Heat loss sanity check

[Furnace]

I'm hoping to build a passive house next year. Planning application is due to be submitted in the next few weeks. I have a PHPP consultant in the mix working with the architect.

In the meantime I've been assessing energy requirements and on the basis that the cheapest energy is the one you don't use, I've spent some time in learning where it appears to be worth spending time, money and energy.

I started with Jeremy Harris' spreadsheet to understand the orders of magnitude of the heat losses. Many people have different ways of wanting to look at data and I'm no different, so I wanted to break the losses into discrete areas:

• Walls
• Windows
• Doors
• Roofs
• Air via the MVHR
• Air via infiltration (leakiness)

 

Jeremy's spreadsheet appeared to address air energy losses, but based on an ACH and MVHR efficiency. That doesn't seem right to me. I've modified the sheet (attached to this post) so it has 2 sources of air heat loss - one that uses 30m3/h per person and a heat recovery efficiency ratio (MVHR); and another that uses the air tightness ACH factor and no heat recovery (leakiness).

Using this approach suggests that the air leakiness losses are about equal to the sum of all losses from roof, walls, windows and doors. If my calcs are correct, it would make sense to focus even greater efforts on air tightness. I understand that Justine Bere's recent build had a blower test result of 0.15 - that makes a big difference to the heat loss - far more than speccing better windows, for example.

Does any of that make sense to you folk?

Kind regards

Mark

• Drag files here to attach, or choose files...
•  

•  

• Accepted file types: bmp, svg, jpg, jpeg, gif, tif, pdf, txt, png, odt, mp3, mp4, mov, qt, xls, xlsx, ods, doc, docx
• Max total size: 5.86 MB

Uploaded Files

Heat loss calculator.xlsHeat loss calculator.xlsx

[SteamyTea]

7 minutes ago, Furnace said:

Does any of that make sense to you folk

Does to me.

 

[Furnace]

4 minutes ago, SteamyTea said:

Does to me.

 

I've been following @TerryE's insightful blog and I notice you are a regular contributor, so I take your comment as a glowing endorsement of my findings 😉

[ProDave]

I found Jeremy's spreadsheet very accurate, far more so that the SAP calculations (with the same input figures) arrived at.

 

I thought the idea was just play with the air change rate.  A totally sealed house without mvhr would be 0.  Then add the air change rate of the MVHR to see how much is lost through that.

 

Any further refinement beyond that is unlikely to tell you much.  At the end of the day, you build it as well and as air tight as you can and if you meet the figures given by the estimate then you can be pretty sure you have not made any mistakes.

 

But all very interesting from an academic point of view.

[SteamyTea]

2 minutes ago, Furnace said:

I've been following @TerryE's insightful blog and I notice you are a regular contributor, so I take your comment as a glowing endorsement of my findings 😉

Just make sure you use kW and kWh the right way around.

 

Modeling is very useful and Jeremy initially used what-if scenarios to work out where best to spend money.

 

One think that a lot if people do is overglaze a building. Southern aspects are generally not too much of a problem, but East and especially West facing can be.

Also worth remembering when it comes to overheating that PV can suck up a fair bit of energy. So that is worth modelling.

 

My view is that it is worth setting a target for energy usage, then see if you can achieve it, got to be better that initially setting up all the components and then changing the values to see what happens.

Nay have to have a play with that once the holiday week is over.

 

Also try not to fall into the trap of using just W or kWh per metre squared as the ultimate aim (which is really the same as not initially setting a target).

Volume to energy use on s important, especially if air losses start to dominate.

And keep an eye in the architect, make sure you know of they are talking about total area or habitable area. It can make a big difference.

[Furnace]

6 minutes ago, SteamyTea said:

One think that a lot if people do is overglaze a building.

Yup. The PHPP consultant (he's built his own PH) has been on top of this and advising the architect on that point from the word go. It's very easy to run into overheating problems. We've got some fin walls to block the late summer west sun, and overhangs for the summer midday heat. First cut of the architect's design passed PHPP so we're on a reasonable  footing I think.

