Hi,

I tought about insulating my foundation by the exterior with medium density polyurethane foam (2.5 inches), both in the under grade and above grade portions. I tought it was a great idea since it would both insulate, provide a vapor, water and freezing protection to the foundation.

Wow, I tought it would be a great and durable way.

The party ended when I red that very interesting Building Science article (see figure 5).

http://www.buildingscience.com/docu...term=vapor

That author think that the thermal bridge is so huge at the brick veener that it would almost be like, on a thermal insulation perspective, if it would not have insulated at all! He said he tried a lot of things over a 25 years frame, but none has provided an efficient solution. 

So, here is my question: don't insulating the sill plate with polyurethane foam on the inside would provide that needed efficient thermal break? Any other solution?

Thank you very much!!

Read this for more/better ideas. 

Your proposal is most-similar to Case 11.  You have lower R, and higher perm rating than Case 11, but read the writeup- the thermal bridging problem isn't solved by interior spray foam on the band joist & sill.

Insulating the sill plate from the interior is necessary, but not sufficient.  It doesn't buy you a whole lot of thermal break unless you add some insulation down the interior face of the concrete as well.   The R-value of concrete is on the order of R1 per foot, and the siding (even brick) is slightly more insulating (but not much) so if you put 1" XPS sheathing on the top half on the interior you'll have a true ~R5 thermal break on that path, while preserving a drying path toward the interior for ground moisture.  But 2" of unfaced EPS (~R8) or 2-2.5" of closed cell spray foam all the way down to the slab would be even better, and using waterproofing below grade on the exterior.  Any time you put foam on the interior, code requires you also install an ignition barrier, so the foam + studwall with unfaced batting gets you fully there while adding R to the stackup.

Note the locations of capillary breaks at the footing/wall and foundation sill/wall in the various stackups in that document- if you don't have them built in you have to work at keeping the drying capacity higher than some of those stackups. Medium density foam perm ratings vary a lot- what specific vendor/product were you looking at?

And what is your climate/location?

Hi Dana,

I'm in zone 4b.

You might be interested to read that Canadian study they made few years ago about different kind of insulation materials on exterior foundation walls:

http://www.nrc-cnrc.gc.ca/eng/ibp/i...s-n36.html

Polyurethane performed best.

The foam product is unique in its ability to protect the footing and direct water past it. It was the only product that showed no evidence of water around the footing. (The board and semi-rigid products simply rest on the footing and are not expected to control water movement in this area.) This finding suggests that when SPF is used, and the footing protected, dampproofing of the concrete is not required, even at the lowest level of the wall.

That being said, I'm very suspicious about the effects of the thermal bridge. It sounds like it "kills" part of the huge benefits of having your foundation on the warm side of the house.

A survey of new home warranty programs across Canada showed that the combined action of water and soils on basements was responsible for most major basement failures in new homes in 1994 and 1995.¹ Frost action on basement walls was cited as a contributing factor in 40% of the failures;

The coldest concrete temperature, measured at 270 mm below grade during the heating season, was about 11°C. Obviously, no freeze/thaw cycles were observed in the concrete. As well, portions of the wall system near grade that would normally experience freeze/thaw conditions were protected from melt or rainwater by the fibre-cement board.

Well, that thermal bridge tells me that the foundation could freeze somewhere and I could lose a significant part of the benefit because the foundation wall gets less exposed to the inside warm (half of the wall has 1" thick of polyurethane foam). Regarding the perm of the specific material, here is their numbers for 50 mm tickness:

37 ng/Pa.s.m² ( 0.65 Perm )

 

Doesn't the study talks about the spf on the OUTside of the wall or am I misreading?

FBBP, indeed. The outside (both above and undergrade) part of the foundation.

This comes as no surprise. Placed on the exterior of the concrete the lower the vapor permeance the better, since there is no drying to the exterior for the sub-grade portions. They only compared SPF to two densities of (far more permeable) EPS, not lower perm XPS, but note in first line of the summary.

"Only small differences were found among the different products in their ability to provide sustained thermal performance — each employs a different water-management strategy. EPS Type 1 was shown to be suitable for application to the exterior of basement walls."

From a COST point of few building up the R with low density Type-I EPS is less than half that of closed cell spray foam.

But exterior foam on the foundtion with a brick-clad wall will always suffer the very significant thermal short, which is why moving to interior foam (and keeping it permeable to allow the foundation to pass ground moisture to the interior, protecting the foundation sill), is a better approach. With other types of cladding, adding foam between the sheathing & siding to make a continuous plane with the exterior foundation foam can work.

Dana, I've red a lot about these very important issues (especially over the long term) and let me suggest what I thing is the best (or least worse, if you prefer methods).

1- Don't finish the basement! I know it's not energy efficient, not comfortable and not practical, but it can avoid you tons of problems.

