Can I ask - what's the best energy calculation treatment for basements?

[Alan Ambrose]

As per title  - I'm initially looking at Mr Harris' spreadsheet, but also curious about SAP / PHPP treatment?

 

I can see that PHPP talks about unheated vs. heated basements. In practice, I'll probably lay UFH pipes but only use them if we feel the need.

 

For unheated, am I right in thinking that there's an assumption of a constant heat loss through the ground floor just like the non-basement case?

 

Maybe, total basement wall and floor area x constant heat flow per m^2 assuming inside & ground temperatures are fixed?

 

What does that all do to 'form factor' - does it all fall apart in the case of house + basement? I think 'form factor' sort of implicitly assumes that floor, walls & roof have the same temperature gradient? 

[SteamyTea]

1 hour ago, Alan Ambrose said:

What does that all do to 'form factor'

Is form Factor the ratio of volume to area?

Edited November 28, 2023 by SteamyTea

[Alan Ambrose]

PHPP form factor apparently is total outside surface area (floor, roof, walls etc) / treated floor area (i.e. sum of internal area of rooms, not including internal walls etc)

[Alan Ambrose]

Also, what's the best treatment for unheated spaces like lobbies?

[Gus Potter]

9 hours ago, Alan Ambrose said:

What does that all do to 'form factor' - does it all fall apart in the case of house + basement? I think 'form factor' sort of implicitly assumes that floor, walls & roof have the same temperature gradient? 

Jeremy did a great spreadsheet, but it it a preliminary assessment tool.

 

Basements are a different animal.

 

Take a basement that is cut and filled into sandy soil, drained all round the outside, a type C drained protection, the mobile ground water is kept clear of the insulated walls. You can maybe with a bit of caution / pragmatism treat the walls as part of the floor slab.. there is "but" later.

 

But if you have ground water swilling around the outside of your walls it will tend to cool them much more.

 

The form factor is only applicable to ground bearing floor slabs, could be your basement slab.. but not the walls. I may have an old version of Jeremy's sheet? On ground bearing floor slabs an input value is the exposed perimeter (P) vs the area of the slab (A)... P/A ratio. Basically the larger the slab the less exposed edge area, the P/A ratio drops which means it performs better... that is why on big industrial buildings sometimes in the past we only put insulation round the edges and down sides.

 

If you are doing preliminary pricing / conceptual calcs I would add 25 -30% to the insulation in the basement walls until you know much more about the ground (type of soil) and ground water flow.

 

Here is a good link that give some U - values of typical soils. When doing detailed calcs these need to be converted back into thermal resistance, an R value. But these values don't account for mobile ground water.

 

https://polyfoamxps.co.uk/what-u-value-is-required-from-a-ground-floor-construction/

 

Once you get a handle on these bits an bobs you can then decide how much heating you want to install in a basement.

 

8 hours ago, Alan Ambrose said:

Also, what's the best treatment for unheated spaces like lobbies?

How big is the lobby, if just a few m sq then probably none but if larger who wants to come out a room and be hit with icy air.

 

In some ways if it's an entrance to your house I would want some flexibility here to heat it plenty at times.. when your visitors come in from the cold they want a bit of .. hey it's nice and warm here while they take off their coats and hats. First impressions matter.

[Conor]

Can't remember how our architect modelled our basement but the heatlosses are far less than predicted. I think good drainage around and under plays a big part I'll dig out the report so you know how not to do it.

[SteamyTea]

What you don't want to happen with flowing water is the possibility of it freezing.

Liquid water has a conductivity of around 0.55 W/m.K, ice, 2.25 W/m.K.

But the real kicker is the latent heat of fusion at 334 kJ/kg (liquid is 4.18 kJ/kg.K).  That will sap the heat out of a basement.

 

Really just a case of adding more thickness of insulation, or making an underground dam to divert flowing water away from the basement.

I worked on a simple project in North London, increase the size of the basement swimming pool. The main contractor found that there was an underground river that was filling the pool.  They dug up the road upstream of the building, then poured thousands of tonnes of concrete into the excavated channel.

Not sure if they worried too much about where the water was diverted to, but it stopped the hotel swimming pool being filled.

 

Bit about Frost Line

https://nhbc-standards.co.uk/4-foundations/4-3-strip-and-trench-fill-foundations/4-3-3-ground-conditions/

Edited November 29, 2023 by SteamyTea

[Alan Ambrose]

OK thanks all, much appreciated and thanks for the pointers, links and considerations. Like a lot of subjects, the answer appears to be 'it's complicated'

 

BTW Jeremy's spreadsheet just has floor area x U-value and assumes steady state ground and inside temperature. There's no allowance for perimeter losses and it comes up with a constant heat loss rate over time.

