Help confirming exterior rockwool insulation amount
I am renovating an old, post-and-beam house that sits on concrete piers, and has no insulation nor sheathing. Summers are humid and fairly warm with temperatures between 23C/71F and 31C/88F. Average winter temperatures are between -5C/23F and 3C/37.4F with lots of snow. At night and morning, temperatures once in a while dip down to at most -15C/5F. I think this would be similar to US climate zone 5 (I’m in Japan).
My question concerns insulation exterior/interior ratio.
I will be adding plank sheathing with a mechanically fastened WRB rated for weather and air sealing (Wütop Trio UV 2SK). After that, 4 inches of rigid rockwool insulation on the exterior, a rain screen, then wood cladding. On the interior another 4 inches of the same rigid rockwool in the wall cavity between the posts (which are about 4 inches). That should give me total 33.6 R-value.
According to this article (https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing), the above amount of exterior insulation is for climates more extreme than mine.
However, my calculations (probably mistaken) indicate that the wall sheathing temperature will reach the dew point during the winter.
E.g. 72F inside 45% humidity = 49.5F dew point.
With R-16.8 on the outside, and with an average winter temperature of 23, the sheathing temperature would be:
47.5 = 23 + [(16.8 / 33.6) x (72 – 23)]
And of course, on those even colder nights, the effect would be more pronounced.
My questions are:
1. Is my calculation correct?
2. If so, would 4 inches of continuous exterior rockwool, and no interior insulation, be enough for this sort of climate? I.e. go full PERSIST technique and avoid condensation problems completely. I will be heating with a wood stove and would like to get it to a “Pretty Good House” level (floors that aren’t cold especially). I will be insulating the floor and roof from the outside as well (i.e. it will be a warm roof).
3. Alternatively, given that the wall will be completely vapor open (can dry in both directions), and I will use plank sheathing which is not damaged by water so easily, is this amount of condensation not worth worrying about?
Regards,
Stephen
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I think your interior humidity in your calculation is too high. You would have to humidify to get it that high in 23 degree weather.
https://www.greenbuildingadvisor.com/article/are-dew-point-calculations-really-necessary
Also considering the (edit) vapor openness of your assembly I would certainly not worry about it.
I agree, but I think that both temperature and humidity are too high. We generally design for 70 degrees, but with a very well insulated house, 67-68 degrees is often warm enough for comfort. Your comfort is related to the mean radiant temperature of the walls and floors. Increased insulation and very good windows will increase the mean radiant temperature, allowing lower thermostat settings for good comfort. Your very vapor-open assembly should have no problems, even with minor condensation in the coldest weather.
Stephen,
You don't need to worry about the ratios if the exterior insulation is permeable. From Martin's article https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
"What if I want to install exterior mineral wool?
Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"
Thanks so much for your responses. Yes, I realize the humidity level is high. I was basing it on the house I’m in now which is quite small, and has a wood stove with kettles on the top so they tend to humidify the room quite a bit (usually around 40% but sometimes higher).
What is the best way to calculate how much heating I will need to heat this house?
I was originally thinking of adding in-floor radiant heating since the wife hates cold feet, but I’m wondering if a smallish 6kW wood stove would be enough to keep the rooms and floor warm.
For reference, I will be putting in about the same amount of insulation in the floor and (2nd floor vaulted) roof as in the walls, and all windows will be replaced with double-glazed units. Floor space is 1432 ft2 (786 1st floor, 646 2nd floor).
Here is a simple UA calculator from western Washington university:
https://www.energy.wsu.edu/Documents/2018%20Heating%20System%20Sizing%20Worksheet~20230215.xlsx
Software like coolcalcs should also give you a pretty close manual j:
https://www.coolcalc.com/
You could also download beopt for a more comprehensive energy usage:
https://www.energy.gov/eere/buildings/building-energy-optimization-beopt-software
I’d start there, if you’re unsure use all three and see if your numbers get close. There is a bit of junk in junk out, but they are pretty simple calcs and should be about the same from all of them, if there is a large disparity then maybe engage someone to run your calcs
if you have the local ambient temperature over the year you can do a rough calculation yourself.
Excel will be your friend..