So I guess every readers will not like that idea. Me neither. So let me tell you what I think is the best solution if you want to insulate and finish your basement AND you live in a cold climate.

- First, be sure that rainwater goes away from your house. That mean good gutters that drain at least 6 feets away from your foundation. That also mean a positive slide soil around your foundation that drains water away at least 4 to 6 feets away too.

- Second, protect your foundation if some water still goes in contact with it. The best material I've found so far is Delta membrane. It's made of plastic, wich is durable, efficient and resistant. It provides you with both a water and damp barrier, so the foundation will be dry. Be sure to apply that ONLY in the sub soil part of your foundation. The above part must be able to dry fast, so the protective coating must have a very high perm rating.

- Third, be sure that your French drain is doing a good job and is properly installed.

- Fourth, use medium density polyurethane from the interior side of your basement and apply a continue coat from the sill plate to the base of your basement wall. A 1.5 to 2.5 thick layer should do a good job. Since on winters, the moisture tend to goes from the inside to to outside, it will provide a barrier. On the summers, it's the opposite. But above soil part will do the job since it's permeable. If you have some water that goes on the sill plate (because of your exterior walls leaks or anything), keep an uninsulated hole in the middle of your sill plate to let it dry fast enough if it gets wet.

- Fifth, have an airtight building. A lot of water vapor comes from air leaks.

- Sixth, let the inside humidity created by occupants goes out fast. That means efficient bathroom fan, dryer fan to the outside, etc.

- Seventh, since your house is airtight, have a good air exchanger and use it especially in winter (when the outside air is dry). In the summers, use it when the exterior is colder than the inside (nights).

- Lastly, try keep the inside moisture level from 35% to 50%. Try to never go higher than 60%. Use a dehumidifier if needed.

I've also tought about using the apron method of outside insulation. It might provide an additionnal drainage layer and protect an additional part of your foundation protected from freeze (40% of foundation degradation is from freeze), but haven't found a lot of documentation to support that. Ask the Builder also recommand using a trench drain, wich is well explained on his website.

Any comments or additional suggestions are welcome.

Insulating the basement and putting in a studwall to carry the fire barrier isn't the same as finishing the basement. It's simply putting the basement within the thermal & pressure envelope of the building, which has many long term benefits. Insulating the basement raises the wintertime temp, lowering the wintertime RH. (Air-sealing an insulated basement reduces summertime RH issues in humid climates as well.)

Insulating just the first-floor joists and leaving the foundation uninsulated just lowers the temp of the basement, raising the average basement RH and puts the joist edges at higher risk of mold/rot.

Not all medium density foams have the same perm rating, nor the same K value. A generic prescription of "...1.5 to 2.5 inches should do a good job..." is highly non-specific to any particular climate, whole house U value, or specific to any a particular design goal. Medium density spray foam isn't cheap either, compared to Type-I or Type-II EPS used in a similar fashion. Some medium density foams can be a good way to air seal and insulate band-joists & foundations to moderate or relatively high R though. For US climate zones 5 & higher there's an economic argument for R20+ if when less expensive methods & materials are used, or about 2x what you'd get out of a couple inches of medium density SPF. (There's also an economic argument for putting R8+ under the basement slab in those zones in new construction if using low-cost EPS.) Whether you finish the basement turning it into LIVING space or not, air sealing & insulating the basement (or crawl space) converting it to CONDITIONED space is "worth it" on a number of levels.

Yes, yes, and yes, on air-sealing, controlling bulk water, controlling interior humidity, ventilation, etc. (Although I prefer HRV/ERV to exhaust-only ventilation schemes in a truly tight house. ) Air sealing is the single most cost effective method of boosting the energy efficiency of the house, while adding durability by reducing moisture-related risks to wooden structures. Don't leave it to chance or make assumptions on new consruction- MEASURE it. Under 2 ACH/50 is good, under 1 ACH/50 is better.

Note, not all climates will have summertime humidity issues related to air leakage, even if much of the eastern US would. Summertime dew points in the Rocky mountains & westward in the US are well below what would raise the humidity of conditioned space air to unhealthy levels. That's not to say there still aren't valid energy, durability and indoor air quality reasons for building tight in those areas.

If you don't handle bulk water issues first, insulating the interior of the foundation with anything that reduces it's drying capacity carries some risk to the foundation sill and band joist. This would include medium density foam & EPS.

Also, even 35% RH is on the high side for January in zones 6 & 7 depending on the wall stackup- controlling it to 30% during cold weather by operating the ventilation under dehumidistat control would be safer for studwalls & sheathing and still comfortable/healthy for humans. Under 25% starts to be less comfortable & less healthy. Holding the line at 60% RH should be measured at the COOLEST part of the house in summer, not the average. In most homes keeping the basement at 60% RH results in ~50-55% RH on the warmer upper floors. Anyone with dust-mite allergies needs to keep bedroom RH below 50% RH to keep mite populations under control.