between the rafters would be as full of cellulose. With enough insulation board above, the entire depth of cellulose (and the roof decking above) would be kept above the dew point in winter.
Venting above the insulation board is to allow moisture to escape from the underside of the metal roofing, and from the strapping that secures the foam board to the roof, and anchors the metal roofing.
It does not look like there is that much existing insulation in the flat part of your attic space. if you pull it out, you can remove any insect and rodent leavings, and check all the structure and wiring for any damage. some time with the shop vac might be a good idea. If you leave it in, it would complicate the calculation to verify that you have enough exterior insulation to keep temperature of the interface to the exterior foam above the dew point.
Catmandeux's calculation a few posts ago is based on the ratio of inside insulation to outside insulation, to keep the temperature of the vapor barrier interface at design conditions above the dew point.
While it might seem that more is better, with your situation, you want consistent thickness of cellulose in the envelope. When heating the building, if you have more insulation in the attic, the interface at the inside of the foam boards could end up cooler than the dew point, and cause issues.
Referencing the foam board layer outside the roof deck, more is better. I would shoot for 6 or more inches. You will, of course, have to plan how to fasten the first layer of strapping over the foam to the existing roof deck and rafters. Careful placement of long screws seems like it could work. You will also need to protect the edges of the foam.
The whole discussion on baffles, drilling holes, and air flow within the wall cavity would only make sense if you were planning to add insulation inside the existing slanted wall, and cover with drywall or similar.
Ok this all makes a lot of sense.
We are fairly certain we understand our plan now - I'll write it here just to share:
- we are going to pull off the top board of the wall and the bottom board of the slant (what we have done in the one section in the photos) so we are able to easily extract all of the blown in fiberglass.
- we will remove all the cellulose from the attic floor and repair what is needed accordingly.
- we will apply netting to the roof rafters for the cellulose to be blown in.
- we will repair the boards or caulk depending on when the insulation contractor determines is best in the order of things.
- we will have cellulose blown into the walls, the wall slant, and inside the netted area.
- once spring is here, we will have the roof stripped and apply foam board (thickness tbd according to the calculations above), ensuring we have enough to reach the appropriate R-value and strapping on the vertical and horizontal to allow for venting (as well as ice shield and roof underlayment).
Thank you so much for all of your guidance here. We feel much more confident in this plan and really appreciate the help. We will likely be figuring out our basement insulation plan here also once we get to that and would love for you to weigh in on that also.
Sounds like you have a pretty good plan.
https://www.youtube.com/watch?v=DpxLi_Rm30s
A somewhat related video. I found it looking for an example of trim details for foam boards on the exterior side of a roof deck. They have metal roofing over foam board, and have put spaced strapping on a slant, with gaps between to enable roof venting air flow. the needs of your specific roof panels will dictate spacing and orientation of your strapping.
You will have a few details to handle with the roof, including venting at the peak and at eaves, and covering the end of the foam boards. Might be a good idea to talk with your roofer over the winter to work out the details. (If you will be your own roofer you will enjoy some learning and planning to fill your spare time...)
Inserting six inches of foam (as an example) and two layers of 2x strapping will mean the metal roofing is nine inches further away from the house than it is, (minus the shingles, if you get them out of the way) which will mean the bottom edge of the steel roofing will be further away from the peak than it is now. Since you will be including a vented peak, it is likely you can make this up with space at the top and a wider vented ridge cap.
If my geometry is close, you will end up with something like 13 inches of space at the peak to cover. ( I guessed a 12:12 roof pitch and that 9 inches of a hypotenuse takes equal opposite sides of 6.36 inches)
You might come up with a way to make the bottom end trim vertical, which would then move the roofing up instead of out, but the point is you and/or your roofer will have some planning to do.