Q&A with Shawn McIsaac - True Thermal Performance

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Shawn McIsaac is a Building Enclosure Engineer specialising in the evaluation and design of building window, wall and roof systems on buildings of all types. Shawn prides himself on working with clients through difficult and unique building envelope challenges. After featuring on PrefabNZ Innovation Bites webinar, Shawn has generously answered the audience questions posed below.

1. Is there any literature involved on where to place external joinery in order to block thermal bridging?

Line up the glass with the middle of the insulation. That is going to produce the best result. Even getting the glass aligned with the outside face of the insulation would be good. Check "fp innovations energy efficient building enclosures"

2. The names/link of the 1970' Canadian studies on super-insulated buildings (now passive house) versus thermal mass (bio-climatic?) architecture?

Have a read here.

3. What are the fixing options for standard cladding systems over a continuous external insulation layer. ie weatherboards, profiled metal? Assume these need to be approved to TA satisfaction, provided by insulation/batten/cladding supplier?

Mostly alternative solutions right now (Oculus can do that engineering work for any project). Have a look at "attaching cladding with long screws by RDH" they have a number of different options that work.

4. In what instances would one use the ETICS (External Thermal Insulation Cladding System: Polymer plaster with glass fibre netting embedded over insulation board adhered with thermal screws onto steel or timber studs) and what situations would it be a bad idea?

ETICS or EIFS is a terrible idea over stud framing. It can work (and does really well) over precast or masonry veneer. I've seen a few buildings that have EIFS over studs that have survived either by luck or clever design.

5. Can you point me towards any external insulation options that are more environmentally friendly than PIR/XPS?

Mineral fibre! It's just hot rocks and natural things. Check "Rockwool Cavitymate" available from Forman's in NZ

6. Would European highly insulated thermal break windows be the best choice for NZ if they where available here?

They are available here and maybe. Even the best thermally broken aluminium suite is going to struggle to match a uPVC of fibreglass frame. There are other complications with those types of windows so it's really the best window for the project.

7. Do you recommend using WUFI in the thermal study?

Yes and no. Use it to make sure you haven't completely botched things in the middle of the wall but WUFI 1D won't give you the effective wall value. You'd want to look at WUFI 2D, THERM, Heat 2D, or Flixo.

8. How would a steel structural designers get more knowledge about design for thermal bridging?

Just assume you don't get any thermal performance from a steel stud wall. Abandon that idea now. Insulate to the exterior, continuously, and you'll beat the pants off the timber stud guys.

9. What about curtains instead of multiple glazing? This would be most effective at night time when temperatures drop and views outside are inconsequential. There is no data available on different curtain types as far as i know. It would be good if they could be brought into the insulation standard.

No, stop using curtains to make up for bad windows. These should all be taken away and burned. Canada banned curtains in the 1990's (not banned but frowned upon), as they made everyone's windows mouldy. A curtain blocks the air from the interior from reaching the windows. This makes the frames colder than they would be otherwise, so this increases the condensation on the windows. Yes, you'll be marginally warmer with some curtains but not that much. CCHRC looked at these types of options here.

10. Where do timber windows sit relative to aluminium?

Way better. Really great option for low rise homes, even some mid rise. At higher exposures you might consider a timber window with aluminium skin.

11. How do I calculate/understand the impact of a very high ceiling (compared to a standard 2.4m) in terms of keeping the thermal heat?

Overall you can ignore the effects of stratification for the thermal calculation. In practice, you'd want to mitigate that effect with air flow moving air from top to bottom or bottom to top. A ceiling fan is an easy solution to this.

12. Is low E glass worthwhile in comparison to a thermal break?

Yes, probably. You're going to lose the effectiveness of the low-e if you don't have a thermal break. Like putting a good engine into a car without a steering wheel. Novel, but not that smart.

13. Can it be cheaper to build to the passive house standard given the exemptions for heat pumps in the healthy homes act?

We are finding that yes. The HHA calculation is ok for a standalone house built ~1950's but otherwise seems to either overestimate, underestimate depending how you look at it.

14. Actually we should stop using steel studs for climate change reasons. Upfront carbon emissions waaaaay too high.

I'll toast to that. Timber has only one major flaw, it burns.

15. You dont talk about the detailing of the position of the window within the wall frame. Are the figures you quote regarding spending money on windows rather than wall insulation still relevant with windows detailed in the standard way with windows being on the outside of cladding and outside insulation line?

A little too detailed for the audience but in general yes you can defeat all the savings by being stupid.

16. Interested to know where you got research re comparative return on investment on windows v walls? Always a challenge when speaking with clients- easier if have some evidence-based research to refer to.

I did a quick sample building but you can calculate it! It's really easy actually. Heat loss = U-value X Area or Area/R-value. Add up all your areas, multiply by the U-value of each assembly and you get the relative heat loss. Then you can modifiy the values and see the impact.

