Cavity blockwork Vs Timber frame Garden Room (1)

[Mag2.0]

Hi

I'm planning the build of a garden room / office space which will be used year round. Therefore comfort will be a significant factor.
The building will not be subject to building regulations although I plan to have it conform to a traditional house build.

I have looked at 2 structural options as follows:
This will be a self build project so labour cost is not an issue for the building fabric.

Option 1: Rendered blockwork + 100mm insulated cavity + blockwork inner

Option 2: Composite clad timber frame + PIR insulation

Has anyone previous experience with considering the 2 options?
My garden backs onto an unused field with overgrowing bushes and weeds. The chosen options must be able to withstand this as well as extended periods of damp conditions (lack of sunlight in the area).

My cost comparisons shows the cost difference is negligible between the 2 build types in terms of materials.

Option 1
+ Traditional materials / building techniques (familiarity)
+ Heavier more "robust" building
+ Less maintenance.
+ 2 layers between inside & out; more = better?
- Larger foundations required to support the load
- Longer construction time

Option 2
+ Quicker build time
-1 layer between inside & out.
-Less able to install heavy shelving units
-Need to have a period if dry weather to ensure no damage to timber elements ie OSB boards
-Greater building shrinkage.


Experiences & data is much appreciated!

Regards

 

[Notch7]

I built my garden room, 4.8m x 4.2m in timber:

95 x 45 stud walls, 50mm celotex in between, 18mm OSB then 50mm externally over studwork, then tyvek housewrap, then 50 x 25 battens, then siberian larch cladding.

I used a warm flat roof with celotex

Its been totally dry ever since it was built, zero indication of any damp whatsoever.

Even though I hardly use it in the winter, there is no problem at all with condensation or damp appearing internally


On your project you could clad the exterior with nomoreply cement board, so it would look same as the blockwork option

my walls are 210mm overall excluding internal finish

My wife might be taking it over this year as a music teaching studio in which case I will install a air con / heating unit

 

[^woody^]

The only significant differences are, the thicker masonry walls (less internal floor area) and more care required for timber frame detailing and constructing.

Anything else is just a mere difference that only needs considering if its specifically relevant and important to you.

 

[Mag2.0]

The only significant differences are, the thicker masonry walls (less internal floor area) and more care required for timber frame detailing and constructing.

Anything else is just a mere difference that only needs considering if its specifically relevant and important to you.

Hi Woody.

The marginal loss of internal floor space is not an immediate concern for us.
There must however be differences in building performance that i'm not aware of that would make a practical difference for us.
For example might be, heating time for the building, over heating during summer months.

 

[mrrusty]

Here's my build https://community.screwfix.com/threads/garden-room.256842/

There is nothing I would do different. The 122mm SIP panels are well insulated and have proved incredibly stable - no movement/shrinkage. Working with SIPs for the first time - you have to be very accurate for flatness and squareness as they are not forgiving - fine for me as I work like that anyway.

I used pad foundations mainly because I didn't want a digger in the garden with the mess/waste. They have so far proved to be very successful, but we have firm clay under the top soil - in fact our 1902 house stands on shallow corbelled brick foundations on the same clay layer and has done for 120 years.

With regard to building in bad weather - we got the membrane on the external walls promptly and used cabershield plus on the floor as it is waterproof for the duration of the build

My walls are also about 210-220mm - from in to out, PB + 25mm batten + 122SIP with 100mm XPS core + housewrap +50mm batten +cembrit plank

 

[^woody^]

There must however be differences in building performance that i'm not aware of that would make a practical difference for us.

There are differences, but in context of this being a small garden building, they won't make any significant impact on performance or use.

You've listed the key things already. If none of them matter to you enough to choose one over the other, then it does not matter which you build as the finished building and your use of it will be the same.

Heating won't matter. You will be heating the same amount of air however its built, and if insulation values are the same, you will use the same amount of energy to keep it heated. The amount of glazing will affect heat gain in summer not the wall construction. The type of heating will affect how fast it heats up in winter.

 

[Mag2.0]

There are differences, but in context of this being a small garden building, they won't make any significant impact on performance or use.

You've listed the key things already. If none of them matter to you enough to choose one over the other, then it does not matter which you build as the finished building and your use of it will be the same.

