Timber Frame Pros Cons

[puntloos]

3 hours ago, Patrick said:

?OK, originally this was about TF pros and cons,

 

 

Ha, I might actually start a new thread on that one, but honestly I think 'heat management' is one of the #1 subjects out there when it comes to major failures of a house design, so regardless of where this is being worked out I am interested in the discussion. 

 

As an engineer in my former life I understand some of these words, but frankly the "actual implications" so far elude me. It's clear(?) that:
 

- Solar gain is a major issue, so you better have some way from stopping the sun even entering your house

- TF-based houses can have the same, or very similar characteristics.

- If you want to create a large amount of heat (or cold) storage, effectively increasing the heat inertia (am I making that term up?) of your house, you can use "cellulose based" as well as "silicon based" to great effect, I can't tell who the ultimateabsolute winner is, but you can have good results with either.

 

Fun fact, I spoke to a structural engineer yesterday at "Don't Move, Improve" and they suggested Cross Laminated Timber for extra structural rigidity, although they suspected it would be harder to get as a self-builder.. Thoughts?

[Patrick]

@puntloos

When it comes to LVL and CLt and other forms of engineered timber, it is best to look abroad for examples. UK  is not exactly a place for timber innovations nor are the professionals over here very knowledgable on average (I'm sure there are exceptions). Austria is a fabulous place for any engineered timber products. They building stadiums with it.

[Jeremy Harris]

16 minutes ago, Sensus said:

 

But that's not what you said: you said that thermal mass is a myth. Period. That it does not exist, that it has no units by which it can be measured.

 

For someone who has spent the last several pages telling us how important it is to be careful with units and terminology, your revised statement is rather a large departure. ?

 

 

Not at all.  I stand by what I wrote all those years ago;  "thermal mass" is a myth.  It has no meaningful units of measurement that are internationally recognised (those quoted so far are just units for heat capacity, rearranged, nothing more).

 

When someone can come up with a meaningful way of measuring "thermal mass" using internationally recognised SI units, that makes coherent sense, then I am more than happy to consider reviewing that old post from 2016.

 

Right now, it's very easy to pick apart the arguments put forward for using the term, as it seems to mean different things to different people.  For example, many assume that more mass in the structure = more thermal mass.  The comparison between silica aerogel and concrete disproves this.  SAP uses the term as an analogy for heat capacity, which begs the question as to why SAP didn't just stick to using heat capacity, as it is units of heat capacity that are being used to denote "thermal mass" in their definition.  It doesn't seem to make any sense to invent a new term to describe something that has been understood and rationalised into our system of units for a century or more.

 

On a positive note, I would really like to see someone come up with a meaningful term to help describe thermal stability.  I believe that many think that adding mass to a structure increases thermal stability, when in reality it's more important to add heat capacity and adequate thermal conductivity to materials inside the structure to enhance thermal stability.  It is most probable that volume and surface area, rather than mass, are the most important factors after heat capacity and thermal conductivity in this regard. 

 

 

 

[MikeSharp01]

33 minutes ago, Sensus said:

factor know as thermal mass, which you have very clearly asserted is a myth, and has no units.

 

Does it have units then? Or A. is it mascerading as something else that got the units already or B. An amalgum of units (ie other concepts combined) that could be simplified to something else that has units or C. Is it just a non unitised concept used a simple hook on which to hang more complex thinking / discourse. I guess that, in the end, the words are the things that resonate with people as most people understand the ideas of Thermal and Mass, which they probably think of as 'a mass' rather than the fundamental unit 'mass'. This may be at the root of our comprehension problem as the actual unit mass of something tells you little, if anything, about its ability to store heat until you combine it with its heat capacity per unit mass. So one is forced back on the presumption that while thermal mass might be a useful idea in facilitating a focus / context for a problem it is probably not useful in providing calculated solutions in building physics.

 

 

[Ferdinand]

I do so enjoy having 3 sports to watch on a Sunday :-).