- all areas to the "outside" with the associated R/U values. It is a bit of guesswork if it comes to ground temperature. It will give you a "whole house" number - not room by room - but it a starting point.
- ubakus.net can also do a "whole house" calculation. It has an english option, you define first the walls etc. and then the house. The tricky part is to tell the programm your climate.
- some air exchange assumption
With such a thick layer of exterior insulation being installed I would suggest looking into an exterior clip system like Cascadia clips to anchor the cladding to.
https://www.cascadiawindows.com/products/cascadia-clip
For cold floor issues, how is the floor built now? Can you add a layer of rigid insulation on top of the floor boards and then another layer of OSB and then the finish flooring?
Are you planning on having a blower door test done before you put up any interior drywall or wood paneling?
Thank you everyone for your responses and links to calculation tools and suggestions. Lots to look through and learn.
rockies63, regarding the floor, it is currently just floor boards directly on joists (no subfloor) and I will replace it completely. I will add 50mm rigid rockwool under all sleepers, 100mm between sleepers, then 50-70mm between joists, taped WRB for an air barrier, then a 12mm diagonal plank subfloor, then 12-24mm flooring.
It will be built in the Japanese style so you have 100mm sleepers then 50-70mm joists across that. Without the topmost joists, it looks like the attached picture.
> With such a thick layer of exterior insulation being installed I would suggest looking into an exterior clip system like Cascadia clips to anchor the cladding to.
I was under the impression that long screws through the rainscreen battens would be enough. However, I would be interested to know more about the risks of this approach.
In Japan there is a type of rigid rockwool board that is sandwiched between two layers of wood wool cement board which you can stack between regular rockwool boards to avoid compression and is supposed to help with heavy cladding. It looks like the attached picture.
Regarding a blower door test, yes, I would like to, if I can find someone to do it.
That picture looks awful lot like a crawlspace. Is that gap underneath open to the outdoors or inside the house?
As for the exterior 4" rigid, it will be somewhat squishy so best to use heavier strapping (2x equivalent so around 40mm thick. Wood cladding is light weight no issue with long screws supporting it. The challenge with any exterior rigid MW install is getting the strapping flat, you have to adjust every screw as MW is still somewhat squishy, for that those MW panels with the facer would definitely help.
Another option is to install horizontal strapping (ie 2x4 equivalent) on edge and install regular MW batts between them on the outside. Essentially an inside out Mooney wall. Depending on the siding type, some can be nailed right to this or strap it out if you need different orientation. This gets you pretty much the 95% effectiveness of continuous insulation without any of the cost premium of the rigid MW or dealing with long screws. I've done this for an over-roof and was pretty simple.
Akos, thank you for the advice. The picture I attached is something I found online to show the sleepers/insulation arrangement I plan to follow and doesn't illustrate my own situation. In my case, I don't have a foundation like that, but rather the house is on pier foundations in soil, so I have a crawlspace that is completely open to the outdoors.
Regarding the strapping with insulation between, thank you for the suggestion. I'm planning to do that for the roof (but oriented in line with the roof pitch), so it is good to hear that others have done so. However, the only unfaced rockwool that I have been able to find here is the rigid board type, so for the walls I thought I might as well go for the continuous insulation design since there are no cost savings that I could get from using cheaper fluffier rockwool.
Given the crawlspace and amount of insulation in my floor, would you say in-floor heating would be necessary to keep the floor temperature at around 72?
I found this comment on GBA which makes me wonder whether non-cold floors without floor heating is realistic: https://www.greenbuildingadvisor.com/article/radiant-floor-heating#comment-225132
StephenN,
Apples and oranges, as that discussion is about slab temperatures, but even so, recent discussions on well insulated slabs with no radiant heat had posters measure their temperatures, and they tracked that of the room above to within three degrees.
Malcolm, thank you for the pointer. Generally speaking, with a floor that is completely exposed (as mine will be), how much more insulation (compared to an insulated slab or sealed crawl space construction) would you say is necessary to keep the floor tracking the room temperature?
Concerning this, I came across a comment by Akos here (https://www.greenbuildingadvisor.com/question/cold-floor-insulation#comment-142834) which seems to suggest it is difficult to do so without floor heating.