Heating won't matter. You will be heating the same amount of air however its built, and if insulation values are the same, you will use the same amount of energy to keep it heated. The amount of glazing will affect heat gain in summer not the wall construction. The type of heating will affect how fast it heats up in winter.

@mrrusty Thanks, great read so far!

@^woody^ That actually makes perfect sense. Great break down of the topic into the fundamentals.

I think I'd decided to go for a rendered blockwork build. I will however be cladding the front with timber to soften the building.

 

[Mike13]

I think I'd decided to go for a rendered blockwork build. I will however be cladding the front with timber to soften the building.

If you do that then insulate outside the blocks so that the thermal mass helps even out the internal heating demands.

 

[23vc]

One of the main reasons people go for timber frames for these (besides lack of bricklaying skill) is you can get away with massively less digging out/disposal of soil. You’re looking at a trench 1m deep and wider than the wall for masonry - not trivial to dig out or dispose of. Versus a timber frame where you can do a slab, some pads, ground screws etc. Just putting it out there if you haven’t already got it covered.

 

[Mag2.0]

If you do that then insulate outside the blocks so that the thermal mass helps even out the internal heating demands.

Hi Mike,
Can you give me some more details on how that works please?

From my understanding, having the "thermal mass" of the blockwork means that the heating system is warming up the cold blockwork.
later in the day the now warm blockwork then radiated this back into the room as the room becomes colder than the blockwork.

In a normal cavity build the insulation sits to the outside of the internal leaf which gives the same effect as that above?

Regards

 

[Mag2.0]

One of the main reasons people go for timber frames for these (besides lack of bricklaying skill) is you can get away with massively less digging out/disposal of soil. You’re looking at a trench 1m deep and wider than the wall for masonry - not trivial to dig out or dispose of. Versus a timber frame where you can do a slab, some pads, ground screws etc. Just putting it out there if you haven’t already got it covered.

Hi 23vc. Thanks, always worth pointing out. Yeh already factored that in. Adds around £600 to the build.
No building regs so could get away with digging down to competent strata (around 600mm from my porch build).

 

[^woody^]

The principle of thermal mass - ie heating up the walls which then give off the heat later, or the walls acting as a barrier by being heated by the sun to keep the inside cool, is largely academic with such a small building.

There are too many other factors at play - the main ones being the comparatively large area of roof to walls plus the roof design, the large ratio of openings to walls, the use of the building, the heat required to heat up the walls before it starts escaping, and the relatively thin walls which will presumably be cavity and insulated, not solid 600mm thick.

 

[mrrusty]

If you do that then insulate outside the blocks so that the thermal mass helps even out the internal heating demands.

The thermal mass can sometimes work against you. An outbuilding that isn't heated all the time will sometimes get very cold. When the weather changes overnight, as it often does in the UK when a warm front comes in, from zero degrees and clear/cold to much warmer and raining, and because the building with high thermal mass takes some time/heat energy to warm up, you can easily have a situation where the outside air is 100% humidity at say 12 degrees, and the blockwork is still very cold and well below the dew point, so if any of that warm wet air from outside gets inside, you get instant condensation on all the surfaces.

For an outbuilding that isn't always heated, the ideal situation is good insulation but very low thermal mass, so the temperature of the fabric of the building tracks ambient as quickly as possible.

 

[Mag2.0]

Hi,

Thank you for the input thus far on this topic particularly from some of the more seasoned participants on this community.

Is there an accepted building option which comprises of a single skin block/brick construction + insulation? My concern is moisture migration both in and out of the building.

With timber construction a Vapour control barrier is used to ensure no moisture penetrates the fabric of the building which also allowing it to exit.
With cavity construction, the cavity acts as the protection measure.

I've seen external insulation becoming more popular of late although usually installed on solid masonry walls.

I feel like i'm missing an option which is neither cavity or timber construction which i could consider.
Note, i'd be more comfortable with building out of block and would also meet the criteria of 'non combustible'.

Regards

M

 

[23vc]

You’ve kinda answered your own question; that separation is key, whether via a cavity, or the batten gap between external cladding and a timber frame. A single skin of brick needs to be expected to get saturated through to the inside face, which makes directly fixing anything onto it without a damp bridge tricky