 

Cricket, F1 and Buildhub Debate by people more knowledgeable than I, which is also a learning experience.

 

Off now to apply the next layer of paint to the car bumper.

 

 

 

.

Edited September 8, 2019 by Ferdinand

[ToughButterCup]

Conscious that some may prefer a slightly less detailed treatment of the same concept, can I draw attention to this thread (2016) ? 

 

[Jeremy Harris]

30 minutes ago, K78 said:

 

I don’t pretend to understand this nearly as much as you. 

 

A concrete block house with EWI was always the example I thought of. Surely the concrete blocks retain heat better than a timberframe with EWI? The storage heater comparison always made sense to my simple mind. 

 

 

It depends on how you define "retain heat".  If you use mass and heat energy, then timber has a specific heat capacity of about 1.8 J· g^-1·K and concrete has a specific heat capacity of about 0.88 J· g^-1·K, so timber holds about twice as much heat for a given mass as concrete.

 

If you ignore the "thermal mass" thing, and switch to thinking in terms of "thermal volume" (I made that up, just to match!), then things change the other way.  Timber has a volumetric heat capacity of about 935 kJ/ m²·K, whereas concrete has a volumetric heat capacity of about 2,112 kJ/m²·K

[puntloos]

20 minutes ago, Sensus said:

 

That's definitely one for a thread on its own, I'd have thought!

 

I used to be Design & Tech Director for a timber frame company that designed and supplied buildings in CLT, SIPs and conventional timber frame, and whilst I know all the arguments for and against CLT (yes, structural rigidity is one of the 'pluses'), I've yet to come across a situation where there was a clear cut case in favour of its use.

 

There seems always to have to be an element of 'we want to use CLT because we want to use CLT'.

 

Although to be fair, I guess you could probably say that for many other forms of construction, in many cases...

 Sorted.  would love to hear your opinion!

[Jeremy Harris]

7 minutes ago, Sensus said:

 

Well, I've quoted it several times already, from the same website you yourself are happy to quote from:

 

SAP uses the kappa (k) value to determine thermal mass. 'k' is the measure of the heat capacity per unit area in kJ/m²K of the 'thermally active' part of the construction element:

k = 10^-6 Σ_i p_i c_i d_i

p_i = the density of the layer 'i' in the construction (kg/m³)
c_i = the specific heat capacity of the layer 'i' (J/kg K)
d_i = the thickness of the layer 'i' (mm)

The calculation is done over all the layers of the construction element, starting at the inside surface and stopping at whichever of these conditions occurs first (including its occurrence part-way through a layer):

• half way through the construction
• an insulating layer
• a maximum thickness of 100mm

 

Of course, I'm not a scientist, but I believe that all the units used above are recognised SI?

 

 

From the same source, and making the same point that I've repeatedly made:

 

Quote

The 'k' value is a relatively crude way of determining thermal mass. It makes assumptions about the extent of the thermally active volumes of a material and ignores the effect of thermal conductivity in calculating the period over which heat is absorbed and emitted from the material.
 

 

I'd also add that just rearranging the terms for heat capacity does not magically create a new property, much as it might be considered useful, within the context of a limited view assessment process like SAP, to do so, really just to indicate that what was being described was not a complete expression for the property.

 

I'd add that it is very useful to look at the SAP worksheet and see exactly the limited purpose for which SAP uses this parameter they've made up.  It's not what many might assume when hearing the term, as I'm sure you already know.

[ToughButterCup]

@Sensus, I'm guessing those who are still with this thread might need a bit of help.... ?

[Jeremy Harris]

One problem here is that I think we may have clash of disciplines, and different, but equally valid, viewpoints that different professions have when debating a particular topic. 

 

Two or three of us in this thread have scientific qualifications that relate directly to measurement and the use of defined units.  Whilst architecture training imparts a wealth of knowledge and experience in building design, fundamentally based on the three cornerstones that Vitruvius laid down Firmitas, Utilitas et Venustas (strength, utility and beauty) I doubt that there is a strong focus on the careful and precise use of units of measurement and their definitions.