 

So, in an ideal world, I should model the ground conditions, ground water presence & flow, temperature fluctuations at various depths and seasons and the full thermal properties of the basement construction 

I'm just delving into PHPP modelling and I see they have some resources and calculation methods here:

 

https://passipedia.org/basics/building_physics_-_basics/thermal_bridges/tbcalculation/examples/heatedb

https://passipedia.org/basics/building_physics_-_basics/thermal_bridges/tbcalculation/examples/unheatedb

 

But I was hoping for an easy answer....

 

I'll post back here with any progress I make.

 

 

[Thorfun]

I did quite a bit of research into basement u-value calculations as the surrounding earth means insulation levels don’t need to be as thick to get the same u-values. But it was so long ago I can’t remember the formula!

 

the information was on here though in a post/thread of mine. You can try searching by author and if I manage to find it I’ll post it up for you. 

[Alan Ambrose]

OK will look for it, thanks.

 

>>> But it was so long ago I can’t remember the formula!

 

If I could remember even half of what I used to know, I would really be quite smart

 

 

[Alan Ambrose]

OK I've found the answer (not really). But PHPP does have a bunch of calcs for:

 

+ standard slab

+ unheated basement (i.e. basement outside thermal envelope)

+ heated basement (i.e. basement inside thermal envelope with interior temperature assumed to be the same as the rest of the building)

+ US-style suspended floor above crawl space

+ some way of coping with combinations of these.

 

They include ... adjustment for soil type / perimeter losses / ground water depth & flow / seasonal ground temperature and delay!

I'm not sure how anyone measures ground water flow rate in the typical soil investigation?

 

Exactly what methods PHPP uses I have not determined yet. But if you accept it as a black box, then it would seem to do most of the necessary calcs.

I'm not sure yet if there's a workaround for 'partly heated basement'.

 

[Alan Ambrose]

>>> ground water depth & flow

 

It's also struck me that if you have reasonable insulation outside the basement RC and a membrane external to that - then the ground water effect should be minimal.

 

[AliG]

Can you just use Jeremy's calculator, but assume a higher outside temperature.

 

Heat loss is driven by the difference between outside and inside temperature, so assuming 20C inside, the heat loss at 0C is roughly double the loss at 10C.

 

In practice heating costs go up even more as at temperatures above 10C thereat generated by solar insolence, people and electrics can generate much of the energy required to heat a space.

 

The figures from Jeremy's calculator have proven pretty accurate for our house, I even used it to guess our pool's energy consumption and for my parent's house.

 

The underground temperature is considered to be pretty constant at around 8C. Near to the ground ti varies more, but under a basement it would hardly vary at all.

 

I suspect if you calculated using Jeremy's calculator and a constant 8C outside temp you would get pretty close. You might also want to adjust for the fact there is no roof, above it will be a heated space. You could just set the roof size to  as if the temperature above is the same or higher there will be no heat losses up the way.

 

All this would of course go out of the window if running water is drawing away the heat.

 

 

 

[Thorfun]

On 30/11/2023 at 12:33, Alan Ambrose said:

OK will look for it, thanks.

 

>>> But it was so long ago I can’t remember the formula!

 

If I could remember even half of what I used to know, I would really be quite smart

 

 

btw, quoting people the way you do doesn't notify them that they've been quoted! 😉 

 

that's why I've only just seen this post.

[Thorfun]

On 30/11/2023 at 12:33, Alan Ambrose said:

OK will look for it, thanks.

 

>>> But it was so long ago I can’t remember the formula!

 

If I could remember even half of what I used to know, I would really be quite smart

 

 

here's the link to my thread

 

 

has some great information from @A_L in there that aided me in my calculations.

[Alan Ambrose]

>>> btw, quoting people the way you do doesn't notify them that they've been quoted! 😉

 

@Thorfun - ah yes I do that when I want to actively avoid bothering someone - but then if they come back to the thread they can answer if they want.

 

Thanks for the thread reminder - I had seen it but will read it through again and make sure I understand everything this time

 

 

[Thorfun]

7 minutes ago, Alan Ambrose said:

>>> btw, quoting people the way you do doesn't notify them that they've been quoted! 😉

 

@Thorfun - ah yes I do that when I want to actively avoid bothering someone - but then if they come back to the thread they can answer if they want.

 

Thanks for the thread reminder - I had seen it but will read it through again and make sure I understand everything this time

 

 

No bother for me. Can just ignore the notification! 😂