17. Retrofitting insulation in an alteration project: adding insulation on the outside (over an existing 90mm timber-framed wall without cavity and a plaster system)? Would this work or any moisture issues created?

Don't guess. It sounds plausible it could work but you'd need to get some details right and do a lot of other things right too or it could all go to custard.

18. Most eco homes seem to utilise concrete floors to maximise free solar heat gain - what is the best tool to include this?

Insulation is the best tool. Thermal mass alone isn't going to help you.

19. So how effective are the strips of thermal break insulation on the outside of steel stud framing? (blue strips EPS) better than nothing? or better to go with a thinner solid sheet continuous over the steel stud (like a R board or FC sheet)?

Modelling seems to show about a 2 degree improvement on the internal faces of the steel studs. Call that a 10% improvement versus a 70% reduction from the steel studs.

20. How do you work with double insulation with plywood or thermal break in the middle?

Good yes. No bridging means you get what you paid for. Very robust durable, moisture resistance is good. If you're going to do precast, do it like this.

21. If thermally broken aluminium windows are used in a project, do they also need to be pushed back to the insulation line to make them the most effective?


22. Thermally broken window frames seem to be quite expensive for what they are. There also appear to be few options on the market. Why haven't thermally broken window frames become the standard (much as per double glazing), and how can NZ encourage the industry to move this way?

Supply and demand. If 90% of your revenue is from non-thermally broken, those 10% cost that much more. Now if 90% is thermally broken, there isn't a cost increase really. I can't see this taking off without governement incentive. I don't think code changes is the right answer, I'd prefer to see a rebate program from EECA to offset the margin cost increase from choosing a better window frame.

23. When using external insulation, what's you experience around fixing through the insulation for cladding etc

Have a look at "attaching cladding with long screws by RDH" they have a number of different options that work

24. Do you see an issue with where the acceptable methods prescribes where the window should sit in the frame? As it appears to almost negates the thermal break.

It does. That detail was created to mitigate water damage that was occuring in homes at the time. Thermal performance wasn't a consideration.

25. Are there any NZ options for Insulated RAB Board such as ZIP R in the US?

A few, Outright insulation, Airdon, Kingspan all make versions of this you could implement

26. I am struggling to find environmentally friendlier alternatives to PIR/XPS for external insulation, can you point me towards any options?

Mineral fibre! It's just hot rocks and natural things. Check "Rockwool Cavitymate" available from Forman's in NZ

27. Is there any literature on where to place the joinery (outside/inside of the frame; timber/steel frame) according to cladding? Or are there a rule of thumb?

Align the glass with the insulation

28. What's your view on mixing insulation types in a wall build up? i.e Kingspan vapour tight insulation on the outside and fibrous insulation inside the framing. Good idea or is it risky from a moisture vapour point of view?

It can be a great idea or a disaster. Rules of thumb, more vapour open materials as you move outwards in the wall.

29. Add straw to the banned list....?

Maybe. I'm never one to ban anything, but somethings have their place. The right tool for the right job. Straw in a convention centre roof? Maybe not in hindsight. Straw or wood fibre in the walls of a house? Probably not that much worse off.

30. Rather than insulating just the external face of a steel column, is it better to insulate around the entire column to reduce thermal bridging?

Just to the exterior side if it's continuous. I can see an argument to wrap the whole thing if it's just localised but you might also want a vapour retarder layer in there too to prevent condensation on the steel.

31. Do you use WUFI? What is your opinion on the effectiveness/accuracy of the modelling this tool provides?

Yes. Very good. It's been calibrated back to real-world physical models. Check "BRANZ SR344"

32. If the 12mm insulation over 89 LGSF studs are replaced with 40-50mm PIR (instead of Pink Batts (or similar), won't THAT be the best solution. With R0.38 -0.68 thermally broken windows off-course.

Yes, that's sounding like a good wall to me. You might actually get a compliant wall from that.

33. Is it correct to assume that you would be better to spend money on up-speccing glass than frames? given their surface area.

Have a play with "". It does depend a little bit on the size and ratio of each window. A ranch slider, maybe glass is better, a bathroom window, definitely the frame. Good LinkedIn content post, I'll fire that one up in the next few weeks.

34. We have had issues with thermally broken sliding doors whereby the internal and external aluminium sections move differentially causing doors to clip each other. Particularly with dark colour aluminium on west faces, Have you come across this before?

No, I've never heard of that. Sounds like the design of that particular door system not thermally broken systems in general.

35. What is your opinion on the thermal performance of Timber window details and what are the key items to consider in detailing them successfully?

Good thermal performance! Ok-ish durability. I'd want to see a sealer or coating of some sort in the glazing pocket and drain holes from the system. Internally glazed with a shimmed butyl tape would be ideal. Water sitting in a wood window isn't going to end well so drained and sealed where water might get. Also maybe not on highly exposed elevations.

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