[SteamyTea]

One way to put a number, but it would be unitless, is to assume a house that changes temperature in step with any energy input, is given the number 1.  A house that does not change temperature, no matter what energy input is given to it, is given the number 0.

We could, for a laugh, call 1 Annabelle and 0 Barbara, we we dislike numbers.  Though it would make subdivisions a bit tricky i.e. a Barbara Barbara Annabelle, or Annabelle Barbara Annabelle.

This would still require some testing and a lot of data collection.  And then another unitless number to adjust for location, orientation, size, shape etc.

Or we can just use the existing Thermal Inertia formula.

That can give us a number of answers regardless of which parts are missing, it is what simultaneous equations are for.

I don't know why it is not used, probably ignorance.

[SteamyTea]

7 minutes ago, Sensus said:

6'0"

Isn't that 6 foot

[Jeremy Harris]

27 minutes ago, Sensus said:

 

As a scientist, I'm sure you'll agree that almost every form of measurement has limitations to its practical accuracy.

 

 

 

|I would, but to qualify that I would hope that the methods of measurement I have used are at least one order of magnitude more accurate than needed, so that errors in measurement do not compound to create larger errors during analysis, and that the definitions of the units used, are several orders of magnitude better, primarily to minimise the probability of errors caused by measurements taken with different apparatus compounding other measurement errors.  For example, I have a couple of NPL calibrated reference thermometers, that I use to calibrate other temperature sensors (primarily DS18B20s).  I need to be confident that whichever of the NPL thermometers I chose to use will behave in exactly the same way as the other.

 

27 minutes ago, Sensus said:

 

Your job as a scientist is to measure things.

 

Mine as an architect is to design buildings that work. I know that I don't need a fantastic level of accuracy to do that. In fact, it quickly reaches a point (due to cost to the client, if nothing else) were attempting to impose greater levels of accuracy, or more sophisticated analysis, causes more harm than benefit.

 

 

That is key to the different perspective we have on this, IMHO.  When designing our house I wanted more than architecture, I wanted the very best understanding of the behaviour of the house as a complex system as was possible.  The architecture part of the overall system design was just one element of the house as a system, I needed to understand the thermal response parameters in detail, so that I could design an appropriate ventilation, heating and cooling system, and provide controls that were easy to use and effective. 

 

The control challenge proved to be far and away the most difficult, as when the thermal time constant of the house increases, the temperature control system can no longer just be wholly reactive.  Traditional, rather crude, weather compensation doesn't work at all, and anyway heating control is not the difficult part.  I have partially addressed the control challenge by just reducing the switching hysteresis to +/- 0.1°C, but that is not a 100% solution.  It works very well for heating control, but not so well for cooling control,  and a small degree of manual intervention is required, by looking at the weather forecast and making minor adjustments to try to pre-empt external weather changes.  Heating isn't an issue because the rate of change of decreasing external temperature is not sufficiently rapid as to cause the control system any problems.  However, the cooling requirement can change very rapidly, and a bit unpredictably.  Even with solar control film on much of the glazing, and deliberately designed in overhangs to shade some windows, solar gain through the glazing remains an issue.  Had the planners allowed us to fit external shutters or blinds that would have been a near ideal solution, as automating those to control solar gain quickly would be easy. 

 

The key point I'm trying to make is that we have a timber frame house that is very thermally stable in cold weather and has a very low heating requirement, yet that makes it very susceptible to small increases in incidental heating.  Just having guests around inevitably causes the cooling system to come on after a short time, just from the additional body heat.  The impact of even small amounts of solar gain is much the same, and all of that solar gain comes in through the glazing (I've yet to be able to detect whether it's day or night by just looking at the temperature sensors on the inner surface of the external walls).

