flooring of attic

The Barrier

We're currently insulating our attic in our new build. We have 8 inch joists and we're putting 200mm of high density rigid insulation between them. These will be backed with 38mm of insulated plaster board. We had intended laying a floor on top of the joists but our builder reckons it would kill the timber i.e stop it from breathing thus resulting in dry rot down the line. Any comments or advice?

Poor Uncle Tom

I believe filling the joist space and closing it off on top and bottom will not allow the timbers to 'breath'. There should be a ventilation gap left above the insulation similar to the 50mm gap required between the felt and insulation for the rafters. Can you not counter batten over the joists before fitting the flooring?

De.Lite.Touch

The Barrier wrote: »

We're currently insulating our attic in our new build. We have 8 inch joists and we're putting 200mm of high density rigid insulation between them. These will be backed with 38mm of insulated plaster board. We had intended laying a floor on top of the joists but our builder reckons it would kill the timber i.e stop it from breathing thus resulting in dry rot down the line. Any comments or advice?

Think twice about doing it this way.

You're going to affect your timbers as anything above the insulation runs the risk of getting very cold and attracting moisture from the air - i.e. getting wet!
Water in a cold unventilated space has nowhere to go.
As Tom says, the only way to combat this is to leave a ventilation space.
For this to work, there must be movement of air, difficult to achieve in the location you're talking about.
And this doesn't really address this issues with the service pipes and water tanks.

IMO and given the amount of work you'll be doing if you do as you suggest, why not allow the insulation to follow the line of the pitched roof.
You'll need to ven at eaves and ridge and keep a 50mm air gap per Part F, P. 28, Diagram 11, D - Ceiling following the pitch of the roof.

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf

So for a 175mm rafter you use 125mm HD insulation cut to fit and placed.
If you want more insulation depth it can be built up by fixing 40mm warmboard to the inner face of the rafters.

This way you have a warm and dry attic in which to store things and you're services are kept as warm as the house.
If we get another winter like last winter you'll soon see the benefit of doing it this way.
Plus you avoid all that messing around with piercing vapour checks and insulation by dowlighters and suchlike serving the floor below.

-- De.Lite.Touch

The Barrier

Thank you for the advice. Could i still proceed with the insulation and not floor the flooring. I presume he rigid board insulation
wont impinge on the timbers.

De.Lite.Touch

The Barrier wrote: »

Thank you for the advice. Could i still proceed with the insulation and not floor the flooring. I presume he rigid board insulation
wont impinge on the timbers.

I still think this isn't the best choice.
You're still going to freeze your attic in the winter and risk condensate forming on cold serfaces.
Unless you have a continuous vapour check and eaves ventilation this could be a serious problem.

Removing the flooring out material doesn't change the internal microclimate that a well insulated ceiling will cause in your attic.
You will still have to ensure all your services are insulated even more than they are to deal with the additional drop in temperature and keep the same safety margin.
Your water tank should have no insulation beneath it but have insulation all around it continuous with the floor insulation and an insulated cover above it.
All pipes to and form all tanks should be insulated, including the overflow pipes.

If you have downlighters and/or naked vent extracts from bathrooms feeding into the attic the vapour check is breached.
Worse than that, warm moist air is entering the very cold attic under pressure - a recipe for problems.
I make this last point because I've seen it happen, where a lazy builder didn't finish off properly.

I think its better to follow the line of the roof with the insulation as I said in my previous post.
Its your call.

-- De.Lite.Touch

creedp

I'm somewhat confused with the advice given here. I'm currently building a 1 1/2 storey house and was of the opinion that the current thinking if you were not using the attic for habitable purposes was to insulate at joist level - using up to 300mm of rockwool or alternatively fill between joists with Rockwool and place rigid insulation on top that could also double as a floor for storage purposes. I was aware of the importance of ensuring an airtight ceiling to prevent warm air entering attic.

Here it seems the advice is that this approach is potentially damaging to the joists, i.e. OK to insulate between and over joists with rockwool but not to floor out the attic. The best approach is therefore to insulate at rafter level either following the slope of the roof all the way down to the eaves or to slab the attic where roof is low leaving a void behind the slabs which could be used for services etc. If opting to go with this approach how much insulation would be needed at joist level, even from a regulation perspective? It seems to me this would require an upgraded heating system as if the insulation at ceiling level is reduced then more heat will escape into attic area which is good for the attic, stored goods, services but not very good from a cost of heating perspective.

Maybe I'm not seeing the full picture here - is it possible that if you insulate at rafter level therefore keeping the attic area warmer, then the incidence of condensation is reduced allowing you to cover the joists with insulation and flooring? In this context, insulating at refter level is just an additional layer of insulation and expense but the upside is a more comfortable attic space and a lower risk of condensdation damage. Finally, if insulating at joist level, then I presume that there are no regulation stipulating the U value of the rafter insulation - other than to consider that the more the better!

I would appreciate if someone could clarify this issue for me before I get to this stage in the build because my builder is not planning to insulate the rafter and as I have quite a spacious attic I most definitely want to floor it.

All comments appreciated

Regards

Poor Uncle Tom

creedp wrote: »

I'm somewhat confused with the advice given here. I'm currently building a 1 1/2 storey house and was of the opinion that the current thinking if you were not using the attic for habitable purposes was to insulate at joist level - using up to 300mm of rockwool or alternatively fill between joists with Rockwool and place rigid insulation on top that could also double as a floor for storage purposes. I was aware of the importance of ensuring an airtight ceiling to prevent warm air entering attic.

Here it seems the advice is that this approach is potentially damaging to the joists, i.e. OK to insulate between and over joists with rockwool but not to floor out the attic. The best approach is therefore to insulate at rafter level either following the slope of the roof all the way down to the eaves or to slab the attic where roof is low leaving a void behind the slabs which could be used for services etc. If opting to go with this approach how much insulation would be needed at joist level, even from a regulation perspective? It seems to me this would require an upgraded heating system as if the insulation at ceiling level is reduced then more heat will escape into attic area which is good for the attic, stored goods, services but not very good from a cost of heating perspective.

Maybe I'm not seeing the full picture here - is it possible that if you insulate at rafter level therefore keeping the attic area warmer, then the incidence of condensation is reduced allowing you to cover the joists with insulation and flooring? In this context, insulating at refter level is just an additional layer of insulation and expense but the upside is a more comfortable attic space and a lower risk of condensdation damage. Finally, if insulating at joist level, then I presume that there are no regulation stipulating the U value of the rafter insulation - other than to consider that the more the better!

I would appreciate if someone could clarify this issue for me before I get to this stage in the build because my builder is not planning to insulate the rafter and as I have quite a spacious attic I most definitely want to floor it.

All comments appreciated

Regards

The issue is the insulation, imo.

Quilt insulation is breathable, filling between the joists with this and putting in a floor on top is acceptable.

Rigid board insulation is not breathable, using rigid board insulation for the same task to fill between joists and then providing a floor overhead is not allowing an air gap for the timbers to breathe.

Whether you insulate at rafter level or joist level is entirely up to you but remember to follow the rules regarding breathability.