10 minutes ago, SteamyTea said:

Also worth remembering when it comes to overheating that PV can suck up a fair bit of energy. So that is worth modelling.

Do you mean the heat effect of having PV on the roof, thus raising the heat gain through the roof?

[SteamyTea]

7 minutes ago, Furnace said:

Do you mean the heat effect of having PV on the roof, thus raising the heat gain through the roof

No, the opposite.

PV at say 20% efficiency at 1000 W/m² input means that only 800 W/m² can get through to heat the roof up.

 

So you can get away with a bit less insulation.

Edited December 19, 2022 by SteamyTea

[Furnace]

27 minutes ago, ProDave said:

I thought the idea was just play with the air change rate.  A totally sealed house without mvhr would be 0.  Then add the air change rate of the MVHR to see how much is lost through that.

I don't know the history of the calcs, but the issue I had was that it only calculated the losses from MVHR (and it used air change rate rather than an amount per occupant). By far the bigger energy loss is from air leakiness since there is no heat recovery. In my example leakiness is 10 times the heat loss of the MVHR. The numbers suggest an uncompromising approach to air tightness is hugely beneficial compared to tweaking a window's U-value

Cheers

[JohnMo]

You've used your 50 pa ach as your infiltration rate, but it's all not. It should about a 1/4 of that.  You are basically adding heat loss.

 

I would be careful with your form factor, i.e. you seem to have a big wall/roof area compared to floor area. So the likelihood of meeting passivhaus specs becomes more difficult.

 

As said the spreadsheet as is pretty much spot on.  At -9 the daily heat losses is only a couple of kWh out.

[Furnace]

6 minutes ago, SteamyTea said:

No, the opposite.

PV at say 20% efficiency at 1000 W/m² input means that only 800 W/m² can get through to heat the roof up.

 

So you can get away with a bit less insulation.

That's an excellent point. Safari Landrovers had a second "skin" over the roof with an air gap beneath to prevent the occupants being baked. I guess it therefore depends if they're "in" or "on" roof panels.
Cheers

[Furnace]

3 minutes ago, JohnMo said:

You've used your 50 pa ach as your infiltration rate, but it's all not. It should about a 1/4 of that.  You are basically adding heat loss.

Thanks for that. Is the 1/4 factor used to simulate "typical" conditions for infiltration?

[ProDave]

3 minutes ago, Furnace said:

I don't know the history of the calcs, but the issue I had was that it only calculated the losses from MVHR (and it used air change rate rather than an amount per occupant). By far the bigger energy loss is from air leakiness since there is no heat recovery. In my example leakiness is 10 times the heat loss of the MVHR. The numbers suggest an uncompromising approach to air tightness is hugely beneficial compared to tweaking a window's U-value

Cheers

Jeremy created the spreadsheet for modelling his own build and then shared it with the forum.  Quite a lot of people have used it since.

 

I built my house as well as I could, but only got an official air test right at the end.  I was disappointed with the result of 1.4 yet the tester was excited as it was the best he has ever tested.  You might think 1.4 would make my heat loss high?  Well in practice the heat i put into the house closely matches the results from the spreadsheet.  You will only get the "leakage" air loss when it is blowing a gale, and in any event the air is likely to take the easy route, through the fan assisted mvhr unit and that is certainly how it appears to be.

 

If you built the house with just trickle vents or other leaks, you can never say for certain what the air change rate is as it will depend a lot on where the leaks are, and how strong and even which way the wind is blowing.  

 

The lesson is do the best you can for air tightness. Just one of many aspects to detail properly.

[SteamyTea]

5 minutes ago, Furnace said:

guess it therefore depends if they're "in" or "on" roof panels

Not really, the efficiency is not affected that much.

In roof an also change the SHC and the thermal inertia times.

[Furnace]

1 hour ago, ProDave said:

Jeremy created the spreadsheet for modelling his own build and then shared it with the forum.  Quite a lot of people have used it since.