 

Our house could just as easily be a brick and block house, with similar thermal performance and a similarly long thermal time constant, and it would have the same system design challenges.  The method of construction isn't really relevant, as it's the thermal time constant, together with the means by which the house is ventilated, heated and cooled, and the level of incidental heat gain, that matters the most.

[Dreadnaught]

To quote something I read this morning by Jae Cotterell (co-author of the Passive House Handbook, and the architect of at least one of the houses featured in Ben Adam Smith's House Planning Help Podcast) (my emphasis): 

 

Quote

Building science is a poorly taught (if at all) to architects and builders within the UK context. A greater understanding of building science is essential if we want to build healthy and sustainable low energy buildings, especially as we increase levels of insulation, use timber frame construction, make use of off-site technologies, and adopt emerging system solutions.

 

Source: https://www.linkedin.com/pulse/timber-passivhaus-unforeseen-consequences-jae-cotterell/.

[SteamyTea]

1 minute ago, Sensus said:

50 minutes ago, SteamyTea said:

Isn't that 6 foot

feet

What it is called is irrelevant.

So shall call it 1.8288 metres

[Jeremy Harris]

12 minutes ago, Dreadnaught said:

To quote something I read this morning by Jae Cotterell (co-author of the Passive House Handbook, and the architect of at least one of the houses featured in Ben Adam Smith's House Planning Help Podcast) (my emphasis): 

Quote

Building science is a poorly taught (if at all) to architects and builders within the UK context. A greater understanding of building science is essential if we want to build healthy and sustainable low energy buildings, especially as we increase levels of insulation, use timber frame construction, make use of off-site technologies, and adopt emerging system solutions.

 

Source: https://www.linkedin.com/pulse/timber-passivhaus-unforeseen-consequences-jae-cotterell/.

 

 

Three or four years ago I was besieged with requests from architects to come and have a look at our build.  I've no idea why this happened, I can only assume that it may have been mentioned on an architects forum or discussion group somewhere.  All told around ten of them came for a wander around, and most were really useful people to chat to, as my understanding of architecture was necessarily pretty limited.  A couple stood out because of the questions they asked, and one, in particular, kept coming back for further visits and discussions, and those discussions focussed entirely on building science, mainly thermal properties, the impact of thermal bridging, heating and ventilation systems, trade offs between cost and performance, etc.  I've since discovered that this particular architect has a very strong interest in low energy house design and construction, and has gone so far as to volunteer to provide free labour on a couple of low energy builds.

 

I did hear remarks from several of those that came to visit that they had little experience with designing low energy homes, and one thing that stood out was that most of them had very little understanding as to how sensitive a low energy home can be to fairly small heat gains.  This was well-illustrated whilst I showed one group of four around, as after they had been in the house for about 30 minutes or so, the MVHR automatically switched to boost and full cooling mode, as their body heat had warmed the house up by a degree or so.

[SteamyTea]

2 minutes ago, Sensus said:

but who took a very different view on what that meant to PassivHaus

So they took a different view on the PassivHaus standard.

Can start to see where it all went wrong.

[SteamyTea]

2 minutes ago, Sensus said:

I prefer Annabelle.

You can't, only because I say you can't.  So you are wrong.

[Jeremy Harris]

@Dreadnaught posted a link earlier to an article that was pretty interesting, and I followed another link from that discussing the relevance of "thermal mass" to passive house design.  This quote struck me as interesting, in part because if accurately mirrors things I've measured, particularly the absence of any significant stabilising effect beyond a depth of about 50mm in any internal material (50mm was, coincidentally, the exact figure I'd come up with):

 

Quote

In conclusion, my sense is that the role of thermal mass is less significant in a Passivhaus where diurnal swings have already been significantly addressed. The most important design approaches are to avoid the summer heat reaching the interior at all (through appropriate glazing, decrement delay and fixed shading measures) and rigorous application of the fabric first principles. Higher thermal mass will typically lower the overheating percentage, although it is a more modest impact than many perceive, but it could also exacerbate heat retention. Given all these factors, I feel that a medium thermal mass construction solution such as PH15 (a timber frame alongside natural insulation materials like wood fibre and cellulose), is an appropriate specification. I would further consider including some additional thermal mass on the interior, using thinner depth finishes up to 50mm.

 

The only issue I have with this is terminology, as I believe that the author really means heat capacity, and specifically the inclusion of about 50mm thickness of high heat capacity materials within the thermal envelope, that have a sufficiently good thermal conductivity to be able to work as a thermal buffering system.  My experience suggests that the effect of such materials is modest.  It is worth having, but control of incidental heat gain, and particularly solar gain through glazing, has a massively greater impact than just increasing the heat capacity of internal structures.  Part of that is due to the relatively small ∆_T, as you want any thermal stabilisation system to start working quickly, and all internal surfaces within a low energy/passive house tend to be at equilibrium, so any thermal buffering effect doesn't start to take effect until after the internal air temperature has changed to a level a fair bit above, or below, that which feels comfortable. 

 

In short, I think we need better ways to control solar gain through glazing, and, in part, that needs planners to be prepared to accept external features that may not be commonplace, such as large overhangs, the fitting of brise soleils,  external blinds or shutters, or the adoption of variable transmissivity glass solutions, like Sage glass.  As I've mentioned before here, I wish that we'd bitten the bullet and fitted Sage glass.  Given that fitting external shutters or blinds was not an option, I feel sure that Sage glass would have been a cost-effective way to control solar gain, without adversely impacting on either the small winter benefit it gives, or affecting the external appearance of the house to any significant degree.

[Cpd]

Having followed this from the start....... I can only hope that I will forget 86% of it by tomorrow but still cannot decide which 14% I should retain in my now befuddled  head...... 

 

[SteamyTea]

For a laugh, and to possibly finish this off, I did a small model of my house today.

Knowing the size of it, how much energy I use, the temperature differences, and importantly, the distribution of those temperature differences, and the mass of my storage heater, I can get a good match.

I also took into account air changes per hour, which is an untested unknown, so I assumed 1 ACH.

The really interesting thing is that the net heating load on the house is 60 times larger than the air heating load.

So it costs a lot to heat up a slab and keep it stable.

The other interesting thing (well to me) is that the cooling load would be about half the heating load (not that I cool, I just open a window).

So I am quite happy considering that my house is 32 years old, is nothing special, is in a warmer than average part of the country, and is timber framed.

 

[joe90]

9 minutes ago, SteamyTea said:

The really interesting thing is that the net heating load on the house is 60 times larger than the air heating load.

 

For those of us who do not have a degree, could you explain the above (in words of one syllable fir me please). 

[SteamyTea]

6 minutes ago, joe90 said:

For those of us who do not have a degree, could you explain the above (in words of one syllable fir me please). 

Timber frame is better.

 

Goes back to something I said a few years ago.  I had a hunch that brick/block/concrete house would be naturally cooler and need more heating.  They also do not benefit from being significantly cooler in hot weather.  To get that benefit, they need to be very large and have even more mass (think wine cellars).  Then you pay for that in the winter by needing more heating.

 

 

Edited September 9, 2019 by SteamyTea

[ToughButterCup]

18 hours ago, Cpd said:

Having followed this from the start....... I can only hope that I will forget 86% of it by tomorrow but still cannot decide which 14% I should retain in my now befuddled  head...... 

 

By any standards, this thread contains ,for the most part, a good discussion. Good because anyone who chooses can engage with the core ideas and make their own mind up.  

Were I not full-time on my own on my build, I'd have a good go at summarising it : the process of doing that well would teach me a great deal.

Here's a go at a one (OK, 2) liner;

 

Because the  term  Thermal Mass is both commonly used and seen by some as problematic, its use should be qualified. That qualification should be given with reference to factors commonly understood to be involved in Thermal Stability. 

 

In other words, ask the person using the term for clarification.