 

I built my house as well as I could, but only got an official air test right at the end.  I was disappointed with the result of 1.4 yet the tester was excited as it was the best he has ever tested.  You might think 1.4 would make my heat loss high?  Well in practice the heat i put into the house closely matches the results from the spreadsheet.  You will only get the "leakage" air loss when it is blowing a gale, and in any event the air is likely to take the easy route, through the fan assisted mvhr unit and that is certainly how it appears to be.

 

If you built the house with just trickle vents or other leaks, you can never say for certain what the air change rate is as it will depend a lot on where the leaks are, and how strong and even which way the wind is blowing.  

 

The lesson is do the best you can for air tightness. Just one of many aspects to detail properly.

That's good to know. I was looking at worst case losses, but can certainly see that there are many variables that can influence the leakage.
Cheers

 

[Iceverge]

I went on a similar voyage of mathematical discovery myself. 

 

Airtightness is the big 'un so to speak. Nothing else makes as much difference. Also it's cheap if planned properly. 

[Furnace]

14 hours ago, SteamyTea said:

Not really, the efficiency is not affected that much.

In roof an also change the SHC and the thermal inertia times.

I was thinking of the shading affect the panels have on the roof fabric. A panel mounted proud of the roof would transmit less heat to the roof compared to an "in roof" setup. I'll speak to the PHPP chap and see how all this might affect the calcs. Much of the roof will be panels, so it could well be a meaningful factor. Thanks for mentioning it.
Cheers

[TerryE]

Some post build improvements try to make a house airtight without bothering with MVHR.  Big mistake IMO, as the net result is a stuffy / smelly environment.  The V in MVHR is very important for occupants' comfort: having a fresh environment without bleeding heat loss. 

 

I did my own version of J's spreadsheet, but the results are very much the same.  All of the material losses are the big ticket items and something like the PHPP calculator just adds a huge amount of complexity on immaterial detail IMO.

 

The devil is in the detail, especially in avoiding thermal bridging flaws in design, and quality cockups in construction e.g. insulation missing or badly installed; lack of attention to detail in air-tightness sealing.  I am delighted with the overall characteristics of my MBC house: the build is performant by design and in detail of construction: e.g the passive slab with UFH in the slab; the larson strut twin-wall with cellulosic filler; ... the whole build was low energy by design, and in construction detail.  MBC has some worthy competitors, and there are other build approaches, so it's very much your choice.  However, I would emphasise that you are better off with a construction approach that is low energy by design rather than a convention build with some attempts to tweak it to get this goal as an afterthought.    Also having the high thermal capacity internal to the insulated envelope (e.g. the warm slab approach) gives an extremely stable environment. 

 

We've just had our 5th anniversary since moving in and overall and we remain delighted at how well our house performs as a lived-in environment and how it has met all of our expectations.

[SteamyTea]

15 minutes ago, Furnace said:

A panel mounted proud of the roof would transmit less heat to the roof compared to an "in roof" setup

There is still an air gap for cooling with inroof systems.

 

And they look so much better.  When I went to see Jeremy's house, even though I knew it had 6 kW_p of modules on the roof, I did not notice them at all. 

 

Building on what Terry has said, factory built houses are much better.  You would not buy a new car if it was built on your drive.  Or a fridge that came in bits.

 

I do find the whole airtightness detailing a bit strange.  Almost as if the industry think it is an afterthought.  It as to be designed in like everything else.  The production manager at Frame UK (company that make hundreds of TF houses down in Redruth) said an interesting thing to me when he took me around the factory.

"We design in all the services, then the plumbers come along and drill holes tough the walls"

Now I suspect your Architect is not that up on site work practices, so it comes down to you to enforce the design ethics and the quality control.  Not a nice task as it can easily alienate a crew (I know how hard it is to change work practices), but it has to be done right, there is, realistically, no second chance.

[Furnace]

6 minutes ago, TerryE said:

I did my own version of J's spreadsheet, but the results are very much the same.  All of the material losses are the big ticket items and something like the PHPP calculator just adds a huge amount of complexity on immaterial detail IMO.

Thanks for that Terry. As far as energy losses are concerned, I also think a pretty simple spreadsheet covers the major contributors without getting into too much detail. The only modification I've made to J's approach is to include the infiltration (leakiness) losses, however these are very dependent on wind speed, direction, orientation on the site so I'm not sure how useful it is.  However, addressing leakiness at build time can only help and is very cost effective compared to speccing ultra-low U-value glazing, for example

I believe the area where PHPP will help me is in managing overheating risks. I'm inclined to go for certification too, partly to have as a declaration to motivate all those involved in the design and build.

I look forward to picking your capacious brain in the future.

Cheers

[Furnace]

3 minutes ago, SteamyTea said:

Building on what Terry has said, factory built houses are much better.  You would not buy a new car if it was built on your drive.  Or a fridge that came in bits.

I'm completely sold on a modular build. Building sites are not generally the place to execute precision work that cannot be easily remedied if substandard. Given the significant risk to a successful build by "site error", I'm very willing to pay a premium to largely remove this risk. A package that includes slab plus frame is my current thinking.

I'm going to have "Air Tightness Hero" T-shirts printed to motivate anyone on site, and will be here all the time anyway as I live here.

Thanks so much for your input. I'll be back for more for sure.

Cheers

 

[Iceverge]

51 minutes ago, SteamyTea said:

factory built houses are much better

 

Maybe so but if you've got hatchet men erecting them you can end up with a ball of chalk too. 

 

I've heard of TF kits being 100mm out of plumb over 10m during the construction boom here in the 2000's. Similarly on a TF site I saw the crew had torn the airtightness membrane in several places. Maybe it was going to be patched up later but it didn't bode well. 

[TerryE]

1 hour ago, SteamyTea said:

"We design in all the services, then the plumbers come along and drill holes through the walls"

 

One of the things that we did on our build was to ban all trades from penetrating the external airtightness membrane.  I did all of the internal joinery / carpentry, kitchen and white goods fitting myself, and Jan and I did all of the plumbing between us.  We had electrics, BT, satellite and a propane backup rings, but we discussed all breeches needed with the tradesmen and we put in the necessary conduit / ducting ourselves, doing all of the air-tightness sealing.  It was then just a matter of adding foam fill and silicon caps to all ducts before doing the air-tightness test.   

 

Incidentally, we got 0.6 on the first run, and I had forgotten to do the final window adjustment.  (The tester was amazed.)  About a month later, I went around on a really windy day and found that ~5 windows and one door needed the seal stops adjusting, so I suspect that our current actual is a bit better than the 0.6 test result.

[Furnace]

6 hours ago, TerryE said:

we discussed all breeches needed with the tradesmen and we put in the necessary conduit / ducting ourselves, doing all of the air-tightness sealing

I think that's a very wise approach.

[Kelvin]

I don’t think I’d bother getting air tightness hero t-shirts printed up if you’re expecting the trades that visit your site to wear them unless it’s just for you. Just explain to the various trades what the goal is, what to do and what not to do. Put some signs up referencing air tightness. Brief every new face to site personally and have someone else on point to brief people if you aren’t around. 
 

Create a no blame culture so that if the barrier inevitably gets breeched it gets highlighted straight away and rectified. Have plenty of fixing material available and readily accessible. Make up some notification sheets for each trade and hang them in a visible location probably where the drawings are and encourage the trades to document any breeches and whether it was rectified or not. Do a visual inspection every day and double-check any areas before they are boxed in. 

[Iceverge]

50 minutes ago, Kelvin said:

 

Create a no blame culture

 

 

Very important this. People will only hide things if they think there's a b*llocking coming and in fairness nearly everyone is very diligent. 

 

" Do your best and if there's any holes let me know and I'll sort them out". 

 

We had none in the end AFAIK apart from when I put my foot through it from the attic.